PNG fails on Emscripten

[LloydJHowarth]

On the last main branch png files fail to load but jpg, bmp all work

The only related exception I can find is the following:
missing function: png_sig_cmp

My build script:

emcmake cmake .. -G Ninja \
    -DSDLIMAGE_BMP=ON \
    -DSDLIMAGE_JPG=ON \
    -DSDLIMAGE_PNG=ON \
    -DSDLIMAGE_AVIF=OFF \
    -DSDLIMAGE_WEBP=OFF \
    -DSDLIMAGE_GIF=OFF \
    -DSDLIMAGE_JXL=OFF \
    -DSDLIMAGE_LBM=OFF \
    -DSDLIMAGE_PCX=OFF \
    -DSDLIMAGE_PNM=OFF \
    -DSDLIMAGE_QOI=OFF \
    -DSDLIMAGE_SVG=OFF \
    -DSDLIMAGE_TGA=OFF \
    -DSDLIMAGE_TIF=OFF \
    -DSDLIMAGE_XCF=OFF \
    -DSDLIMAGE_XPM=OFF \
    -DSDLIMAGE_XV=OFF \
    -DSDLIMAGE_TESTS=OFF \
    -DSDLIMAGE_VENDORED=ON \
    -DSDLIMAGE_DEPS_SHARED=OFF \
    -DBUILD_SHARED_LIBS=OFF \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_INSTALL_PREFIX="$INSTALLPATH" \
    -DCMAKE_C_FLAGS="-fwasm-exceptions -sSUPPORT_LONGJMP=wasm" \
    -DCMAKE_CXX_FLAGS="-fwasm-exceptions -sSUPPORT_LONGJMP=wasm" \
    -DSDL3_DIR="$MODULES_DIR/SDL/install_$PLATFORM/lib/cmake/SDL3"

Which enables png and has been the same build script I've had for 6 months,
Works on all other platforms that I've tested.

[LloydJHowarth]

After checking my exported symbols it is actually present, so I'm unsure why this isn't working now?

76:png_sig_cmp
Other png_methods...

Has something changed in SDL3_Image over the past lets say 2 weeks ish? I've noticed some commits relevant to png.

[LloydJHowarth]

Ahh but my build log it states:

2>EXEC: Warning  : undefined symbol: png_sig_cmp (referenced by root reference (e.g. compiled C/C++ code))
2>EXEC: Warning  : undefined symbol: png_write_end (referenced by root reference (e.g. compiled C/C++ code))
2>EXEC: Warning  : undefined symbol: png_write_flush (referenced by root reference (e.g. compiled C/C++ code))
2>EXEC: Warning  : undefined symbol: png_write_image (referenced by root reference (e.g. compiled C/C++ code))
2>EXEC: Warning  : undefined symbol: png_write_info (referenced by root reference (e.g. compiled C/C++ code))
2>emcc: Warning  : warnings in JS library compilation [-Wjs-compiler]
Other png_methods...

So maybe libpng isn't being linked or something?

[madebr]

Let's do some sanity checking first.

After applying this patch, what appears in the configure log?

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -907,6 +907,7 @@ if(SDLIMAGE_PNG)
             message(STATUS "Dynamic libpng: ${SDLIMAGE_DYNAMIC_PNG}")
             target_compile_definitions(${sdl3_image_target_name} PRIVATE "LOAD_LIBPNG_DYNAMIC=\"${SDLIMAGE_DYNAMIC_PNG}\"")
         else()
+            message(STATUS "target_link_libraries(${sdl3_image_target_name} PRIVATE PNG::PNG)")
             target_link_libraries(${sdl3_image_target_name} PRIVATE PNG::PNG)
         endif()
     endif()

What backends are enabled/disabled according to CMake? (These are the last 3 lines of CMake configuration)

Finally, there were patches to the libpng git submodule. Make sure you've updated them all. (git submodule update --recursive --init)

[LloydJHowarth]

Output:

-- SDL3_image backends:
-- - enabled:  stb ani bmp jpg png apng
-- - disabled: imageio wic avif gif jxl lbm pcx pnm qoi svg tga tif webp xcf xpm xv

There was a warning:

-- Renaming
--     C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/external/zlib/zconf.h
-- to 'zconf.h.included' because this file is included with zlib
-- but CMake generates it automatically in the build directory.
CMake Warning (dev) at external/zlib/CMakeLists.txt:152 (add_library):
  ADD_LIBRARY called with SHARED option but the target platform does not
  support dynamic linking.  Building a STATIC library instead.  This may lead
  to problems.
This warning is for project developers.  Use -Wno-dev to suppress it.

