diff options
author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-16 19:23:18 +0000 |
---|---|---|
committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-16 19:23:18 +0000 |
commit | 43a123c1ae6613b3efeed291fa552ecd909d3acf (patch) | |
tree | fd92518b7024bc74031f78a1cf9e454b65e73665 /src/image/png/paeth.go | |
parent | Initial commit. (diff) | |
download | golang-1.20-43a123c1ae6613b3efeed291fa552ecd909d3acf.tar.xz golang-1.20-43a123c1ae6613b3efeed291fa552ecd909d3acf.zip |
Adding upstream version 1.20.14.upstream/1.20.14upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'src/image/png/paeth.go')
-rw-r--r-- | src/image/png/paeth.go | 71 |
1 files changed, 71 insertions, 0 deletions
diff --git a/src/image/png/paeth.go b/src/image/png/paeth.go new file mode 100644 index 0000000..9ed6300 --- /dev/null +++ b/src/image/png/paeth.go @@ -0,0 +1,71 @@ +// Copyright 2012 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package png + +// intSize is either 32 or 64. +const intSize = 32 << (^uint(0) >> 63) + +func abs(x int) int { + // m := -1 if x < 0. m := 0 otherwise. + m := x >> (intSize - 1) + + // In two's complement representation, the negative number + // of any number (except the smallest one) can be computed + // by flipping all the bits and add 1. This is faster than + // code with a branch. + // See Hacker's Delight, section 2-4. + return (x ^ m) - m +} + +// paeth implements the Paeth filter function, as per the PNG specification. +func paeth(a, b, c uint8) uint8 { + // This is an optimized version of the sample code in the PNG spec. + // For example, the sample code starts with: + // p := int(a) + int(b) - int(c) + // pa := abs(p - int(a)) + // but the optimized form uses fewer arithmetic operations: + // pa := int(b) - int(c) + // pa = abs(pa) + pc := int(c) + pa := int(b) - pc + pb := int(a) - pc + pc = abs(pa + pb) + pa = abs(pa) + pb = abs(pb) + if pa <= pb && pa <= pc { + return a + } else if pb <= pc { + return b + } + return c +} + +// filterPaeth applies the Paeth filter to the cdat slice. +// cdat is the current row's data, pdat is the previous row's data. +func filterPaeth(cdat, pdat []byte, bytesPerPixel int) { + var a, b, c, pa, pb, pc int + for i := 0; i < bytesPerPixel; i++ { + a, c = 0, 0 + for j := i; j < len(cdat); j += bytesPerPixel { + b = int(pdat[j]) + pa = b - c + pb = a - c + pc = abs(pa + pb) + pa = abs(pa) + pb = abs(pb) + if pa <= pb && pa <= pc { + // No-op. + } else if pb <= pc { + a = b + } else { + a = c + } + a += int(cdat[j]) + a &= 0xff + cdat[j] = uint8(a) + c = b + } + } +} |