BUG: Findings collected while porting ~60 filters — reported together in one issue

[physwkim]

Following up on #6569: while porting SimpleITK's filter surface to another language, I kept a ledger of everything in the ITK sources that looked wrong or surprising. It has grown to 83 suspected bugs and 118 quirks (originally 30 and 24; follow-up batches B31–B83 and Q25–Q118 are merged into the tables below) — too many to file individually without flooding the tracker, so I am reporting them together here for triage. Happy to split any subset into standalone issues if that is more useful; I am not planning PRs for all of these, so please treat this as raw material.

PR status (maintainer triage — updated 2026-07-11 by @hjmjohnson)

Ledger item	PR	Author	State
B10 (MultiLabelSTAPLE tie increment)	#6579	@physwkim	Merged
B18 (FastApproximateRank last axis)	#6580	@physwkim	Merged
B19 (AttributeMorphologyBase sort range)	#6581	@physwkim	Merged
B25 (ApplyToImageMetadata null inverse)	#6583	@dzenanz	Merged
B31 (IterativeInverseDisplacementField stale error bar)	#6576	@physwkim	Merged
B32 (InverseDisplacementField subsampler direction)	#6577	@physwkim	Merged; adjacent geometry gaps split to #6582, addressed by #6588
B32 follow-up (lattice centering, output-direction API, subsampling guards)	#6588	@hjmjohnson	Draft, CI green — closes #6582
B2, B3, B4, B9, B11, B12, B20, B23, B27, B29 (NaN / UB-cast guards)	#6589	@hjmjohnson	Ready for review, CI green (17/17)
B34, B35, Q25–Q38 (displacement-field / warp / demons batch-2+3)	branch bug-displacement-field-batch2-6575	@hjmjohnson	Local, 9 commits, tested — PR pending
B24 (FastMarchingUpwindGradient AllTargets duplicates)	branch bug-fastmarching-alltargets-6575	@hjmjohnson	Local, tested (fail-before/pass-after GTest) — PR pending
B40–B45 (LabelMap / label-statistics correctness)	#6590	@hjmjohnson	Draft, CI running — 6 commits, 3 fail-before GTests + 136-test regression
B63, B67–B69, Q82, Q84 (NrrdIO)	ITK bug-nrrdio-6575 + 4 teem branches on sourceforge	@hjmjohnson	Local/pushed upstream — vendored fixes sent to Gordon Kindlmann
B33 (ConfidenceConnected empty re-estimation)	#6578	@physwkim	Merged
Precursors	#6569 (JHMI marginals + derivative)	@physwkim	Open, CI fully green, awaiting review
	#6572 (ScaleLogarithmicTransform Jacobian)	@physwkim	Merged

All six draft PRs carry a single commit authored by @physwkim; each regression test was verified locally to fail on the pre-fix code and pass on the fix. Remaining items B1–B9, B11–B17, B20–B24, B26–B30 and the 24 quirks are unclaimed.

How these were found, and what was checked before filing

Each filter was reimplemented from the .hxx sources and the two implementations compared on synthetic inputs; every divergence was traced back to the ITK code path that caused it. Before filing, every item was re-checked against main at e46eb723a5: the cited lines re-read in context, declared types and constructor defaults verified, and each claimed consequence re-derived from the quoted code. Items whose original analysis did not survive that re-check were corrected or dropped — some of my own earlier conclusions turned out to be wrong, so I would not be surprised if a few of the remaining ones are too. Corrections welcome.

One item is flagged inline as source-analysis only (B28: non-termination identified from the loop structure, not run to a live hang).

Suspected bugs — wrong results, NaN, or UB in live code paths

Row status: ✅ completed (merged) · 🔄 in progress (open PR or prepared branch) · ⬜ not started.

Per-item detail blocks live in the collapsed comments below: B1–B30 in a dedicated comment, B31–B83 and Q25–Q118 in the follow-up batch comments.

