Fix gamut mapping returning black/greenish for white pixels

colorconv/colorconv.go

@@ -384,6 +384,7 @@ func (c *ConvertColor) gamutMapChromaScale(L, a, b float64) (r, g, bl float64) {
 	hi := 1.0
 	var mid float64
 	var foundR, foundG, foundB float64
+	found := false
 	// If even fully desaturated (a=b=0) is out of gamut, we'll clip
 	for range 24 {
 		mid = (lo + hi) / 2.0
@@ -392,14 +393,15 @@ func (c *ConvertColor) gamutMapChromaScale(L, a, b float64) (r, g, bl float64) {
 		r0, g0, b0 := c.LabToSRGBNoGamutMap(L, a2, b2)
 		if inGamut(r0, g0, b0) {
 			foundR, foundG, foundB = r0, g0, b0
+			found = true
 			// can try to keep more chroma
 			lo = mid
 		} else {
 			hi = mid
 		}
 	}
 	// If we never found a valid in-gamut during binary search, try a= b =0
-	if !(inGamut(foundR, foundG, foundB)) {
+	if !found {
 		r0, g0, b0 := c.LabToSRGBNoGamutMap(L, 0, 0)
 		// if still out-of-gamut (very unlikely), clip
 		return clamp01(r0), clamp01(g0), clamp01(b0)

prism/meta/icc/pcs.go

@@ -188,25 +188,34 @@ func NewUniformFunctionTransformer(name string, f func(unit_float) unit_float) *
 }
 
 type XYZtosRGB struct {
-	c *colorconv.ConvertColor
-	t func(l, a, b unit_float) (x, y, z unit_float)
+	c         *colorconv.ConvertColor
+	t         func(l, a, b unit_float) (x, y, z unit_float)
+	map_gamut bool
 }
 
 func NewXYZtosRGB(whitepoint XYZType, clamp, map_gamut bool) *XYZtosRGB {
 	c := colorconv.NewConvertColor(whitepoint.X, whitepoint.Y, whitepoint.Z, 1)
 	if clamp {
 		if map_gamut {
-			return &XYZtosRGB{c, c.XYZToSRGB}
+			return &XYZtosRGB{c, c.XYZToSRGB, true}
 		}
-		return &XYZtosRGB{c, c.XYZToSRGBNoGamutMap}
+		return &XYZtosRGB{c, c.XYZToSRGBNoGamutMap, false}
 	}
-	return &XYZtosRGB{c, c.XYZToSRGBNoClamp}
+	return &XYZtosRGB{c, c.XYZToSRGBNoClamp, false}
 }
 
 func (n *XYZtosRGB) AddPreviousMatrix(m Matrix3) {
 	n.c.AddPreviousMatrix(m[0], m[1], m[2])
 }
 
+// CanAbsorbMatrix reports whether it is safe for the pipeline optimizer to
+// fuse preceding matrices into this transformer. When gamut mapping is
+// active, the fallback path converts via Lab which requires the input to be
+// actual XYZ values, so matrix absorption would break that invariant.
+func (n *XYZtosRGB) CanAbsorbMatrix() bool {
+	return !n.map_gamut
+}
+
 func (c *XYZtosRGB) Transform(l, a, b unit_float) (unit_float, unit_float, unit_float) {
 	return c.t(l, a, b)
 }

prism/meta/icc/pipeline.go

@@ -64,7 +64,7 @@ func (p *Pipeline) finalize(optimize bool) {
 		// Check if the last transform can absorb previous matrices
 		if len(p.transformers) > 1 {
 			last := p.transformers[len(p.transformers)-1]
-			if apm, ok := last.(*XYZtosRGB); ok {
+			if apm, ok := last.(*XYZtosRGB); ok && apm.CanAbsorbMatrix() {
 				p.transformers = p.transformers[:len(p.transformers)-1]
 				for {
 					m := p.remove_last_matrix3()