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Use stb's paeth predictor #540

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Use stb's paeth predictor #540

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27 changes: 10 additions & 17 deletions src/filter.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -318,24 +318,17 @@ impl Default for AdaptiveFilterType {
}
}

/// Adapted from stb_image.h 2.30
fn filter_paeth_decode(a: u8, b: u8, c: u8) -> u8 {
// Decoding seems to optimize better with this algorithm
let pa = (i16::from(b) - i16::from(c)).abs();
let pb = (i16::from(a) - i16::from(c)).abs();
let pc = ((i16::from(a) - i16::from(c)) + (i16::from(b) - i16::from(c))).abs();

let mut out = a;
let mut min = pa;

if pb < min {
min = pb;
out = b;
}
if pc < min {
out = c;
}

out
// This formulation looks very different from the reference in the PNG spec, but is
// actually equivalent and has favorable data dependencies and admits straightforward
// generation of branch-free code, which helps performance significantly.
let thresh = c as i16 * 3 - (a as i16 + b as i16);
let lo = a.min(b);
let hi = a.max(b);
let t0 = if thresh >= hi as i16 { lo } else { c };
let t1 = if thresh <= lo as i16 { hi } else { t0 };
t1
}

#[cfg(feature = "unstable")]
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