#	Site	One-line symptom
⬜ B1	ProjectionImageFilter	Collapsed-axis origin shift uses the axis index, unsigned; axis 0 wraps to a ~2³¹·spacing shift
🔄 B2	N4BiasFieldCorrectionImageFilter::SharpenImage	Degenerate bin range → 0.0/0.0 = NaN → NaN→int cast (UB) → unchecked histogram index — fix: #6589
🔄 B3	SLICImageFilter	Empty cluster → 0/0 NaN centroid → NaN→integer index cast (UB) — fix: #6589
🔄 B4	WienerDeconvolutionImageFilter	Pf == Pn → complex division by zero → NaN output (guard passes on inf) — fix: #6589
⬜ B5	CannySegmentationLevelSetFunction	Propagation weight 0 → advection generated from a never-allocated speed image
⬜ B6	GridImageSource	Kernel loop walks only the first line per axis; origin cancels out of the pattern
⬜ B7	MinMaxCurvatureFlowFunction (3-D)	acos taken on a gradient rescaled to length r, not 1 — wrong polar angle for radius ≥ 2
⬜ B8	MinMaxCurvatureFlowFunction	Neighborhood scales divide by the stencil radius but only ±1 neighbors are sampled — update r² too small
🔄 B9	AdaptiveHistogramEqualization	Constant input → iscale == 0 → NaN everywhere — fix: #6589
✅ B10	MultiLabelSTAPLEImageFilter	Confusion-matrix increment walks into the next row on any voting tie — fix: #6579 (merged)
🔄 B11	STAPLEImageFilter	All-background inputs → p_denom == 0 → NaN, unguarded — fix: #6589
🔄 B12	LabelOverlapMeasuresImageFilter	false_positive_error guard tests the wrong quantity — NaN reachable — fix: #6589
⬜ B13	ThresholdMaximumConnectedComponentsImageFilter	Initial bisection midpoint is (upper−lower)/2; unsigned wrap when it lands below lower
⬜ B14	ScalarImageKmeansImageFilter	UseNonContiguousLabels interval underflows to 0xFFFFFFFF for > 255 classes
⬜ B15	LabelVotingImageFilter	Undecided label max_label+1 wraps to 0 for a UInt8 image using all 256 labels
⬜ B16	MaskedFFTNormalizedCorrelationImageFilter	Pixel squaring runs in the input integer type — int32 overflow before the cast to real
⬜ B17	IterativeDeconvolutionImageFilter	Output regions clobbered before PadInput/CropOutput read them — OutputRegionMode silently ignored
✅ B18	FastApproximateRankImageFilter::SetRank	Rank forwarded to only the first Dim−1 per-axis filters — last axis always median — fix: #6580 (merged)
✅ B19	AttributeMorphologyBaseImageFilter	stable_sort end iterator off by one — last raster pixel never sorted — fix: #6581 (merged)
🔄 B20	BoxSigmaImageFilter	Radius 0 → 0.0/0.0 → NaN for every pixel — fix: #6589
⬜ B21	LevelSetNeighborhoodExtractor	Strict sign tests: a pixel exactly on the contour neutralizes its neighborhood; grid-aligned contour starves the outward march
⬜ B22	StructureTensorImageFilter (AnisotropicDiffusionLBR)	Tensor smoothing K_ρ applied along axis 0 only (single-direction filter, SetDirection never called)
🔄 B23	CoherenceEnhancingDiffusionImageFilter	Constant input → ∞ rescale → NaN tensors → negative step count via UB cast → zero iterations by accident — fix: #6589
🔄 B24	FastMarchingUpwindGradientImageFilter	Duplicate target entries make AllTargets termination unreachable — fix: branch bug-fastmarching-alltargets-6575
✅ B25	Transform::ApplyToImageMetadata	Null GetInverseTransform() (singular or non-linear transform) dereferenced without a check — fix: #6583 (merged)
⬜ B26	PatchBasedDenoisingImageFilter (POISSON)	static_cast<PixelValueType>(0.99999) is 0 for integer types — step size collapses to 1e-5 always
🔄 B27	PatchBasedDenoisingImageFilter	Update write-back casts an unclamped negative float to an unsigned pixel type — UB — fix: #6589
⬜ B28	UniformRandomSpatialNeighborSubsampler	Search loops forever when the search box is exactly the query point (kernel-bandwidth path)
🔄 B29	GaussianRandomSpatialNeighborSubsampler	Negative Gaussian variate cast to unsigned int — UB; rejection loop relies on the wrap — fix: #6589
⬜ B30	RegionBasedLevelSetFunction	Reinitialization smoothing term silently degenerates to a bare Laplacian when CurvatureWeight == 0
✅ B31	IterativeInverseDisplacementFieldImageFilter	smallestError carries over from the previous pixel when the mapped point starts outside the buffer — output depends on raster order — fix: #6576 (merged)
✅ B32	InverseDisplacementFieldImageFilter	the field-subsampling resampler never gets the input's direction — with non-identity direction cosines the spline landmarks are not on the input lattice at all — fix: #6577 (merged)
✅ B33	VectorConfidenceConnectedImageFilter	a re-estimation pass that visits zero pixels produces NaN statistics, which invert the Mahalanobis guard — the next flood includes every pixel in the image — fix: #6578 (merged)
🔄 B34	WarpImageFilter	the same-information fast-path test omits size, so a valid larger-output configuration dies in an iterator containment check instead of taking the general path
🔄 B35	ExponentialDisplacementFieldImageFilter	the field is divided by 1 << numiter — UB when numiter >= 31; unreachable by default (cap 20, clamped), but an explicit SetMaximumNumberOfIterations(>= 31) makes it live
⬜ B36	FastMarchingImageFilterBase::UpdateNeighbors	the neighbor loop gates on the center being strictly interior, so a border seed never propagates along the boundary-normal axis — a face seed confines the march to that face, a corner seed marches nowhere
⬜ B37	FastMarchingImageFilterBase (NoHandles)	marched-alive voxels never receive a connected-component label, so the facing-neighbor label test always passes and NoHandles degenerates to Strict
⬜ B38	FastMarchingImageFilterBase::Solve	the zero-speed guard substitutes a finite coefficient, giving zero-speed pixels an arrival time ≈ 2⁵² instead of the old filter's ∞ — speed barriers leak
⬜ B39	ScalarToRGBColormapImageFilter / ColormapFunction	NumericTraits<float/double>::min() (smallest positive) used as a "most negative" seed in two places — wrong rescale for non-positive float images, and a ≈FLT_MAX default input range
🔄 B40	MergeLabelMapFilter (KEEP)	the deferred-collision deque is never cleared between inputs — with ≥ 3 inputs one LabelObject ends up aliased under two label keys — fix: #6590
🔄 B41	LabelMapContourOverlayImageFilter (SLICE_CONTOUR)	the slice-radius loop tests the write cursor against m_SliceDimension where it means the read index — SliceDimension == 0 draws no contour at all — fix: #6590
🔄 B42	LabelMapMaskImageFilter (Crop)	an empty/zero-line object set leaves the bounding-box seeds untouched — signed overflow (UB), then a 2×2 region at index 2⁶³−1 — fix: #6590
🔄 B43	AttributeUniqueLabelMapFilter	the "remove objects without lines" pass is while (it.IsAtEnd()) — inverted, never executes — fix: #6590
🔄 B44	LabelMap::PushLabelObject	on a completely full label range the gap scan finds nothing, skips SetLabel, and silently overwrites the entry at the object's own label instead of throwing "map is full" — fix: #6590
🔄 B45	StatisticsLabelMapFilter	weighted elongation/flatness: no eigenvalue clamp, one shared guard, no ratio > 0 check — an indefinite weighted covariance yields NaN where ShapeLabelMapFilter yields 0 — fix: #6590
⬜ B46	NormalizedCorrelationImageFilter	both divisions unguarded — a constant template or a locally-constant neighborhood produces NaN
⬜ B47	TransformIOBaseTemplate::CorrectTransformPrecisionType	find("float") result used unchecked — a transform-type string naming neither precision throws std::out_of_range past every ITK handler
⬜ B48	NiftiImageIO::Read	rescale cast buffer sized per voxel, consumed per component — heap over-read for any multi-component image with scl_slope/scl_inter
⬜ B49	NiftiImageIO::Read	RescaleFunction is given the voxel count — only the first numElts of a multi-component/complex buffer are rescaled
⬜ B50	NiftiImageIO::Read	RAS↔LPS conversion never checks the 3-component requirement its own exception text names — 2/4-component vectors get a stride-3 sign flip across voxel boundaries
⬜ B51	NiftiImageIO	spatial-dimension inference for intent-vector images never consults dim[1] — a 2-D vector image one row tall reads back as 1-D
⬜ B52	niftilib nifti_read_buffer (vendored)	maps every non-finite float/double/complex component to 0 on read wherever isfinite is a macro (glibc: always) — NaN/±Inf pixels silently destroyed
⬜ B53	GiplImageIO::Write	INT/UINT images throw "Pixel Type Unknown" only after the target file has been truncated and the header written — destroys existing data, leaves a 256-byte stub
⬜ B54	GiplImageIO::Read	success test is !bad(); a short pixel block sets failbit/eofbit, never badbit — truncated file reads as success over an uninitialized tail
⬜ B55	GiplImageIO::Read (compressed)	success flag is p != nullptr on the caller's always-non-null output buffer — gzread's byte count is never inspected
⬜ B56	GiplImageIO::Write	header image_type is derived from the component type alone while the byte count multiplies by components — a vector image writes a scalar header plus N× the bytes, reading back as garbage
⬜ B57	VTKImageIO::Read	ReadBufferAsBinary's bool (false on short read) is discarded — truncated BINARY file reads as success
⬜ B58	VTKImageIO::InternalReadImageInformation	every sscanf return is discarded over uninitialized locals — an under-filled DIMENSIONS/SPACING/ORIGIN line sizes the image from indeterminate values (UB)
⬜ B59	VTKImageIO::InternalReadImageInformation	attribute dispatch is an ordered substring test — SCALARS vector_field float matches the vector arm and is misparsed as a 3-component VECTORS line
⬜ B60	VTKImageIO ASCII write	precision(16) is set on a throwaway header stream; pixel data is emitted through a second stream at the default 6 significant digits — silent precision loss
⬜ B61	ImageIOBase::ReadBuffer (ASCII)	extraction variable declared outside the loop, stream state never checked — one bad/overflowing field latches into every later component (5 99999 7 8 as short reads [5, 32767, 32767, 32767])
⬜ B62	VTKImageIO::CanReadFile	four GetNextLine calls outside the try block — a .vtk shorter than four lines throws "Premature EOF" through the factory instead of returning false
🔄 B63	NrrdImageIO::Write metadata dispatch	NRRD_-prefixed keys are matched by strncmp of the candidate's length — NRRD_space directions is swallowed by the space handler, and unmatched NRRD_ keys (including the reader's own NRRD_pixel_original_axis) are silently dropped — fix: branch bug-nrrdio-6575
⬜ B64	MetaIO ElementDataFile = LIST (vendored)	three unguarded paths: short slice list → success with uninitialized tail; negative file dimension → m_DimSize[-1]/m_SubQuantity[-1] OOB reads; == NDims → zero-length slice reads, still success
⬜ B65	MetaIO M_ReadElements (vendored)	a LOCAL compressed header without CompressedDataSize seeks to file start and inflates its own header text over the uninitialized pixel buffer — success, garbage pixels
⬜ B66	MetaIO MET_PerformUncompression (vendored)	unconditionally return true — truncated/corrupt/wrong-container streams and short output all report success
🔄 B67	NrrdIO per-axis field parsers (vendored)	sizes/spacings/thicknesses/axis mins/axis maxs parse dim + 1 values into a NRRD_DIM_MAX(16)-element stack array — a 16-D header with a 17th token is a stack buffer overflow — fix: teem branch bug-nrrd-parse-axis-overflow (pushed to sourceforge; regenerates into ITK NrrdIO) + ITK-side bug-nrrdio-6575
🔄 B68	NrrdIO nrrdOriginCalculate (vendored)	the gotMin loop tests axis[0]->min for every axis (literal 0, not the loop variable) — origins computed from NaN mins, or usable mins ignored — fix: teem branch bug-nrrd-origin-calculate (pushed to sourceforge; regenerates into ITK NrrdIO) + ITK-side bug-nrrdio-6575
🔄 B69	NrrdIO rnParse_kinds (vendored)	the none token arm assigns the axis's centering, not its kind — kinds: none … clobbers a previously parsed centers: value — fix: teem branch bug-nrrd-kinds-none-centering (pushed to sourceforge; regenerates into ITK NrrdIO) + ITK-side bug-nrrdio-6575
⬜ B70	PNGImageIO::WriteSlice	the file is opened "wb" — truncating it — before the component-type switch whose default: throws; writing an int16 image destroys any existing .png and leaves it zero-byte
⬜ B71	PNGImageIO::WriteSlice	height comes from GetDimensions(1) alone, no axis beyond it is consulted — a 3-D image writes only its first slice, with no error
⬜ B72	PNGImageIO::WriteSlice	any component count > 4 declares 4 channels (PNG_COLOR_TYPE_RGB_ALPHA) while row pointers advance by the actual count — each row's trailing components are silently dropped
⬜ B73	HDF5ImageIO::WriteImageInformation	the C-string meta-data branch calls constCstringObj->GetMetaDataObjectValue() under a guard that admits constCstringObj == nullptr — null dereference for a MetaDataObject<char *>
⬜ B74	HDF5ImageIO::ReadImageInformation	every geometry dataset's length is taken on trust — spacing[i] / Dims[i] are indexed to numDims regardless of the file's actual dataset length, reading past the end of a heap vector before any pixel is touched
⬜ B75	HDF5ImageIO::Read	the file datatype is passed as H5Dread's memory type, making the conversion the identity — a VoxelData stored STD_I16BE is copied without a byte swap and read back byte-swapped, silently
⬜ B76	JPEGImageIO::WriteSlice	the "prefer centimetres" density branch tags the JFIF header density_unit = 0 ("aspect ratio only") while storing real dots-per-cm values — the reader's own density_unit > 0 guard then discards them, so the spacing is lost on a round-trip through ITK
⬜ B77	JPEGImageIO::Read	the CMYK→RGB conversion assumes all four channels are inverted; that holds for plain CMYK, but for a YCCK source libjpeg un-inverts C/M/Y and passes K through — a white YCCK pixel decodes to black
⬜ B78	TIFFImageIO::ReadVolume	each kept page is written at its raw directory index while the volume depth is sized from m_SubFiles — one ignored or untagged page before a SUBFILETYPE == 0 page and the last slice is written past the end of the buffer
⬜ B79	TIFFImageIO::ReadGenericImage	the scanline buffer is sized from the current directory, but width/SamplesPerPixel/BitsPerSample stay pinned to directory 0 — a multi-page file whose later pages are narrower over-reads the buffer
⬜ B80	TIFFImageIO::ReadImageInformation	the 32-bit arm of the component-type ladder has no default:, and CanRead never constrains SampleFormat — a 32-bit SAMPLEFORMAT_VOID page is read as whatever m_ComponentType already held
⬜ B81	TIFFImageIO::ReadGenericImage	PHOTOMETRIC_MINISWHITE is read out un-inverted — the // check inverted comment above the plain-copy grayscale arm never grew a body
⬜ B82	TIFFImageIO::GetFormat	MINISBLACK/MINISWHITE map to GRAYSCALE regardless of SamplesPerPixel, so a 2-sample grey+alpha page is declared 1-component and each row keeps only its first half
⬜ B83	TIFFImageIO::InternalWrite	PHOTOMETRIC_RGB is written for every component count other than 1 — a 2-component image becomes an RGB-tagged file with SamplesPerPixel = 2 and no EXTRASAMPLES

Reproduced quirks and inconsistencies

These were verified the same way but may be judged intended, historical, or not worth changing — listed for the record, one line each, citations available on request (I have exact file:line and analysis for every row).

