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Diffstat (limited to 'libgo/go/image/png/paeth.go')
-rw-r--r-- | libgo/go/image/png/paeth.go | 70 |
1 files changed, 70 insertions, 0 deletions
diff --git a/libgo/go/image/png/paeth.go b/libgo/go/image/png/paeth.go new file mode 100644 index 0000000..37978aa --- /dev/null +++ b/libgo/go/image/png/paeth.go @@ -0,0 +1,70 @@ +// 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 + +// 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 = pa + pb + if pa < 0 { + pa = -pa + } + if pb < 0 { + pb = -pb + } + if pc < 0 { + pc = -pc + } + 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 = pa + pb + if pa < 0 { + pa = -pa + } + if pb < 0 { + pb = -pb + } + if pc < 0 { + pc = -pc + } + 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 + } + } +} |