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# Source file src/image/png/paeth.go

## Documentation: image/png

```  // Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style

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
}
}
}

```

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