Verified quirks Q1–Q118 (table)

#	Site	Behavior
⬜ Q1	Deconvolution family	The same KernelZeroMagnitudeThreshold ε is compared against |H| by Inverse but against the |H|²-carrying denominator by Tikhonov/Wiener — "regularization → 0 equals inverse" fails in the band ε ≤ |H| < √ε.
⬜ Q2	DiscreteGaussianDerivativeImageFilter	Never calls FlipAxes(), unlike DerivativeImageFilter/GradientImageFilter (which do) — odd-order derivatives carry the opposite sign vs. its siblings. Also the inter-axis intermediate is Image<OutputPixelType>, so integer outputs truncate per axis pass (the local comments say "real to real").
⬜ Q3	ThresholdSegmentationLevelSetImageFilter	The class doc (.h:63-65) states positive = inside — the opposite of the convention its own base class documents (itkSegmentationLevelSetImageFilter.h:82-84: negative inside).
⬜ Q4	Segmentation level-set functions	CurvatureSpeed is overridden to PropagationSpeed only by GeodesicActiveContour and ShapeDetection; Threshold/Laplacian/Canny keep the base-class constant 1 — an undocumented asymmetry across siblings.
⬜ Q5	BinaryContourImageFilter	Non-foreground pixels keep their original values verbatim (.hxx:164-179) — they are not normalized to BackgroundValue, so several distinct non-foreground input values all survive into the output.
⬜ Q6	SimpleContourExtractorImageFilter	InputForegroundValue == InputBackgroundValue marks every foreground pixel as contour (the neighborhood scan includes the center pixel, .hxx:84).
⬜ Q7	BinaryReconstructionByErosionImageFilter	Built on the complement via BinaryNot; BackgroundValue is consumed only as a self-consistent internal sentinel — the parameter has no effect on the output.
⬜ Q8	AnisotropicDiffusionImageFilter	An unstable time step warns and proceeds; the clamp is present but commented out (.hxx:68).
⬜ Q9	ThresholdMaximumConnectedComponentsImageFilter	m_LowerBoundary is written once in the constructor and read only by PrintSelf — dead configuration with no setter.
⬜ Q10	MinMaxCurvatureFlowFunction	SetStencilRadius clamps to ≥ 1 (.hxx:43), making the radius == 0 early-outs (.hxx:217-220, 276-279) unreachable dead code.
⬜ Q11	ScalarImageKmeansImageFilter	Overrides the estimator's defaults in GenerateData (SetMaximumIteration(200), threshold 0.0, .hxx:91-92), invisibly to users; and KdTreeBasedKmeansEstimator's loop tests the limit pre-increment after the refinement pass (.hxx:327-347) — MaximumIteration = n runs n+1 passes while GetCurrentIteration() reports n.
⬜ Q12	SLICImageFilter	Enforce-connectivity relabels an undersized component to the previously-encountered label, whatever it is (.hxx:544-546); there is no final relabel/compaction pass; per-pixel distances accumulate in float.
⬜ Q13	BoxMean/BoxSigmaImageFilter	The window crops at the image border and divides by the cropped count (itkBoxUtilities.h:322-324, 499-501) — border statistics use fewer samples. A ZeroFluxNeumannBoundaryCondition is declared (L215/389) but never used, suggesting replication was once intended.
⬜ Q14	BinShrinkImageFilter	Throws when a shrink factor exceeds the axis size where ShrinkImageFilter silently clamps to 1 (.hxx:289-296 vs itkShrinkImageFilter.hxx:265-267); integer outputs round half-up while float outputs truncate (RoundIfInteger, .h:135-149).
⬜ Q15	AntiAliasBinaryImageFilter	A constant input (even all-maximum) shifts to 0 under the max − (max−min)/2 iso-value and the strict > 0 sign test (.hxx:109; base .hxx:605) — the output is uniformly negative, i.e. "all inside".
⬜ Q16	ReinitializeLevelSetImageFilter	Narrow-band mode pre-fills the whole output with ±NumericTraits<PixelType>::max() and only overwrites band points (.hxx:206-225) — far-field pixels keep ±max rather than a distance-like magnitude.
⬜ Q17	FastMarchingUpwindGradientImageFilter	m_TargetValue is overwritten at every accepted point in NoTargets mode; in Some/AllTargets modes the reached test is sticky, so every later accepted point — target or not — keeps advancing it; OneTarget reports the last reached target, not the first (.hxx:154-227).
⬜ Q18	CollidingFrontsImageFilter	ApplyConnectivity's flood fill is seeded on SeedPoints1 alone (.hxx:114-122), so the forced −1e-6 at every SeedPoints2 seed (.hxx:92-97) is dropped from the output unless that seed is face-connected to front 1's region.
⬜ Q19	itk::watershed::Segmenter	Edge lists are sorted with a height-only operator< (itkWatershedSegmentTable.h:73-82); a NaN height compares "equivalent" to everything — a strict-weak-ordering violation (formal UB; benign merge-equal in libstdc++).
⬜ Q20	itkMath.h FloatAlmostEqual	The absolute-difference test (default 0.1 · epsilon) returns equal before the signbit check (itkMath.h:334-352) — 0.0 == -0.0, and near-zero pairs millions of ULPs apart compare equal, partially defeating the documented ULP semantics.
⬜ Q21	MultiphaseDenseFiniteDifferenceImageFilter	Computes the CFL time step then discards it: timeStep = 0.08; // FIXME !!! After all this work, assign a constant !!! Why ?? (.hxx:153-154).
⬜ Q22	Chan-Vese UseImageSpacing	Builds m_ScaleCoefficients that nothing in the Chan-Vese chain reads; the PDE divides by m_InvSpacing from the feature image unconditionally while the re-distancing Maurer map inherits the level-set image's spacing — the flag's only live effect is maurer->SetUseImageSpacing (.hxx:227). Two images with different spacings drive the PDE and the re-distancing with different metrics.
⬜ Q23	ScalarChanAndVeseDenseLevelSet RMS/labeling	m_ReinitializeCounter defaults to 1, so the reinit branch runs every iteration, zeroes the RMS accumulator, and refills it with the re-distancing residual (.hxx:214-248) — MaximumRMSError halting compares against the wrong quantity. CopyInputToOutput labels phi < 0 strictly (.hxx:69), so exactly-zero boundary pixels are excluded from the label.
⬜ Q24	ObjectMorphologyImageFilter	BeforeThreadedGenerateData's fill-then-conditional-copy always reduces to output := input (the fill sentinel is constructed to differ from ObjectValue, .hxx:87-113); UseBoundaryCondition is dead configuration (installed but never enabled, .h:145 even warns "Don't forget to set UseBoundaryCondition to true!"); boundary detection is a hard-coded 3^dim box decoupled from the kernel radius (.hxx:130-198) — dilation is unaffected, erosion diverges substantially from true erosion (the header caveats this).
🔄 Q25	IterativeInverseDisplacementFieldImageFilter	the coordinate-descent probe step is axis 0's spacing on every axis
🔄 Q26	DisplacementFieldJacobianDeterminantFilter	SetDerivativeWeights silently flips UseImageSpacing off — precedence between the two is last-writer-wins, while the class doc describes only one ordering
🔄 Q27	WarpImageFilter	m_OutputSize[0] == 0 alone keys "size unset"; output-grid defaults are unit/zero/identity from neither input; the field is sampled with edge replication, never out-of-domain
🔄 Q28	VectorConnectedComponentImageFilter	zero vectors join anything under the default threshold (no special case); concept macro name typo InputValyeTypeIsFloatingCheck
🔄 Q29	MahalanobisDistanceMembershipFunction	the det < 0 exception branch is unreachable; GetCovariance() never reflects the singular-case substitute
🔄 Q30	FastSymmetricForcesDemonsRegistrationFilter	its difference function is ESMDemonsRegistrationFunction; the identically-named FastSymmetricForcesDemonsRegistrationFunction class is orphaned from its own filter
🔄 Q31	ExponentialDisplacementFieldImageFilter	"take the ceil" comment vs + 1.0-then-truncate code (integral values overshoot by 1); a zero field still runs one squaring iteration
🔄 Q32	WarpVectorImageFilter (as wired by DiffeomorphicDemonsRegistrationFilter)	under the nearest-neighbor-extrapolating interpolator, IsInsideBuffer is always true and m_EdgePaddingValue is dead code
🔄 Q33	ESMDemonsRegistrationFunction	the WarpedMoving and MappedMoving gradient options coincide exactly at a zero displacement field (interior pixels, default linear interpolation, coincident grids)
🔄 Q34	LevelSetMotionRegistrationFilter/Function	dt = 1/maxL1 makes the applied step at the governing pixel exactly Alpha-independent; thresholded pixels count toward the metric while out-of-buffer pixels don't; Halt adds an RMS==0 clause the base can never fire
🔄 Q35	LevelSetMotionRegistrationFunction	one-sided difference steps add index-axis spacing to a physical coordinate — wrong direction and step under non-identity direction cosines
🔄 Q36	demons family	zero-iteration runs report GetRMSChange() == 0 but GetMetric() == NumericTraits<double>::max() in the two filters that don't override GetRMSChange
🔄 Q37	LevelSetMotionRegistrationFunction smoothing	the smoothed moving image is re-quantized to the moving image's own pixel type (u8 in → u8 smoothed) before the gradient; and any axis shorter than 4 pixels is a hard error for this family member only
🔄 Q38	FastMarchingImageFilterBase::IsChangeWellComposed3D	builds its neighborhood iterator over GetRequestedRegion() (empty, never set) where the 2-D sibling and the strict-topology check use GetBufferedRegion() — harmless (only SetLocation+GetPixel are used) but inconsistent (.hxx:779 vs :583/:619)
⬜ Q39	LabelToRGBImageFilter	background is always fixed black (m_BackgroundColor zero-filled at construction); the functor comment promising a "gray pixel matching the background intensity" describes LabelOverlayFunctor, not this class (itkLabelToRGBFunctor.h:93-98)
⬜ Q40	LabelToRGB/LabelOverlay palette	the 0–255 palette constants are rescaled by NumericTraits<output RGB component>::max() rather than a fixed 255 (itkLabelToRGBFunctor.h:110-117) — a double RGB output scales the palette into [0, ≈1.8e308]
⬜ Q41	HessianToObjectnessMeasureImageFilter	eigenvalues are sorted by |λ| with unstable std::sort (equal-magnitude order unspecified, .hxx:78-79); alpha == 0 skips only the R_A factor while beta == 0 (or a zero denominator) zeroes the whole measure (.hxx:111-151) — asymmetric parameter semantics
⬜ Q42	ComplexToModulusImageFilter	computes sqrt(re*re + im*im) in the component type (.h:46-50): ComplexFloat32 overflows to inf above |component| ≈ 1.8e19 and flushes to 0 near denormals, where std::hypot would not
⬜ Q43	RelabelLabelMapFilter	class doc says objects are reassigned via PushLabelObject; the inherited implementation uses SetLabel + AddLabelObject against a counter from 0 (itkAttributeRelabelLabelMapFilter.hxx:64-79)
