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Determine min depth by palette length

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This commit is contained in:
Andrew 2023-05-18 16:54:25 +12:00
parent 27d9e34ff9
commit 4a1cd2db3e
1 changed files with 14 additions and 29 deletions
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43
src/reduction/bit_depth.rs
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@ -36,37 +36,22 @@ pub fn reduced_bit_depth_8_or_less(png: &PngImage, mut minimum_bits: usize) -> O
// Calculate the current number of pixels per byte
let ppb = 8 / bit_depth;
if let ColorType::Indexed { .. } = png.ihdr.color_type {
for line in png.scan_lines(false) {
let line_max = line
.data
.iter()
.map(|&byte| match png.ihdr.bit_depth {
BitDepth::Two => (byte & 0x3)
.max((byte >> 2) & 0x3)
.max((byte >> 4) & 0x3)
.max(byte >> 6),
BitDepth::Four => (byte & 0xF).max(byte >> 4),
_ => byte,
})
.max()
.unwrap_or(0);
let required_bits = match line_max {
x if x > 0x0F => 8,
x if x > 0x03 => 4,
x if x > 0x01 => 2,
_ => 1,
};
if required_bits > minimum_bits {
minimum_bits = required_bits;
if minimum_bits >= bit_depth {
// Not reducable
return None;
}
}
if let ColorType::Indexed { palette } = &png.ihdr.color_type {
// We can easily determine minimum depth by the palette size
let required_bits = match palette.len() {
0..=2 => 1,
3..=4 => 2,
5..=16 => 4,
_ => 8,
};
if required_bits >= bit_depth {
// Not reducable
return None;
} else if required_bits > minimum_bits {
minimum_bits = required_bits;
}
} else {
// Checking for grayscale depth reduction is quite different than for indexed
// Finding minimum depth for grayscale is much more complicated
let mut mask = (1 << minimum_bits) - 1;
let mut divisions = 1..(bit_depth / minimum_bits);
for &b in &png.data {