ebiten: add BlendOperationSubtract and BlendOperationReverseSubtract

Updates #2382

blend.go

@@ -104,12 +104,24 @@ const (
 	// BlendOperationAdd represents adding the source and destination color.
 	// c_out = factor_src × c_src + factor_dst × c_dst
 	BlendOperationAdd BlendOperation = iota
+
+	// BlendOperationSubtract represents subtracting the source and destination color.
+	// c_out = factor_src × c_src - factor_dst × c_dst
+	BlendOperationSubtract
+
+	// BlendOperationReverseSubtract represents subtracting the source and destination color in a reversed order.
+	// c_out = factor_dst × c_dst - factor_src × c_src
+	BlendOperationReverseSubtract
 )
 
 func (b BlendOperation) internalBlendOperation() graphicsdriver.BlendOperation {
 	switch b {
 	case BlendOperationAdd:
 		return graphicsdriver.BlendOperationAdd
+	case BlendOperationSubtract:
+		return graphicsdriver.BlendOperationSubtract
+	case BlendOperationReverseSubtract:
+		return graphicsdriver.BlendOperationReverseSubtract
 	default:
 		panic(fmt.Sprintf("ebiten: invalid blend operation: %d", b))
 	}

image_test.go

@@ -3551,3 +3551,104 @@ func TestImageColorMAndScale(t *testing.T) {
 		})
 	}
 }
+
+func TestImageBlendOperation(t *testing.T) {
+	const w, h = 16, 1
+	dst := ebiten.NewImage(w, h)
+	src := ebiten.NewImage(w, h)
+
+	dstColor := func(i int) (byte, byte, byte, byte) {
+		return byte(4 * i * 17), byte(4*i*17 + 1), byte(4*i*17 + 2), byte(4*i*17 + 3)
+	}
+	srcColor := func(i int) (byte, byte, byte, byte) {
+		return byte(4 * i * 13), byte(4*i*13 + 1), byte(4*i*13 + 2), byte(4*i*13 + 3)
+	}
+	clamp := func(x int) byte {
+		if x > 255 {
+			return 255
+		}
+		if x < 0 {
+			return 0
+		}
+		return byte(x)
+	}
+
+	dstPix := make([]byte, 4*w*h)
+	for i := 0; i < w; i++ {
+		r, g, b, a := dstColor(i)
+		dstPix[4*i] = r
+		dstPix[4*i+1] = g
+		dstPix[4*i+2] = b
+		dstPix[4*i+3] = a
+	}
+	srcPix := make([]byte, 4*w*h)
+	for i := 0; i < w; i++ {
+		r, g, b, a := srcColor(i)
+		srcPix[4*i] = r
+		srcPix[4*i+1] = g
+		srcPix[4*i+2] = b
+		srcPix[4*i+3] = a
+	}
+	src.WritePixels(srcPix)
+
+	colorOperations := []ebiten.BlendOperation{
+		ebiten.BlendOperationAdd,
+		ebiten.BlendOperationSubtract,
+		ebiten.BlendOperationReverseSubtract,
+	}
+	alphaOperations := []ebiten.BlendOperation{
+		ebiten.BlendOperationAdd,
+		ebiten.BlendOperationSubtract,
+		ebiten.BlendOperationReverseSubtract,
+	}
+	for _, cop := range colorOperations {
+		for _, aop := range alphaOperations {
+			// Reset the destination state.
+			dst.WritePixels(dstPix)
+			op := &ebiten.DrawImageOptions{}
+			op.Blend = ebiten.Blend{
+				BlendFactorSourceColor:      ebiten.BlendFactorOne,
+				BlendFactorSourceAlpha:      ebiten.BlendFactorOne,
+				BlendFactorDestinationColor: ebiten.BlendFactorOne,
+				BlendFactorDestinationAlpha: ebiten.BlendFactorOne,
+				BlendOperationColor:         cop,
+				BlendOperationAlpha:         aop,
+			}
+			dst.DrawImage(src, op)
+			for i := 0; i < w; i++ {
+				got := dst.At(i, 0).(color.RGBA)
+
+				sr, sg, sb, sa := srcColor(i)
+				dr, dg, db, da := dstColor(i)
+
+				var want color.RGBA
+				switch cop {
+				case ebiten.BlendOperationAdd:
+					want.R = clamp(int(sr) + int(dr))
+					want.G = clamp(int(sg) + int(dg))
+					want.B = clamp(int(sb) + int(db))
+				case ebiten.BlendOperationSubtract:
+					want.R = clamp(int(sr) - int(dr))
+					want.G = clamp(int(sg) - int(dg))
+					want.B = clamp(int(sb) - int(db))
+				case ebiten.BlendOperationReverseSubtract:
+					want.R = clamp(int(dr) - int(sr))
+					want.G = clamp(int(dg) - int(sg))
+					want.B = clamp(int(db) - int(sb))
+				}
+				switch aop {
+				case ebiten.BlendOperationAdd:
+					want.A = clamp(int(sa) + int(da))
+				case ebiten.BlendOperationSubtract:
+					want.A = clamp(int(sa) - int(da))
+				case ebiten.BlendOperationReverseSubtract:
+					want.A = clamp(int(da) - int(sa))
+				}
+
+				if !sameColors(got, want, 1) {
+					t.Errorf("dst.At(%d, 0): operations: %d, %d: got: %v, want: %v", i, cop, aop, got, want)
+				}
+			}
+		}
+	}
+}