⬜ Q44	LabelMap::HasLabel	doc says the background label also returns true; the implementation is a plain find(label) != end(), which returns false for background (.hxx:151-153)
⬜ Q45	StatisticsLabelMapFilter weighted moments	adds spacing²/12 (the pixel's own second moment) to every central-moment diagonal (.hxx:224-228) while ShapeLabelMapFilter does not — WeightedPrincipalMoments exceed PrincipalMoments by 1/12 per eigenvalue even on a constant feature image
⬜ Q46	StatisticsLabelMapFilter extremum indices	min/max index updates are non-strict (v <= min, v >= max, .hxx:118-127) — on ties the recorded index is the last tie in raster order
⬜ Q47	itk::Statistics::Histogram::Initialize	computes the bin interval and every non-final bin bound in float (itkHistogram.hxx:225-236) — a Histogram<double> gets float-quantized bin edges, which StatisticsLabelMapFilter's Median inherits
⬜ Q48	ImageBase::IsCongruentImageGeometry	scales the coordinate tolerance by this->GetSpacing()[0] only (itkImageBase.hxx:400-405) — the predicate is asymmetric when the two images differ in axis-0 spacing, and axes 1+ are judged against axis 0's scale
⬜ Q49	DivideImageFilter::Div	a near-zero divisor returns NumericTraits<TOutput>::max() regardless of the numerator's sign or magnitude (itkArithmeticOpsFunctors.h:142-151); through NormalizeToConstantImageFilter a zero-sum input sends the entire output to the type maximum
⬜ Q50	VotingBinaryHoleFillingImageFilter	the else-branch stamps it.Set(foregroundValue) for any center value other than backgroundValue (.hxx:138-141) — a third pixel value is silently promoted to foreground without counting toward GetNumberOfPixelsChanged(); the iterative sibling inherits this
⬜ Q51	ExtractImageFilter	the output origin ignores a collapsed axis's start index (and the collapsed↔retained direction coupling), so extracting slice k of an oblique volume reports slice 0's origin (.hxx:74-85, :162-179)
⬜ Q52	CenteredVersorTransformInitializer::ComputeRotation	uses R = M·Fᵀ (.hxx:47), which does not satisfy the covariance-aligning identity R·Cov_f·Rᵀ = Cov_m (that requires Mᵀ·F); with per-image independent eigenvector sign canonicalization the initializer can 180°-flip an axis
⬜ Q53	CompositeTransform	Get/SetParameters, Get/SetFixedParameters, and UpdateTransformParameters all flatten sub-transforms in reverse queue order (last-added first, .hxx:613,665-687,703,734,847) while GetNthTransform(n) indexes forward — two orderings in one class
⬜ Q54	FFTPadImageFilter	SizeGreatestPrimeFactor == 1 pads each axis to an even size (.hxx:64-68) while the class doc says "1 or less … disable the extra padding" (.h:90) — only 0 disables
⬜ Q55	MaskedFFTNormalizedCorrelationImageFilter	pads FFT sizes using only factors 2/3/5 (.hxx:528-545) while FFTConvolutionImageFilter uses the backend's greatest prime factor (11 under PocketFFT) — identical inputs get different padded sizes across the two FFT-based filters
⬜ Q56	RegistrationParameterScalesFromIndexShift	the step scale is computed in continuous-index (voxel) units while MaximumStepSizeInPhysicalUnits defaults to the minimum virtual spacing (mm) — the gradient-descent learning rate divides mm by voxels, dimensionally mixed on anisotropic spacing (.hxx:66-79; itkGradientDescentOptimizerv4.hxx:272)
⬜ Q57	RegistrationParameterScalesEstimator CentralRegionRadius	consulted only by CentralRegionSampling, which is selected only for displacement-field/B-spline transforms (.hxx:346-367, :203-211); linear/affine transforms take Corner/Random sampling and never read it, and for displacement fields the scales estimate is radius-independent
⬜ Q58	ObjectToObjectOptimizerBase weights	optimizer weights within 1e-4 of 1.0 are flagged identity and never multiplied into the gradient (strict >, so exactly 1e-4 is also discarded, itkObjectToObjectOptimizerBase.cxx:154-164); the sibling scales-identity test uses a 100× looser 0.01
⬜ Q59	v4 optimizer weights application	every v4 optimizer validates the weights-array length in the base StartOptimization, but the vnl-based LBFGS/LBFGSB and Amoeba optimizers then apply only scales and silently ignore well-sized weights (itkSingleValuedNonLinearVnlOptimizerv4.cxx:53-58); LBFGS2Optimizerv4, by contrast, does apply them
⬜ Q60	NiftiImageIO	ITK_InputFilterName is encapsulated (:1102) and then wiped six lines later by SetImageIOMetadataFromNIfTI, whose first act is thisDic.Clear() (:630)
⬜ Q61	NiftiImageIO	the "original file was non-orthogonal" warning is dead: stored under ITK_sform_corrected (:632), looked up under nifti_sform_corrected (:2038)
⬜ Q62	NiftiImageIO	getQFormCodeFromDictionary/getSFormCodeFromDictionary results are unconditionally overwritten with NIFTI_XFORM_SCANNER_ANAT (:2077-2078) — yet their StringToInt32 still throws on a non-numeric dictionary value that would have been discarded anyway
⬜ Q63	NiftiImageIO	IMG.NII passes CanWriteFile (extension check lowercases) but WriteImageInformation compares the raw uppercase extension against lowercase spellings only and throws "Bad Nifti file name" (:1173-1202)
⬜ Q64	niftilib (vendored)	a magic-less (Analyze-7.5) header in a single .nii reports nifti_type == 1: convert_nhdr2nim sets ANALYZE, then nifti_set_type_from_names forces NIFTI1_1 whenever fname == iname — true for every single-file case (nifti1_io.c:3730, :3453-3454)
⬜ Q65	GiplImageIO	the magic number is validated only in CanReadFile; ReadImageInformation re-reads the same 4 bytes and discards them, so a file supplied via SetImageIO gets no magic check (:566-584)
⬜ Q66	GiplImageIO	NDims is a population count of non-trivial dims — a 2-D image round-trips as 3-D [nx, ny, 1], and malformed dims like [4, 0, 5, 1] give NDims == 2 with the 5 silently dropped (:294-312)
⬜ Q67	GiplImageIO	image_type is byte-swapped only in the BigEndian arm and min/max are never swapped — unobservable, since the constructor pins BigEndian and all these header fields are read then discarded (:78, :325-328, :443-461)
⬜ Q68	GiplImageIO	registers no read/write extensions (constructor omits AddSupported*Extension) and its real gate is a case-sensitive suffix test — IMAGE.GIPL is not recognized (:1079-1090)
⬜ Q69	GiplImageIO	the compressed path uses gzopen/gzread throughout, which transparently pass through a non-gzip stream — a .gipl.gz containing a plain GIPL file is accepted and read verbatim
⬜ Q70	VTKImageIO::GetNextLine	tolerates 5 consecutive empty lines and throws on the 6th, while its message says "consecutive 5 empty lines" — off by one from its own text (:44-46)
⬜ Q71	VTKImageIO::GetNextLine	tests eof() before using the line, so a header whose final line lacks a trailing newline throws "Premature EOF" even though getline delivered the content (:50-56)
⬜ Q72	VTKImageIO	the 2-D/3-D test is written twice and the first copy is dead — the surviving rule is "2-D iff dims[2] <= 1", so DIMENSIONS 4 1 3 is a one-row-tall 3-D volume (:210-221)
⬜ Q73	VTKImageIO	a round-trip collapses ComplexFloat32 to a 2-component VectorFloat32, and a 1-component vector image to a scalar — the legacy format cannot express either distinction (:701-709, :311-329)
⬜ Q74	VTKImageIO	the component count is never range-checked on either side (writer carries a // todo this should be asserted, :705) — COLOR_SCALARS c 7 reads as a 7-component VectorUInt8
⬜ Q75	ImageIOBase ASCII read	is >> temp with an integer PrintType: a negative literal for an unsigned pixel type wraps rather than failing, and out-of-range 8-bit values narrow silently (−1 → 255, 300 → 44) (itkImageIOBase.cxx:804-811)
⬜ Q76	MetaImageIO	complex pixels cannot round-trip: ElementNumberOfChannels == 2 is indistinguishable from a 2-vector, and the reader forces VECTOR for any channels > 1 (itkMetaImageIO.cxx:241-244, in-source // BUG: 8732)
⬜ Q77	MetaIO (vendored)	MetaImage::CanRead re-tests the suffix case-sensitively against .mha/.mhd while the ITK factory and CanWriteFile match case-insensitively — IMG.MHA is writable but not readable (metaImage.cxx:1184-1199)
⬜ Q78	MetaIO (vendored)	booleans test only value[0] == 'T'|'t'|'1' — trash parses as true, yes as false (metaObject.cxx:1605-1642)
⬜ Q79	MetaIO (vendored)	header-alias precedence is M_Read's apply order, not file order: Origin > Position > Offset, TransformMatrix > Rotation > Orientation, BinaryDataByteOrderMSB > ElementByteOrderMSB — a trailing ElementByteOrderMSB = False loses to an earlier BinaryDataByteOrderMSB = True (metaObject.cxx:1618-1707)
⬜ Q80	MetaIO (vendored)	writes a zlib (RFC-1950) stream but reads zlib or gzip (inflateInit2(…, 47)) — the reader accepts a superset of what the writer produces (metaUtils.cxx:808, :862)
⬜ Q81	vendored gzip readers	NrrdIO's gzio and zlib's gz_look fall back to a byte-for-byte copy when the gzip magic is absent, while niftilib's nifti_is_gzfile is a pure name test — a genuine gzip stream named .nii is never inflated, and a .nii.gz holding raw bytes reads via the copy fallback (gzio.c:501-519; nifti1_io.c:2656-2668) — reviewed, no change (see below)
🔄 Q82	NrrdImageIO	only RAS/LAS/LPS are converted to LPS; every other space value (scanner-xyz, right-up, …) has its geometry used verbatim through an empty default: with no warning (itkNrrdImageIO.cxx:766-786) — fix: branch bug-nrrdio-6575
⬜ Q83	NrrdIO (vendored)	data file: LIST reads filenames until EOF, so a blank line becomes an empty filename that later fails fopen("") — there is no in-band list terminator (parseNrrd.c:1347-1400) — reviewed, no change (see below)
🔄 Q84	NrrdIO (vendored)	formatNRRD_write emits comments and key/value pairs after the data file: field, defeating the writer's own "data file must be last" ordering — latent header corruption for LIST-form writes with comments (formatNRRD.c:783-857; cf. write.c:724-728) — fix: teem branch bug-nrrd-datafile-last (pushed to sourceforge; regenerates into ITK NrrdIO) + ITK-side bug-nrrdio-6575
⬜ Q85	TxtTransformIO	Parameters: lines are parsed by vnl read_ascii, which stops at the first unparseable token and reports success — Parameters: 1 2 oops 4 silently yields [1, 2] (itkTxtTransformIO.cxx:183-186)
⬜ Q86	TxtTransformIO	one transform local is reassigned per Transform: line and buffered parameters apply only once both Parameters and FixedParameters have been seen — unpaired lines bind one transform's parameters to a different transform, silently (:168-232)
⬜ Q87	TxtTransformIO	the #Insight Transform File V1.0 header is swallowed by the #-comment skip and never validated; CanReadFile checks only the .txt/.tfm extension (:148, :39-46)
