perf(graphics): optimize Atkinson dithering with 1D slice and add test

pkg/graphics/graph_engine.go

@@ -165,11 +165,11 @@ func (p *Pipeline) applyAtkinson(gray *image.Gray) *MonochromeBitmap {
 	mono := NewMonochromeBitmap(width, height)
 
 	// Create a working copy for error diffusion
-	work := make([][]int, height)
+	work := make([]int, width*height)
 	for y := 0; y < height; y++ {
-		work[y] = make([]int, width)
+		rowOffset := y * width
 		for x := 0; x < width; x++ {
-			work[y][x] = int(gray.GrayAt(x, y).Y)
+			work[rowOffset+x] = int(gray.GrayAt(x, y).Y)
 		}
 	}
 
@@ -180,8 +180,13 @@ func (p *Pipeline) applyAtkinson(gray *image.Gray) *MonochromeBitmap {
 	// Error is distributed as 1/8 to each neighbor (total 6/8 = 3/4)
 
 	for y := 0; y < height; y++ {
+		rowOffset := y * width
+		nextRowOffset := (y + 1) * width
+		nextNextRowOffset := (y + 2) * width
+
 		for x := 0; x < width; x++ {
-			oldPixel := work[y][x]
+			idx := rowOffset + x
+			oldPixel := work[idx]
 			newPixel := 0
 			if oldPixel > int(p.opts.Threshold) {
 				newPixel = 255
@@ -199,22 +204,22 @@ func (p *Pipeline) applyAtkinson(gray *image.Gray) *MonochromeBitmap {
 
 			// Distribute to neighbors
 			if x+1 < width {
-				work[y][x+1] += diffusedError
+				work[idx+1] += diffusedError
 			}
 			if x+2 < width {
-				work[y][x+2] += diffusedError
+				work[idx+2] += diffusedError
 			}
 			if y+1 < height {
 				if x-1 >= 0 {
-					work[y+1][x-1] += diffusedError
+					work[nextRowOffset+(x-1)] += diffusedError
 				}
-				work[y+1][x] += diffusedError
+				work[nextRowOffset+x] += diffusedError
 				if x+1 < width {
-					work[y+1][x+1] += diffusedError
+					work[nextRowOffset+(x+1)] += diffusedError
 				}
 			}
 			if y+2 < height {
-				work[y+2][x] += diffusedError
+				work[nextNextRowOffset+x] += diffusedError
 			}
 		}
 	}

pkg/graphics/graph_engine_test.go

@@ -145,3 +145,57 @@ func TestPipeline_Resize_Limit(t *testing.T) {
 		t.Errorf("Output width = %d, want 576 (capped)", mono.Width)
 	}
 }
+
+func TestPipeline_Process_Atkinson_DiffusionLogic(t *testing.T) {
+	opts := graphics.DefaultOptions()
+	opts.Dithering = graphics.Atkinson
+	opts.Threshold = 128
+	// Use small width to force manual checking and avoid resize
+	opts.PixelWidth = 3
+	p := graphics.NewPipeline(opts)
+
+	// We test each neighbor independently to avoid cascade interference
+	tests := []struct {
+		name   string
+		source image.Point
+		target image.Point
+	}{
+		{"Right_x+1", image.Point{0, 0}, image.Point{1, 0}},
+		{"Right_x+2", image.Point{0, 0}, image.Point{2, 0}},
+		{"Below_y+1_x", image.Point{0, 0}, image.Point{0, 1}},
+		{"Diag_y+1_x+1", image.Point{0, 0}, image.Point{1, 1}},
+		{"Below_y+2_x", image.Point{0, 0}, image.Point{0, 2}},
+		{"Diag_y+1_x-1", image.Point{1, 0}, image.Point{0, 1}},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			img := image.NewGray(image.Rect(0, 0, 3, 3))
+			// Clear
+			for i := 0; i < 9; i++ {
+				img.SetGray(i%3, i/3, color.Gray{Y: 0})
+			}
+
+			// Set source to 100. Error = 100-0 = 100. Diff = 12.
+			img.SetGray(tc.source.X, tc.source.Y, color.Gray{Y: 100})
+
+			// Set target to 120. 120+12 = 132 > 128 -> White.
+			img.SetGray(tc.target.X, tc.target.Y, color.Gray{Y: 120})
+
+			mono, err := p.Process(img)
+			if err != nil {
+				t.Fatalf("Process failed: %v", err)
+			}
+
+			// Source should be Black
+			if !mono.GetPixel(tc.source.X, tc.source.Y) {
+				t.Errorf("Pixel(%d,%d) should be Black", tc.source.X, tc.source.Y)
+			}
+
+			// Target should be White
+			if mono.GetPixel(tc.target.X, tc.target.Y) {
+				t.Errorf("Pixel(%d,%d) should be White (received diffusion)", tc.target.X, tc.target.Y)
+			}
+		})
+	}
+}