internal/graphicsdriver/blend.go

@@ -39,8 +39,8 @@ type BlendOperation byte
 
 const (
 	BlendOperationAdd BlendOperation = iota
-
-	// TODO: Add more operators
+	BlendOperationSubtract
+	BlendOperationReverseSubtract
 )
 
 var BlendSourceOver = Blend{

internal/graphicsdriver/directx/pipeline_windows.go

@@ -55,6 +55,10 @@ func blendOperationToBlendOp(o graphicsdriver.BlendOperation) _D3D12_BLEND_OP {
 	switch o {
 	case graphicsdriver.BlendOperationAdd:
 		return _D3D12_BLEND_OP_ADD
+	case graphicsdriver.BlendOperationSubtract:
+		return _D3D12_BLEND_OP_SUBTRACT
+	case graphicsdriver.BlendOperationReverseSubtract:
+		return _D3D12_BLEND_OP_REV_SUBTRACT
 	default:
 		panic(fmt.Sprintf("directx: invalid blend operation: %d", o))
 	}

internal/graphicsdriver/metal/graphics_darwin.go

@@ -325,6 +325,10 @@ func blendOperationToMetalBlendOperation(o graphicsdriver.BlendOperation) mtl.Bl
 	switch o {
 	case graphicsdriver.BlendOperationAdd:
 		return mtl.BlendOperationAdd
+	case graphicsdriver.BlendOperationSubtract:
+		return mtl.BlendOperationSubtract
+	case graphicsdriver.BlendOperationReverseSubtract:
+		return mtl.BlendOperationReverseSubtract
 	default:
 		panic(fmt.Sprintf("metal: invalid blend operation: %d", o))
 	}

internal/graphicsdriver/opengl/context.go

@@ -36,7 +36,9 @@ const (
 type blendOperation int
 
 const (
-	glFuncAdd blendOperation = 0x8006
+	glFuncAdd             blendOperation = 0x8006
+	glFuncReverseSubtract blendOperation = 0x800b
+	glFuncSubtract        blendOperation = 0x800a
 )
 
 func convertBlendFactor(f graphicsdriver.BlendFactor) blendFactor {
@@ -64,6 +66,10 @@ func convertBlendOperation(o graphicsdriver.BlendOperation) blendOperation {
 	switch o {
 	case graphicsdriver.BlendOperationAdd:
 		return glFuncAdd
+	case graphicsdriver.BlendOperationSubtract:
+		return glFuncSubtract
+	case graphicsdriver.BlendOperationReverseSubtract:
+		return glFuncReverseSubtract
 	default:
 		panic(fmt.Sprintf("opengl: invalid blend operation %d", o))
 	}