⬜ Q88	TxtTransformIO	an empty FixedParameters vector writes the line FixedParameters: with a trailing space (:238-250, :303-305)
⬜ Q89	HDF5TransformIO	CanWriteFile claims .hdf/.h4/.hdf4/.he4 — HDF4 container names — though Write always creates an HDF5 file, and the lowercase extension table rejects .H5/.HDF5 (itkHDF5TransformIO.cxx:83-84, :424)
⬜ Q90	PNGImageIO::ReadImageInformation is a bare return; on a wrong PNG signature (:334-338) where Read throws "File is not png type: …" for the identical png_sig_cmp (:139-143) — a caller reaching it via SetImageIO (bypassing CanReadFile) gets a fabricated success with m_Dimensions unset. A missing file pointer returns silently the same way (:321-324)
⬜ Q91	PNGImageIO::CanWriteFile passes ignoreCase = false to HasSupportedWriteExtension (:497), unlike the trait's own case-insensitive default; .png and .PNG are both registered (:281-287), so foo.PNG is claimed but foo.PnG is not
⬜ Q92	m_ExpandRGBPalette's in-class initializer is false (itkImageIOBase.h:853) but ImageIOBase's constructor calls Reset(false), which unconditionally sets it true (itkImageIOBase.cxx:28,45) — PNGImageIO never touches the flag, so the header default is dead and every instance starts with palette expansion on
⬜ Q93	HDF5TransformIO's ReadParameters/ReadFixedParameters emit "Wrong data type for " << DataSetName << "in HDF5 File" — no space before in, so the dataset name runs into the word (itkHDF5TransformIO.cxx:171, :217)
⬜ Q94	the rank-1 check in both of those readers raises "Wrong # of dims for TransformType in HDF5 File" (:176, :222) — TransformType is the one dataset the check never covers
⬜ Q95	HDF5TransformIO::Read loops i < transformGroup.getNumObjs() and then openGroup(GetTransformName(i)) (:297-302); getNumObjs counts every link, so one stray dataset beside the 0, 1, … subgroups makes the loop ask for a group N that was never written and the read dies inside libhdf5 rather than reporting an extra object
⬜ Q96	HDF5TransformIO::CanReadFile is h5file.exists("/TransformGroup") (:59-66), i.e. H5Lexists — a dataset of that name passes, the factory hands the file over, and Read's openGroup (:295) throws an H5::Exception instead of the factory's clean "could not create Transform IO object"
⬜ Q97	HDF5ImageIO stores three axis conventions in one group: Dimension/Origin/Spacing in ITK order, fastest-moving first (:1151-1161); only VoxelData's dataspace is reversed (:1170-1182); and Directions writes m_Direction[i] as its row i (:490-510) while that is column i of the direction matrix — so a 4×3 2-D image writes Dimension = [4, 3] and a VoxelData of shape [3, 4], and the Directions dataset is the matrix transpose
⬜ Q98	HDF5ImageIO's ReadScalar (:410) and ReadVector (:479) both raise "Wrong # of dims for TransformType in HDF5 File" — the transform IO's message (Q94), verbatim, in a file that holds no transform. ReadDirections (:521) at least names the image directions
⬜ Q99	VoxelType is written on every Write (:1163-1166) and read on no Read: m_ComponentType is derived from imageSet.getDataType() (:752-753). A file whose VoxelType reads FLOAT over int16 voxels loads as Int16; a file with no VoxelType at all loads fine. The class comment (itkHDF5ImageIO.h:68-71) says it exists to make the file "user friendly with respect to HDF5 viewers", so this is by design — worth stating, since it looks authoritative
⬜ Q100	HDF5ImageIO::CanReadFile is h5file.exists("/ITKImage") (:647-654) — same H5Lexists shape as Q96, so a dataset named /ITKImage is claimed and ReadImageInformation's first openDataSet throws
⬜ Q101	three of ReadImageInformation's meta-data branches are unreachable: WriteScalar(unsigned long) stores NATIVE_UINT tagged isUnsignedLong (:322-342), so no ITK-written dataset is ever NATIVE_INT + isUnsignedLong as :801-805 expects; and on every LP64 platform H5Tequal(NATIVE_LONG, NATIVE_LLONG) and H5Tequal(NATIVE_ULONG, NATIVE_ULLONG) are true, so the arms at :874-897 swallow every 64-bit integer and those at :898-905 never run (the isLLong/isULLong tags inside the surviving arms are what actually distinguish the two)
⬜ Q102	a meta-data dataset of rank > 1 is skipped by continue (:785-789), and the if/else if chain over the fourteen NATIVE_* types ends in an else that tests only metaDataType == strType with no else of its own (:914-922) — so a compound, enum, array or variable-length-sequence dataset leaves no dictionary entry and raises nothing. A subgroup under MetaData, by contrast, is fatal: the loop opens every link with openDataSet (:776-782)
⬜ Q103	HDF5ImageIO's constructor registers .hdf, .h4, .hdf4, .he4 — HDF4 container names — for both reading and writing alongside the HDF5 ones (:36-43), the same eight-name list already noted on the transform side (Q89). Here CanWriteFile is HasSupportedWriteExtension(name) with ignoreCase = true (itkImageIOBase.h:788), so IMAGE.H5 is claimed for writing where the transform IO's case-sensitive CanWriteFile declines it
⬜ Q104	JPEGImageIO::Write throws "JPEG Writer can only write 2-dimensional images" before opening anything (:459-462), where PNGImageIO::WriteSlice silently writes only the first slice (B71) — the two sibling IOs disagree on whether a 3-D source is an error
⬜ Q105	JPEGImageIO::CanReadFile is markedly stricter than PNG's: case-sensitive extension (:102, ignoreCase = false), then the 0xFFD8 magic (:117-130), then a full jpeg_read_header parse (:152) — where PNGImageIO::CanReadFile stops at the 8-byte signature
⬜ Q106	JPEGImageIO::CanWriteFile passes ignoreCase = false (:449) against four registered spellings (:308-314) — .jpg, .JPG, .jpeg, .JPEG are claimed, .Jpg is not. Same shape as PNG's (Q91)
⬜ Q107	m_UseCompression is assigned false in the constructor (:299) and never read: jpeg_set_quality is called unconditionally (:537), so only Quality/CompressionLevel has any effect and UseCompressionOn() is silently inert for this IO
⬜ Q108	Two quirks of the TIFF resolution tags. (a) Spacing survives a round-trip only to float precision: WriteImageInformation stores 25.4 / m_Spacing[i] (:587, :794) into XRESOLUTION, which libtiff holds as a float (tif_dir.h:98; TIFF_RATIONAL/TIFF_SETGET_FLOAT, tif_dirinfo.c:90) and m_XResolution mirrors as float — so a spacing of 1.0 reads back as 1.0000000150…. (b) TIFFReaderInternal::Initialize reads XRESOLUTION with the non-defaulted TIFFGetField (:202) over Clean()'s seed of 1, while the read guard is only unit > 0 && m_XResolution > 0 (:375-385) — so a page with RESOLUTIONUNIT = 2 (inch) and no XRESOLUTION tag yields a spacing of 25.4 rather than the default 1.0. (An absent RESOLUTIONUNIT seeds unit = 1, which takes neither branch, so the plain case is safe.)
⬜ Q109	FastChamferDistanceImageFilter	the anti-raster sweep is for (it.GoToEnd(), --it; !it.IsAtBegin(); --it) (.hxx:194), so flat index 0 is never the center of the backward pass — harmless, because the pass writes to lower-index neighbours and flat index 1 still updates it. The 3-D weights are the float literals 0.92644, 1.34065, 1.65849 (.hxx:38, :41, :44)
⬜ Q110	IsolatedConnectedImageFilter	the bisection bounds and midpoint are typed NumericTraits<InputPixelType>::AccumulateType (.hxx:147, :178-180, :243-245), which for unsigned int is unsigned int — no widening (itkNumericTraits.h:1027). guess = (upper + lower) / 2 (.hxx:231, :296) can therefore overflow for thresholds near UINT_MAX
⬜ Q111	STAPLEImageFilter	contrary to the common description of the algorithm, there is no p = q = 0.99999 seeding: W is seeded from the rater mean (.hxx:116), the prior g_t is computed once before the loop (:123), per-rater convergence is (dp)² , (dq)² ≤ 1e-14 (:51, :215-220), and the foreground test is a two-sided ε-window with ε = 1e-10 (:94). Worth stating because implementations that copy the papers' seeding will not reproduce ITK
⬜ Q112	BinaryMedianImageFilter	the majority rule is strict > against neighborhoodSize / 2 (.hxx:121, :136-143), so a tie goes to background; and the output is always exactly ForegroundValue or BackgroundValue — a pixel whose value matches neither is not passed through
⬜ Q113	ChangeLabelImageFilter	the change map is std::map<InputPixelType, OutputPixelType> (.h:57), so keys are quantized through the pixel type and a repeated key is last-wins (.h:77); operator() performs a single lookup (.h:92-101), so a→b together with b→c is not transitive — a maps to b
⬜ Q114	BSplineDecompositionImageFilter	the causal-coefficient initialization switches between an accelerated truncated sum and the exact mirror-boundary sum on horizon < dataLength, where `horizon = ceil(log(tol)/log(
⬜ Q115	PatchBasedDenoisingImageFilter noise-model fidelity	on the first iteration the output equals the input, so GAUSSIAN's 2·(in − out) (.hxx:2015) and POISSON's (in − out)/(out + 1e-5) (:2049) are both identically zero. With SimpleITK's default NumberOfIterations = 1, neither noise model contributes anything; RICIAN's Bessel-ratio term (:2031-2032) is nonzero and is unaffected
⬜ Q116	PatchBasedDenoisingImageFilter internals	three of a kind: ResolveSigmaUpdate writes m_KernelBandwidthSigma (.hxx:1854, :1860) before returning, and the caller then compares |sigmaUpdate| against the already-updated sigma (:1427); m_MinProbability = 2.2e-306 (:43) is added to the entropy probability and to its first and second derivatives (:1734-1738); and on the skipped-query path the subsampler's offset keeps its zero initialization rather than being recomputed by ComputeOffset (itkSpatialNeighborSubsampler.hxx:140, :147-151)
⬜ Q117	JoinSeriesImageFilter size mismatch	asymmetric and undocumented. Each input's requested region is derived from the primary input's size (.hxx:209-216), so a subsequent input that is smaller throws InvalidRequestedRegionError from region propagation, while one that is larger is silently corner-cropped to [0, primary_size). (Physical congruency is checked separately, with coordinateTol = m_CoordinateTolerance · spacing[0] — the first dimension's spacing for every axis, itkImageToImageFilter.hxx:184-185)
⬜ Q118	TransformToDisplacementFieldFilter linear fast path	the scan-line endpoints are the continuous indices idx[0] and idx[0] + size[0] — one past the last pixel — with alpha = i / size[0] (.hxx:239, :245, :255-256). This is exact rather than off-by-one, because for a linear transform T(p) − p is affine, so interpolating between those two endpoints reproduces the displacement at every interior pixel. Recorded because it reads like an off-by-one and is not