All SDL, Satellites, submodules get pulled and updated every build.

[LloydJHowarth]

Full log if needed (already ran once to update modules and apply the patch)

Running IMAGE
SDL_IMAGE  [Already Installed]
configure: cmake .. -G Ninja -DSDLIMAGE_BMP=ON -DSDLIMAGE_JPG=ON -DSDLIMAGE_PNG=ON -DSDLIMAGE_AVIF=OFF -DSDLIMAGE_WEBP=OFF -DSDLIMAGE_GIF=OFF -DSDLIMAGE_JXL=OFF -DSDLIMAGE_LBM=OFF -DSDLIMAGE_PCX=OFF -DSDLIMAGE_PNM=OFF -DSDLIMAGE_QOI=OFF -DSDLIMAGE_SVG=OFF -DSDLIMAGE_TGA=OFF -DSDLIMAGE_TIF=OFF -DSDLIMAGE_XCF=OFF -DSDLIMAGE_XPM=OFF -DSDLIMAGE_XV=OFF -DSDLIMAGE_TESTS=OFF -DSDLIMAGE_VENDORED=ON -DSDLIMAGE_DEPS_SHARED=OFF -DBUILD_SHARED_LIBS=OFF -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_ "-DCMAKE_C_FLAGS=-fwasm-exceptions -sSUPPORT_LONGJMP=wasm" "-DCMAKE_CXX_FLAGS=-fwasm-exceptions -sSUPPORT_LONGJMP=wasm" -DSDL3_DIR=C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL/install_/lib/cmake/SDL3 -DCMAKE_TOOLCHAIN_FILE=C:\Dev\Hybrid\.github\scripts\Dependencies\Modules\EMSCRIPTEN\upstream\emscripten\cmake\Modules\Platform\Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/EMSCRIPTEN/node/16.20.0_64bit/bin/node.exe
-- Configuring SDL3_image 3.3.5
-- Detecting Target CPU Architecture
-- Detecting Target CPU Architecture - EMSCRIPTEN
-- Performing Test LINKER_SUPPORTS_VERSION_SCRIPT
-- Performing Test LINKER_SUPPORTS_VERSION_SCRIPT - Failed
-- SDL3_image: Using vendored zlib
-- Looking for sys/types.h
-- Looking for sys/types.h - found
-- Looking for stdint.h
-- Looking for stdint.h - found
-- Looking for stddef.h
-- Looking for stddef.h - found
-- Check size of off64_t
-- Check size of off64_t - done
-- Looking for fseeko
-- Looking for fseeko - found
-- Looking for unistd.h
-- Looking for unistd.h - found
CMake Warning (dev) at external/zlib/CMakeLists.txt:152 (add_library):
  ADD_LIBRARY called with SHARED option but the target platform does not
  support dynamic linking.  Building a STATIC library instead.  This may lead
  to problems.
This warning is for project developers.  Use -Wno-dev to suppress it.

-- SDL3_image: Using vendored libpng
-- Building for target architecture: x86
-- Performing Test HAVE_LD_VERSION_SCRIPT
-- Performing Test HAVE_LD_VERSION_SCRIPT - Success
-- Found AWK program: C:/Program Files/Git/usr/bin/gawk.exe
-- Creating standard configuration
-- target_link_libraries(SDL3_image-static PRIVATE PNG::PNG)
-- Performing Test HAVE_WL_NO_UNDEFINED
-- Performing Test HAVE_WL_NO_UNDEFINED - Failed
-- SDL3_image backends:
-- - enabled:  stb ani bmp jpg png apng
-- - disabled: imageio wic avif gif jxl lbm pcx pnm qoi svg tga tif webp xcf xpm xv
-- Configuring done (16.8s)
-- Generating done (0.1s)
CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_CXX_FLAGS


-- Build files have been written to: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/build_
[82/82] Linking C executable showgpuimage.js
[0/1] Install the project...-- Install configuration: "Release"
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/lib/libSDL3_image.a
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/include/SDL3_image/SDL_image.h
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/lib/libzlibstatic.a
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/lib/libpng16.a
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/lib/cmake/SDL3_image/SDL3_imageConfig.cmake
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/lib/cmake/SDL3_image/SDL3_imageConfigVersion.cmake
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/lib/cmake/SDL3_image/SDL3_image-static-targets.cmake
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/lib/cmake/SDL3_image/SDL3_image-static-targets-release.cmake
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/lib/pkgconfig/sdl3-image.pc
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/share/licenses/SDL3_image/LICENSE.txt
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/share/licenses/SDL3_image/optional/LICENSE.zlib.txt
-- Installing: C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL_IMAGE/install_/share/licenses/SDL3_image/optional/LICENSE.libpng.txt

[madebr]

That all looks correct:

• apng is only available when libpng is available ("ordinary static" pngs are handled by stb_image of SDL3)
• target_link_libraries(SDL3_image-static PRIVATE PNG::PNG) is available in the output, so we request linking to it.

