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Tweak alpha factor in luma sort

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Andrew 2023-05-31 19:51:32 +12:00 committed by Josh Holmer
parent 6cd3783775
commit cf9c73bf4a
1 changed files with 4 additions and 1 deletions
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5
src/reduction/palette.rs
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@ -88,7 +88,10 @@ pub fn sorted_palette(png: &PngImage) -> Option<PngImage> {
enumerated.sort_by(|a, b| { enumerated.sort_by(|a, b| {
// Sort by ascending alpha and descending luma // Sort by ascending alpha and descending luma
let color_val = |color: &RGBA8| { let color_val = |color: &RGBA8| {
((color.a as i32) << 18) let a = i32::from(color.a);
// Put 7 high bits of alpha first, then luma, then low bit of alpha
// This provides notable improvement in images with a lot of alpha
((a & 0xFE) << 18) + (a & 0x01)
// These are coefficients for standard sRGB to luma conversion // These are coefficients for standard sRGB to luma conversion
- i32::from(color.r) * 299 - i32::from(color.r) * 299
- i32::from(color.g) * 587 - i32::from(color.g) * 587