Also: LinearAnisotropicDiffusionLBRImageFilter's Selling decomposition bails out after 200 superbase flips by printing "Warning: Selling's algorithm not stabilized." to std::cerr and continuing with a possibly non-obtuse superbase — which can yield negative stencil weights. The 2-D branch guards obtuseness with a debug-only assert; the 3-D branch has no check at all (.hxx:185-263).

AI assistance

• Tool: Claude Code (claude-opus-4-8)
• Role: found the defects while cross-checking an independent reimplementation of these filters against this source; performed the pre-filing re-verification pass (re-reading every cited line on main at e46eb723a5, checking declared types and constructor defaults, re-deriving each consequence)
• Contribution: the inventory above
• Testing: source-level verification against e46eb723a5; behavioral claims cross-checked against the independent reimplementation's pinned tests. B28 is source analysis only (not run to a live hang) and is labeled as such inline.
• Items whose original analysis did not survive re-verification were corrected or dropped before filing.

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[physwkim]

Following up on this issue: continuing the porting work, I have a second batch of findings, all in the displacement-field / warp / vector-filter area. Same methodology and caveats as the original report — source-level reading against master e46eb723a5, independently re-verified before posting, no runtime reproduction. Numbering continues from the original lists (B31–B34, Q25–Q29).

Bugs

#	Filter / site	Finding
B31	IterativeInverseDisplacementFieldImageFilter	smallestError carries over from the previous pixel when the mapped point starts outside the buffer — output depends on raster order
B32	InverseDisplacementFieldImageFilter	the field-subsampling resampler never gets the input's direction — with non-identity direction cosines the spline landmarks are not on the input lattice at all
B33	VectorConfidenceConnectedImageFilter	a re-estimation pass that visits zero pixels produces NaN statistics, which invert the Mahalanobis guard — the next flood includes every pixel in the image
B34	WarpImageFilter	the same-information fast-path test omits size, so a valid larger-output configuration dies in an iterator containment check instead of taking the general path

Quirks / doc

#	Site	Finding
Q25	IterativeInverseDisplacementFieldImageFilter	the coordinate-descent probe step is axis 0's spacing on every axis
Q26	DisplacementFieldJacobianDeterminantFilter	SetDerivativeWeights silently flips UseImageSpacing off — precedence between the two is last-writer-wins, while the class doc describes only one ordering
Q27	WarpImageFilter	m_OutputSize[0] == 0 alone keys "size unset"; output-grid defaults are unit/zero/identity from neither input; the field is sampled with edge replication, never out-of-domain
Q28	VectorConnectedComponentImageFilter	zero vectors join anything under the default threshold (no special case); concept macro name typo InputValyeTypeIsFloatingCheck
Q29	MahalanobisDistanceMembershipFunction	the det < 0 exception branch is unreachable; GetCovariance() never reflects the singular-case substitute

Details below.

B31 — IterativeInverseDisplacementField: stale smallestError makes output raster-order dependent

itkIterativeInverseDisplacementFieldImageFilter.hxx:96 declares the residual bar outside the per-pixel loop:

double smallestError = 0;
InputIt.GoToBegin();
OutputIt.GoToBegin();
while (!OutputIt.IsAtEnd())

and it is (re)assigned only inside the inside-buffer branch (:122-132):

if (inputFieldInterpolator->IsInsideBuffer(mappedPoint))
{
  ...
  smallestError = 0;
  ...
  smallestError = std::sqrt(smallestError);
}

Nothing at the top of the pixel iteration resets it. For a pixel whose initial mapped point falls outside the buffer, the branch is skipped and smallestError still holds the previous pixel's residual. The coordinate-descent probes are individually buffer-guarded (:147, :167), so a probe that lands inside the buffer is compared against that stale bar (:156 if (tmp < smallestError)) and accepted or rejected based on how well the previous pixel converged — the inverse field's values at such pixels depend on raster order.