It appears showgpuimage.js links successfully?! So all symbols must be available there.

Can you please compare the link command between that of showgpuimage.js and that of a failed build?

[LloydJHowarth]

@madebr hmm.. I thought the same everything seems to be in place.

I've just ran a build using release-3.2.6 and png works fine:

-- Configuring SDL3_image 3.2.6
-- Detecting Target CPU Architecture
-- Detecting Target CPU Architecture - EMSCRIPTEN
-- Performing Test LINKER_SUPPORTS_VERSION_SCRIPT
-- Performing Test LINKER_SUPPORTS_VERSION_SCRIPT - Failed
-- SDL3_image backends:
-- - enabled:  stb bmp jpg png
-- - disabled: imageio wic avif gif jxl lbm pcx pnm qoi svg tga tif webp xcf xpm xv

I'm using .NET so I build into SDL3_Image.a file then link that in my .NET project, SDL3_Image builds using the log I gave you previously and seems to be okay, the symbols seem to be present in the dotnet output symbols list, but during the .net project build I get these warnings about undefined symbols:

2>EXEC: Warning : undefined symbol: png_sig_cmp (referenced by root reference (e.g. compiled C/C++ code))
2>EXEC: Warning : undefined symbol: png_write_end (referenced by root reference (e.g. compiled C/C++ code))
2>EXEC: Warning : undefined symbol: png_write_flush (referenced by root reference (e.g. compiled C/C++ code))
2>EXEC: Warning : undefined symbol: png_write_image (referenced by root reference (e.g. compiled C/C++ code))
2>EXEC: Warning : undefined symbol: png_write_info (referenced by root reference (e.g. compiled C/C++ code))
2>emcc: Warning : warnings in JS library compilation [-Wjs-compiler]
Other png_methods...

However in SDL3_Image 3.2.6 < this doesn't happen

Can you please compare the link command between that of showgpuimage.js and that of a failed build?

Can you elaborate please? Cmake isn't my strongest point...

This was all working previously so I'm unsure what's changed or if I need to adapt to anything new, the only thing I've ever used CMAKE for was building SDL3 so I'm a complete beginner 😂

[madebr]

Can you please compare the link command between that of showgpuimage.js and that of a failed build?

Can you elaborate please? Cmake isn't my strongest point...

When building the application (after configuration), you can see all compiler commands in verbose mode.

cmake --build <target-directory>

This was all working previously so I'm unsure what's changed or if I need to adapt to anything new, the only thing I've ever used CMAKE for was building SDL3 so I'm a complete beginner 😂

When building SDL_image, are you starting fresh, or using a cached configuration? Add --fresh to the configuration command to do so.

I just successfully built lastest SDL and SDL_image using emscripten.

emcmake cmake -S source-directory/SDL -GNinja -B /tmp/em-sdl  --fresh
cmake --build /tmp/em-sdl

emcmake cmake -S source-directory/SDL_image -GNinja -B /tmp/em-sdl_image \
 -DSDL3_DIR=/tmp/em-sdl  --fresh \
 -DSDLIMAGE_BMP=ON     -DSDLIMAGE_JPG=ON     -DSDLIMAGE_PNG=ON     -DSDLIMAGE_AVIF=OFF \
 -DSDLIMAGE_WEBP=OFF     -DSDLIMAGE_GIF=OFF     -DSDLIMAGE_JXL=OFF     -DSDLIMAGE_LBM=OFF \
 -DSDLIMAGE_PCX=OFF     -DSDLIMAGE_PNM=OFF     -DSDLIMAGE_QOI=OFF     -DSDLIMAGE_SVG=OFF \
 -DSDLIMAGE_TGA=OFF     -DSDLIMAGE_TIF=OFF     -DSDLIMAGE_XCF=OFF     -DSDLIMAGE_XPM=OFF \
 -DSDLIMAGE_XV=OFF     -DSDLIMAGE_TESTS=OFF     -DSDLIMAGE_VENDORED=ON \
 -DSDLIMAGE_DEPS_SHARED=OFF     -DBUILD_SHARED_LIBS=OFF
cmake --build /tmp/em-sdl_image --verbose

The link command of showgpuimage is:

/media/download/maarten/emsdk/upstream/emscripten/emcc -O2 -g -DNDEBUG  CMakeFiles/showgpuimage.dir/examples/showgpuimage.c.o -o showgpuimage.js  libSDL3_image.a  /tmp/em-sdl/libSDL3.a  external/libpng-build/libpng16.a  external/zlib-build/libzlibstatic.a  -lm

This gives me that libpng16.a has succesfully been added as a dependency.

