vector: re-order some functions.

There are no code changes, just a re-ordering so that these files are
consistent with others in this package: OpOver, OpSrc, Mask.

Change-Id: Ib1d46a8e912dae0c760af655e919b77023688189
Reviewed-on: https://go-review.googlesource.com/30111
Reviewed-by: David Crawshaw <crawshaw@golang.org>

vector/raster_fixed.go

@@ -199,39 +199,39 @@ func (z *Rasterizer) fixedLineTo(b f32.Vec2) {
 	}
 }
 
-func fixedAccumulateOpSrc(dst []uint8, src []uint32) {
+func fixedAccumulateOpOver(dst []uint8, src []uint32) {
 	acc := int2ϕ(0)
 	for i, v := range src {
 		acc += int2ϕ(v)
 		a := acc
 		if a < 0 {
 			a = -a
 		}
-		a >>= 2*ϕ - 8
-		if a > 0xff {
-			a = 0xff
+		a >>= 2*ϕ - 16
+		if a > 0xffff {
+			a = 0xffff
 		}
-		dst[i] = uint8(a)
+		// This algorithm comes from the standard library's image/draw package.
+		dstA := uint32(dst[i]) * 0x101
+		maskA := uint32(a)
+		outA := dstA*(0xffff-maskA)/0xffff + maskA
+		dst[i] = uint8(outA >> 8)
 	}
 }
 
-func fixedAccumulateOpOver(dst []uint8, src []uint32) {
+func fixedAccumulateOpSrc(dst []uint8, src []uint32) {
 	acc := int2ϕ(0)
 	for i, v := range src {
 		acc += int2ϕ(v)
 		a := acc
 		if a < 0 {
 			a = -a
 		}
-		a >>= 2*ϕ - 16
-		if a > 0xffff {
-			a = 0xffff
+		a >>= 2*ϕ - 8
+		if a > 0xff {
+			a = 0xff
 		}
-		// This algorithm comes from the standard library's image/draw package.
-		dstA := uint32(dst[i]) * 0x101
-		maskA := uint32(a)
-		outA := dstA*(0xffff-maskA)/0xffff + maskA
-		dst[i] = uint8(outA >> 8)
+		dst[i] = uint8(a)
 	}
 }
 

vector/raster_floating.go

@@ -145,7 +145,7 @@ const (
 	almost65536 = almost256 * 256
 )
 
-func floatingAccumulateOpSrc(dst []uint8, src []float32) {
+func floatingAccumulateOpOver(dst []uint8, src []float32) {
 	acc := float32(0)
 	for i, v := range src {
 		acc += v
@@ -156,11 +156,15 @@ func floatingAccumulateOpSrc(dst []uint8, src []float32) {
 		if a > 1 {
 			a = 1
 		}
-		dst[i] = uint8(almost256 * a)
+		// This algorithm comes from the standard library's image/draw package.
+		dstA := uint32(dst[i]) * 0x101
+		maskA := uint32(almost65536 * a)
+		outA := dstA*(0xffff-maskA)/0xffff + maskA
+		dst[i] = uint8(outA >> 8)
 	}
 }
 
-func floatingAccumulateOpOver(dst []uint8, src []float32) {
+func floatingAccumulateOpSrc(dst []uint8, src []float32) {
 	acc := float32(0)
 	for i, v := range src {
 		acc += v
@@ -171,11 +175,7 @@ func floatingAccumulateOpOver(dst []uint8, src []float32) {
 		if a > 1 {
 			a = 1
 		}
-		// This algorithm comes from the standard library's image/draw package.
-		dstA := uint32(dst[i]) * 0x101
-		maskA := uint32(almost65536 * a)
-		outA := dstA*(0xffff-maskA)/0xffff + maskA
-		dst[i] = uint8(outA >> 8)
+		dst[i] = uint8(almost256 * a)
 	}
 }