B32 — InverseDisplacementField: subsampling resampler never gets the input's direction

itkInverseDisplacementFieldImageFilter.hxx, PrepareKernelBaseSpline configures the field-subsampling resampler at :119-141:

resampler->SetInput(inputImage);
resampler->SetOutputOrigin(inputImage->GetOrigin());
...
resampler->SetSize(size);
resampler->SetOutputStartIndex(subsampledRegion.GetIndex());
resampler->SetOutputSpacing(spacing);

SetOutputDirection is never called, and ResampleImageFilter's constructor leaves it identity (itkResampleImageFilter.hxx:51 m_OutputDirection.SetIdentity();; no reference image is set, so nothing overrides it). For an input field with non-identity direction cosines two things go wrong at once:

1. the resampler samples along an axis-aligned physical grid; interior sample points can fall outside the input buffer and silently take the default pixel value (zero, :65, applied at :411);
2. the landmark read-back at :168 (sampledInput->TransformIndexToPhysicalPoint(ot.GetIndex(), targetPoint)) uses the subsampled image's (identity) direction, so the target landmarks handed to the kernel transform are those axis-aligned points — not a subset of the input field's lattice.

The kernel-spline inverse is then anchored on landmarks that don't correspond to the field being inverted.

B33 — VectorConfidenceConnected: empty re-estimation pass floods the whole image via NaN statistics

itkVectorConfidenceConnectedImageFilter.hxx: the re-estimation loop recomputes mean/covariance from the previous segmentation with no guard on the visited-pixel count (SizeValueType num{}; at :264):

mean[ii] /= static_cast<double>(num);              // :289
covariance[ii][jj] /= static_cast<double>(num);    // :292

(The initial statistics do guard the equivalent case at :179-184; the loop has no such guard.) The empty case is reachable: the threshold is fixed before the loop (SetThreshold(m_Multiplier), :198/:226; the seed-inclusion boost :209-222 runs pre-loop only), while mean and covariance are recomputed each round (:304-305) — the acceptance interval moves, so a round can exclude even the seeds; the flood loop then simply never executes (:319-323 over an all-background image filled at :314) and the next pass has num == 0.

With num == 0, mean and covariance are 0.0/0.0 = NaN. In MahalanobisDistanceThresholdImageFunction the NaN squared distance then fails the guard at itkMahalanobisDistanceThresholdImageFunction.hxx:113:

const double mahalanobisDistance = (mahalanobisDistanceSquared > 0.0) ? std::sqrt(...) : 0.0;

so the distance clamps to 0.0, and 0.0 <= m_Threshold (:87) is true for any positive threshold — the next flood accepts every pixel in the image. (Mechanism note: the NaN reaches the distance chiefly through the NaN mean — a NaN determinant fails det > singularThreshold in itkMahalanobisDistanceMembershipFunction.hxx:107 and routes to the singular branch, which substitutes a finite inverse; the measurement − NaN-mean differences carry the NaN regardless.)

B34 — WarpImageFilter: same-information fast-path test omits size, so a valid configuration throws

itkWarpImageFilter.hxx:387-391:

this->m_DefFieldSameInformation =
  (outputPtr->GetOrigin()   ... is_equal(fieldPtr->GetOrigin(),   ...)) &&
  (outputPtr->GetSpacing()  ... is_equal(fieldPtr->GetSpacing(),  ...)) &&
  (outputPtr->GetDirection()... is_equal(fieldPtr->GetDirection(), ...));

Size is not part of the test, but the fast path iterates the field over the output's thread region (:294 ImageRegionConstIterator fieldIt(fieldPtr, outputRegionForThread);). The output can legitimately be larger than the field — :428-438 takes the output region from the user's SetOutputSize/SetOutputStartIndex when set — and GenerateInputRequestedRegion explicitly anticipates the mismatch by falling back to the field's largest possible region (:407-410), which silences the pipeline's requested-region check.

What happens then is not a buffer over-read — the iterator's own containment assert catches it (itkImageConstIterator.h:207-212, itkAssertOrThrowMacro(bufferedRegion.IsInside(m_Region)), which throws in release builds and aborts in debug). But the user-visible result is that a configuration the general path handles fine (it clamps field sampling to the field domain, :200-216) instead dies with an internal iterator error ("Region ... is outside of buffered region") whenever the output grid shares origin/spacing/direction with a smaller field. Adding a size/region comparison to the fast-path test would route this case to the general path.

Q25 — IterativeInverseDisplacementField: probe step is axis 0's spacing on every axis

itkIterativeInverseDisplacementFieldImageFilter.hxx:89 is the only per-axis spacing read in GenerateData:

const double spacing = inputPtr->GetSpacing()[0];

and the coordinate-descent loop applies it along every physical axis (:144-146 mappedPoint[k] += step;, :166, :186). For anisotropic fields every axis but the first is probed with the wrong step size.

Q26 — DisplacementFieldJacobianDeterminant: SetDerivativeWeights silently flips UseImageSpacing off

itkDisplacementFieldJacobianDeterminantFilter.hxx:55: SetDerivativeWeights begins with m_UseImageSpacing = false; (unconditionally, not via SetUseImageSpacing). The effective precedence between the two parameters is therefore last-writer-wins: SetUseImageSpacing(true) after SetDerivativeWeights re-derives weights from spacing at update time (:155-168), while the reverse order keeps the explicit weights and drops the spacing request. The class doc (.h:78-82) describes only the first ordering ("if UseImageSpacing is set to TRUE (ON), then these weights will be overridden by weights derived from the image spacing when the filter is updated").

Downstream note: SimpleITK's generated wrapper sets members in yaml order — UseImageSpacing before DerivativeWeights — so a SimpleITK user who sets both gets the UseImageSpacing(true) request silently dropped.

Q27 — WarpImageFilter parameter quirks

Three small ones, all in itkWarpImageFilter.hxx:

1. :428 — if (this->m_OutputSize[0] == 0 && fieldPtr != nullptr) keys "size unset" on element 0 alone; an explicit SetOutputSize({0, 5, 5}) silently discards the 5s and inherits the field's region.
2. :43-48 — constructor defaults are unit spacing, zero origin, identity direction, zero size: borrowed from neither the input image nor the field (asymmetric with ResampleImageFilter's reference-image convention).
3. :200-216 — displacement-field sampling clamps baseIndex/neighIndex into [m_StartIndex, m_EndIndex], i.e. edge-replicates, and never goes out-of-domain; only the input-image probe (m_Interpolator->IsInsideBuffer, :312/:342) can send a pixel to EdgePaddingValue. Worth a doc note since the two lookups follow different out-of-domain policies.

Q28 — VectorConnectedComponent: zero vectors, and a concept-macro typo

itkVectorConnectedComponentImageFilter.h:77-87: the functor is an unnormalized dot product (documented — "Assumes that vectors are normalized", and Math::Absolute deliberately makes antiparallel vectors similar per .h:100-101). A zero vector, however, has no documented story: it yields dotProduct = 0 against anything, so 1 - |0| = 1, and with the wrapped default DistanceThreshold = 1 it joins any neighbor — zero-padding or masked-out regions merge with whatever they touch.

Trivial: .h:145 — itkConceptMacro(InputValyeTypeIsFloatingCheck, ...) misspells "Value" as "Valye".

Q29 — MahalanobisDistanceMembershipFunction: dead det check; GetCovariance hides the substitute

itkMahalanobisDistanceMembershipFunction.hxx:

1. :98-103 — const double det = inv_cov.determinant_magnitude(); followed by if (det < 0.) { itkExceptionStringMacro(...) }. determinant_magnitude() is a product of SVD singular values, hence non-negative; the branch is unreachable (a NaN det also fails < 0.).
2. :92 caches the raw covariance; the singular-case substitution (:114-124) replaces only m_InverseCovariance with aLargeDouble·I. GetCovariance() (a GetConstReferenceMacro) therefore always reports the literal input matrix, with no indication the function is actually operating with the substituted inverse.

(Same AI-assistance disclosure as the issue body applies to this comment.)

[physwkim]

A third batch from the continuing port, this time in the demons-registration / displacement-field-exponential area. Same methodology and caveats as before — source-level reading against master e46eb723a5, independently re-verified before posting, no runtime reproduction. Numbering continues (B35, Q30–Q37).

Bug (latent, explicit-configuration-only)

#	Filter / site	Finding
B35	ExponentialDisplacementFieldImageFilter	the field is divided by 1 << numiter — UB when numiter >= 31; unreachable by default (cap 20, clamped), but an explicit SetMaximumNumberOfIterations(>= 31) makes it live

Quirks / doc

#	Site	Finding
Q30	FastSymmetricForcesDemonsRegistrationFilter	its difference function is ESMDemonsRegistrationFunction; the identically-named FastSymmetricForcesDemonsRegistrationFunction class is orphaned from its own filter
Q31	ExponentialDisplacementFieldImageFilter	"take the ceil" comment vs + 1.0-then-truncate code (integral values overshoot by 1); a zero field still runs one squaring iteration
Q32	WarpVectorImageFilter (as wired by DiffeomorphicDemonsRegistrationFilter)	under the nearest-neighbor-extrapolating interpolator, IsInsideBuffer is always true and m_EdgePaddingValue is dead code
Q33	ESMDemonsRegistrationFunction	the WarpedMoving and MappedMoving gradient options coincide exactly at a zero displacement field (interior pixels, default linear interpolation, coincident grids)
Q34	LevelSetMotionRegistrationFilter/Function	dt = 1/maxL1 makes the applied step at the governing pixel exactly Alpha-independent; thresholded pixels count toward the metric while out-of-buffer pixels don't; Halt adds an RMS==0 clause the base can never fire
Q35	LevelSetMotionRegistrationFunction	one-sided difference steps add index-axis spacing to a physical coordinate — wrong direction and step under non-identity direction cosines
Q36	demons family	zero-iteration runs report GetRMSChange() == 0 but GetMetric() == NumericTraits<double>::max() in the two filters that don't override GetRMSChange
Q37	LevelSetMotionRegistrationFunction smoothing	the smoothed moving image is re-quantized to the moving image's own pixel type (u8 in → u8 smoothed) before the gradient; and any axis shorter than 4 pixels is a hard error for this family member only

Details below.

