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Strengthen Sub and Paeth

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This commit is contained in:
Andrew 2022-12-10 14:51:30 +13:00
parent d70b35053e
commit 63e1aa33b0
1 changed files with 16 additions and 12 deletions
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26
src/filters.rs
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@ -129,13 +129,19 @@ impl RowFilter {
let prev_pixels: Vec<_> = prev_line.chunks(bpp).collect(); let prev_pixels: Vec<_> = prev_line.chunks(bpp).collect();
for i in 0..pixels.len() { for i in 0..pixels.len() {
if pixels[i].iter().skip(color_bytes).all(|b| *b == 0) { if pixels[i].iter().skip(color_bytes).all(|b| *b == 0) {
// If the first pixel in the row is transparent, find the next non-transparent pixel and pretend
// it is the previous one. This can help improve effectiveness of the Sub and Paeth filters.
let prev = match i {
0 => pixels
.iter()
.position(|px| px.iter().skip(color_bytes).any(|b| *b != 0))
.unwrap_or(i),
_ => i - 1,
};
match self { match self {
Self::Sub => { Self::Sub => {
for j in 0..color_bytes { for j in 0..color_bytes {
pixels[i][j] = match i { pixels[i][j] = pixels[prev][j];
0 => 0,
_ => pixels[i - 1][j],
};
} }
} }
Self::Up => { Self::Up => {
@ -153,17 +159,15 @@ impl RowFilter {
} }
} }
Self::Paeth => { Self::Paeth => {
if i == 0 {
pixels[i][0..color_bytes]
.copy_from_slice(&prev_pixels[i][0..color_bytes]);
} else {
for j in 0..color_bytes { for j in 0..color_bytes {
pixels[i][j] = paeth_predictor( pixels[i][j] = match i {
0 => pixels[prev][j].min(prev_pixels[i][j]),
_ => paeth_predictor(
pixels[i - 1][j], pixels[i - 1][j],
prev_pixels[i][j], prev_pixels[i][j],
prev_pixels[i - 1][j], prev_pixels[i - 1][j],
); ),
} };
} }
} }
_ => unreachable!(), _ => unreachable!(),