tomo/backend
tomo/backend
1
0
Fork 0
Code Issues 6 Pull Requests Packages Projects Releases 13 Wiki Activity

Use premultiplied alpha for X canvas

Browse Source
This commit is contained in:
Sasha Koshka 2024-08-12 18:15:15 -04:00
parent d166d88388
commit e4fdde3da1
3 changed files with 33 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

29
x/canvas/canvas.go
View File

@ -132,3 +132,32 @@ func convertColor (c color.Color) xgraphics.BGRA {
A: uint8(a >> 8),
}
}
// For some reason, xgraphics.BGRA does not specify whether or not it uses
// premultiplied alpha, and information regarding this is contradictory.
// Basically:
// - BGRAModel just takes the result of c.RGBA and bit shifts it, without
// un-doing the aplha premultiplication that is required by Color.RGBA,
// suggesting that xgraphics.BGRA stores alpha-premultiplied color.
// - xgraphics.BlendBGRA lerps between dest and src using only the alpha of
// src (temporarily converting the colors to fucking floats for some reason)
// which seems to suggest that xgraphics.BGRA *does not* store alpha-
// premultiplied color.
// There is no issues page on xgbutil so we may never get an answer to this
// question. However, in this package we just use xgraphics.BGRA to store alpha-
// premultiplied color anyway because its way faster, and I would sooner eat
// spaghetti with a spoon than convert to and from float64 to blend pixels.
func blendPremultipliedBGRA (dst, src xgraphics.BGRA) xgraphics.BGRA {
// https://en.wikipedia.org/wiki/Alpha_compositing
return xgraphics.BGRA {
B: blendPremultipliedChannel(dst.B, src.B, src.A),
G: blendPremultipliedChannel(dst.G, src.G, src.A),
R: blendPremultipliedChannel(dst.R, src.R, src.A),
A: blendPremultipliedChannel(dst.A, src.A, src.A),
}
}
func blendPremultipliedChannel (dst, src, a uint8) uint8 {
dst16, src16, a16 := uint16(dst), uint16(src), uint16(a)
return uint8(src16 + ((dst16 * (255 - a16)) >> 8))
}

6
x/canvas/draw.go
View File

@ -46,7 +46,7 @@ func (this *pen) textureRectangleTransparent (bounds image.Rectangle) {
srcPos := pos.Add(offset)
dstIndex := this.image.PixOffset(pos.X, pos.Y)
srcIndex := this.texture.PixOffset(srcPos.X, srcPos.Y)
pixel := xgraphics.BlendBGRA(xgraphics.BGRA {
pixel := blendPremultipliedBGRA(xgraphics.BGRA {
B: dst[dstIndex + 0],
G: dst[dstIndex + 1],
R: dst[dstIndex + 2],
@ -93,7 +93,7 @@ func (this *pen) fillRectangleTransparent (c xgraphics.BGRA, bounds image.Rectan
for pos.Y = bounds.Min.Y; pos.Y < bounds.Max.Y; pos.Y ++ {
for pos.X = bounds.Min.X; pos.X < bounds.Max.X; pos.X ++ {
index := this.image.PixOffset(pos.X, pos.Y)
pixel := xgraphics.BlendBGRA(xgraphics.BGRA {
pixel := blendPremultipliedBGRA(xgraphics.BGRA {
B: this.image.Pix[index + 0],
G: this.image.Pix[index + 1],
R: this.image.Pix[index + 2],
@ -256,7 +256,7 @@ func (context *fillingContext) fillPolygonHotTransparent () {
// fill pixels in between
for x := left; x < right; x ++ {
index := context.image.PixOffset(x, context.y)
pixel := xgraphics.BlendBGRA(xgraphics.BGRA {
pixel := blendPremultipliedBGRA(xgraphics.BGRA {
B: context.image.Pix[index + 0],
G: context.image.Pix[index + 1],
R: context.image.Pix[index + 2],

2
x/canvas/plot.go
View File

@ -32,7 +32,7 @@ func (context plottingContext) plot (center image.Point) {
for y := square.Min.Y; y < square.Max.Y; y ++ {
for x := square.Min.X; x < square.Max.X; x ++ {
index := context.image.PixOffset(x, y)
pixel := xgraphics.BlendBGRA(xgraphics.BGRA {
pixel := blendPremultipliedBGRA(xgraphics.BGRA {
B: context.image.Pix[index + 0],
G: context.image.Pix[index + 1],
R: context.image.Pix[index + 2],