B35 — ExponentialDisplacementField: signed shift UB behind SetMaximumNumberOfIterations ≥ 31

itkExponentialDisplacementFieldImageFilter.hxx:160 (and the inverse at :164):

m_Divider->SetInput2(static_cast<InputPixelRealValueType>(1 << numiter));

numiter itself is unsigned int (:70); the UB kernel is the signed literal 1 — the shift's result type is int, so 1 << 31 is undefined. To be clear about reachability: this is not hittable by default. The constructor caps m_MaximumNumberOfIterations at 20 (:32), and the automatic count is hard-clamped:

numiter = std::min(static_cast<unsigned int>(numiterfloat + 1.0), m_MaximumNumberOfIterations);  // :112

so no input field, however extreme, raises it past the cap. It becomes live only when a user explicitly calls SetMaximumNumberOfIterations with ≥ 31 (clamp target in automatic mode; taken verbatim in non-automatic mode, :121). 1u << numiter or computing the power in floating point would close it.

Q30 — FastSymmetricForcesDemonsRegistrationFunction is orphaned from its own filter

itkFastSymmetricForcesDemonsRegistrationFilter.h:120-121:

using DemonsRegistrationFunctionType =
  ESMDemonsRegistrationFunction<FixedImageType, MovingImageType, DisplacementFieldType>;

with the in-source comment at :117: FIXME: Why is this the only permissible function ?. Meanwhile the identically-named FastSymmetricForcesDemonsRegistrationFunction (itkFastSymmetricForcesDemonsRegistrationFunction.h:47) is a complete, instantiable class that its own filter never uses. It is not the default force function of any filter either (CurvatureRegistrationFilter defaults to MeanSquareRegistrationFunction, itkCurvatureRegistrationFilter.h:96-99); its only uses anywhere in the tree are as an explicit CurvatureRegistrationFilter template argument in two tests (itkCurvatureRegistrationFilterTest.cxx:114, RegistrationITKv3/DeformableRegistration10.cxx:60). Worth either wiring it back to its filter, documenting the ESM substitution, or deprecating the orphan.

Q31 — ExponentialDisplacementField iteration count: not-quite-ceil, and the zero-field case

itkExponentialDisplacementFieldImageFilter.hxx:111-112:

// take the ceil and threshold
numiter = std::min(static_cast<unsigned int>(numiterfloat + 1.0), m_MaximumNumberOfIterations);

For an exactly-integral numiterfloat == 2.0 this yields 3 where ceil would give 2 — a harmless extra squaring, but not what the comment says.

Zero displacement field: maxnorm2 == 0, so the ternary at :106-107 assigns numiterfloat = NumericTraits<...>::min() (the smallest positive normal), which passes numiterfloat >= 0.0 (:109) and truncates to numiter = 1 — the identity field is still divided by 2, squared once, and composed. Correct result, one wasted composition pass.

Q32 — WarpVectorImageFilter: EdgePaddingValue is dead under the extrapolating interpolator

All three IsInsideBuffer overloads of VectorLinearInterpolateNearestNeighborExtrapolateImageFunction return true unconditionally (.h:99-118), and EvaluateAtContinuousIndex clamps the base index and zeroes the out-of-range fractional distance (.hxx:51-67) — out-of-buffer points extrapolate the nearest pixel. WarpVectorImageFilter's per-pixel test (itkWarpVectorImageFilter.hxx:171-186):

if (m_Interpolator->IsInsideBuffer(point)) { ... } else { outputIt.Set(m_EdgePaddingValue); }

is then always-true, and the m_EdgePaddingValue branch is unreachable. The plain filter's default interpolator (VectorLinearInterpolateImageFunction, .h:141) does real bounds checks, so this is configuration-dependent — but DiffeomorphicDemonsRegistrationFilter installs exactly the extrapolating one (itkDiffeomorphicDemonsRegistrationFilter.h:196-199, ctor wiring .hxx:38-39), so for the demons field warper the edge-padding parameter silently does nothing.

Q33 — ESMDemonsRegistrationFunction: WarpedMoving and MappedMoving gradients coincide at a zero field

WarpedMoving hand-computes an index-space central difference over the warped moving image (itkESMDemonsRegistrationFunction.hxx:294-296 — deliberately not via CentralDifferenceImageFunction, per the comment at :206-207), while MappedMoving calls CentralDifferenceImageFunction::Evaluate, which samples the interpolant at p ± 0.5·spacing in physical space (itkCentralDifferenceImageFunction.hxx:264-300). Under linear interpolation, M(p + 0.5s) − M(p − 0.5s) = ½(m[i+1] − m[i−1]) at a pixel centre, so divided by s the two are exactly equal at a zero displacement field — for interior pixels, default linear interpolation on both paths, and coincident fixed/moving grids (the boundary rules and spacing divisors differ). Users choosing between the two enum options may expect a difference the defaults don't deliver; a doc note would cover it.

Q34 — LevelSetMotion: time-step consequences the docs don't state

The only family member whose global time step is data-dependent: dt = 1/m_MaxL1Norm (itkLevelSetMotionRegistrationFunction.hxx:352-367; the other four functions all return a constant m_TimeStep = 1.0). Three consequences:

1. Per-pixel update is speed * gradient[j] / (gradientMagnitude + m_Alpha) (:330) and the L1 norm feeds m_MaxL1Norm (:339, :345-348). At the pixel that attains the max, with one dominant gradient direction, dt · update[k] = ±mSpacing[k] — the (|g| + Alpha) factor cancels exactly, so at the governing pixel the applied displacement is independent of Alpha (other pixels carry the ratio (|g_max|+α)/(|g_p|+α), so it is approximate field-wide). The Alpha doc ("small positive constant to avoid division by zero") does not mention that the global step rescales its attenuation back out.
2. A thresholded pixel still increments SumOfSquaredDifference/NumberOfPixelsProcessed (:313-325 — counted before the guards zero the update), while an out-of-buffer pixel returns before counting (:220-228) — the reported metric mixes the two exclusion policies.
3. LevelSetMotionRegistrationFilter::Halt (.hxx:76-89) adds m_RMSChange == 0.0 && GetElapsedIterations() != 0; the base's convergence test is strictly GetMaximumRMSError() > m_RMSChange (itkFiniteDifferenceImageFilter.hxx:225), which at the default MaximumRMSError == 0.0 can never fire on 0.0 > 0.0 — so this subclass converges on an exactly-zero iteration where every sibling keeps iterating to the count limit.

Q35 — LevelSetMotion: one-sided differences step a physical coordinate by index-axis spacing

itkLevelSetMotionRegistrationFunction.hxx:253 (loop :251-277):

PointType mPoint(mappedPoint);          // physical point
...
mPoint[j] += mSpacing[j];               // index-axis j's spacing added to physical coordinate j

mappedPoint comes from TransformIndexToPhysicalPoint plus the displacement (:213), and mSpacing is the moving image's spacing (:240-245). With a non-identity direction matrix, physical axis j is not index axis j — the probe walks the wrong direction by the wrong step. (Same borrowed-axis pattern as CentralDifferenceImageFunction::Evaluate's point[dim] ± 0.5 * spacing[dim], so it may be a known family-wide convention — flagging it because here it also interacts with UseImageSpacing, which selects both this step and the L1-norm divisor of the time step, :339. The demons/ESM functions have no such toggle; they fold spacing in unconditionally via m_Normalizer, itkDemonsRegistrationFunction.hxx:122-127.)

Q36 — Demons family: zero-iteration metric reporting is inconsistent

SymmetricForcesDemonsRegistrationFilter (.h:118), FastSymmetricForcesDemonsRegistrationFilter (.h:113), and DiffeomorphicDemonsRegistrationFilter (.h:125) override GetRMSChange() to forward to the function; DemonsRegistrationFilter and LevelSetMotionRegistrationFilter do not. For the latter two, a run with zero elapsed iterations reports the base's value-initialized m_RMSChange == 0.0 (itkFiniteDifferenceImageFilter.h:361) — "converged" — while GetMetric() forwards (itkLevelSetMotionRegistrationFilter.hxx:91-103) to the function's constructor sentinel m_Metric = NumericTraits<double>::max() (itkLevelSetMotionRegistrationFunction.hxx:50). Two query methods on the same freshly-run filter thus disagree about whether anything happened.

Q37 — LevelSetMotion smoothing: re-quantization, and a 4-pixel minimum unique in the family

itkLevelSetMotionRegistrationFunction.h:116 instantiates the smoother with one template argument:

using MovingImageSmoothingFilterType = SmoothingRecursiveGaussianImageFilter<MovingImageType>;

so TOutputImage = TInputImage (itkSmoothingRecursiveGaussianImageFilter.h:50): the filter computes internally in a real-typed image but casts back (CastingFilterType, .h:91) — a u8 moving image is smoothed and then re-quantized to u8 before the gradient interpolator reads it (.hxx:181, :185). And through RecursiveSeparableImageFilter it inherits the hard error on any filtered axis shorter than 4 pixels (itkRecursiveSeparableImageFilter.hxx:193-201, in BeforeThreadedGenerateData) — a 3-pixel-thick volume is a throw for LevelSetMotionRegistrationFilter and for no other member of the family (the others take gradients via CentralDifferenceImageFunction, no minimum size).

(Same AI-assistance disclosure as the issue body applies to this comment.)