image/color: Palette.Index can be optimized #43756
Description
What did you do?
image/color
func (p Palette) Index(c Color) int {
// A batch version of this computation is in image/draw/draw.go.
cr, cg, cb, ca := c.RGBA()
ret, bestSum := 0, uint32(1<<32-1)
for i, v := range p {
vr, vg, vb, va := v.RGBA()
sum := sqDiff(cr, vr) + sqDiff(cg, vg) + sqDiff(cb, vb) + sqDiff(ca, va)
if sum < bestSum {
if sum == 0 {
return i
}
ret, bestSum = i, sum
}
}
return ret
}is very slowly
image size 1366x768 --> cost > 2s
What did you expect to see?
func (p Palette) Index(c color.Color) int {
// A batch version of this computation is in image/draw/draw.go.
var mod uint8 = 51
var splitNum = 255 / mod + 1
cr, cg, cb, ca := c.RGBA()
cr8, cg8, cb8, ca8 := uint8(cr), uint8(cg), uint8(cb), uint8(ca)
ri, gi, bi, ai := cr8/mod, cg8/mod, cb8/mod, ca8/mod
rm, gm, bm, am := cr8%mod, cg8%mod, cb8%mod, ca8%mod
if rm > mod/2{
ri += 1
}
if gm > mod/2 {
gi += 1
}
if bm > mod/2 {
bi += 1
}
if am > mod/2 {
ai += 1
}
//ret := int(ri * 36 + gi * 6 + bi)
ret := int(ri * splitNum * splitNum + gi * 6 + bi)
return ret
}will be faster
image size 1366x768 --> cost < 100ms