As a verification, the SDL_image cmake test project also successfully builds and links:

emcmake cmake -S source-directory/SDL_image/cmake/test -GNinja -B /tmp/em-sdl_image-test \
 -DSDL3_DIR=/tmp/em-sdl -DSDL3_image_DIR=/tmp/em-sdl_image -DTEST_STATIC=ON -DTEST_SHARED=OFF --fresh
cmake --build /tmp/em-sdl_image-test --verbose

This shows:

/media/download/maarten/emsdk/upstream/emscripten/emcc   CMakeFiles/main_static.dir/main.c.o -o main_static.js  /tmp/em-sdl_image/libSDL3_image.a  /tmp/em-sdl/libSDL3.a  /tmp/em-sdl_image/external/libpng-build/libpng16.a  /tmp/em-sdl_image/external/zlib-build/libzlibstatic.a  -lm

The additional static libraries are identical.
(This shows us the config files behave identical)

[LloydJHowarth]

When building SDL_image, are you starting fresh, or using a cached configuration? Add --fresh to the configuration command to do so.

It's a fresh github actions runner per build, which pulls all sdl, satellites and submodules (updates everything). I've even tried a manual build on my own computer using --fresh but it's just the same.

The link command of showgpuimage is:

Ahh okay.. I believe this is it.

upstream\emscripten\emcc.bat -fwasm-exceptions -sSUPPORT_LONGJMP=wasm -O3 -DNDEBUG  CMakeFiles/showgpuimage.dir/examples/showgpuimage.c.o -o showgpuimage.js  libSDL3_image.a  C:/Dev/Hybrid/.github/scripts/Dependencies/Modules/SDL/install_/lib/libSDL3.a  external/libpng-build/libpng16.a  external/zlib-build/libzlibstatic.a  -lm && cd ."

It seems it's present.

Does the SDL_Image test attempt to load a png file on Emscripten? Everything with SDL + Image seems to work except loading / handling PNG files yet everything seems to be present apart from the logs I was getting in my .NET build about undefined symbol: png_sig_cmp.. Yet they're present in the final dotnet.symbols file in the build.. Strange.

I think for now I will just fallback to SDL_IMAGE 3.2.6 for now.

[madebr]

Try to reproduce the error in a minimal project. Either it's a bug in your project, or a real bug in SDL_image's build scripts.

[LloydJHowarth]

Tested with a minimal C# project already with the same result.

I personally believe it to be the latter since from 3.2.6, main has had lots of png work done and different backends such as ani & apng which wasn't there before, since those have been added or enabled it's stopped working for me.

3.2.6: enabled: stb bmp jpg png
latest: enabled: stb ani bmp jpg png apng

I'm sure whatever it is will be found & fixed eventually but unfortunately I don't know how to take this further without much C, CMake & Emscripten expertise, I just thought it would be something simple or something forgotten.

I appreciate your time!

[madebr]

Starting from SDL3 3.4 and SDL3_image 3.4, libpng is only needed for animated png's (apng). Support for static PNG's moved to the main SDL3 library.

So by configuring SDL3_image with SDLIMAGE_PNG=ON -DSDLIMAGE_PNG_LIBPNG=OFF you will get png support without animations.

[LloydJHowarth]

I'll run some more tests see what I can find

[LloydJHowarth]

Okay so png broke for emscripten here: a82b6e68201c8b14b33206014d08ecff7b875fef [Cleaned up PNG loading logic]

So I decided to retry build with a82b6e6 again but with these options added:

-DSDLIMAGE_ANI=OFF 
-DSDLIMAGE_PNG_LIBPNG=OFF 

Now png is working as intended, just tried the same with the latest main branch and that works too!

Maybe it's a conflict with SDLIMAGE_PNG_LIBPNG & SDLIMAGE_PNG both being on with Emscripten.
Maybe similar export symbols or something? No Idea.

Well it works now!

[madebr]

Great!

btw, if the only thing from SDL_image you need is PNG support, SDL3 3.4 brings SDL_LoadPNG.
In that case, you can drop the SDL_image dependency.