golang paeth 代码
文件路径：/src/image/png/paeth.go
/ 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
}
}
}