72 lines
1.7 KiB
Go
72 lines
1.7 KiB
Go
// Copyright 2012 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package apng
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// intSize is either 32 or 64.
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const intSize = 32 << (^uint(0) >> 63)
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func abs(x int) int {
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// m := -1 if x < 0. m := 0 otherwise.
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m := x >> (intSize - 1)
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// In two's complement representation, the negative number
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// of any number (except the smallest one) can be computed
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// by flipping all the bits and add 1. This is faster than
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// code with a branch.
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// See Hacker's Delight, section 2-4.
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return (x ^ m) - m
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}
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// paeth implements the Paeth filter function, as per the PNG specification.
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func paeth(a, b, c uint8) uint8 {
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// This is an optimized version of the sample code in the PNG spec.
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// For example, the sample code starts with:
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// p := int(a) + int(b) - int(c)
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// pa := abs(p - int(a))
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// but the optimized form uses fewer arithmetic operations:
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// pa := int(b) - int(c)
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// pa = abs(pa)
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pc := int(c)
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pa := int(b) - pc
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pb := int(a) - pc
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pc = abs(pa + pb)
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pa = abs(pa)
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pb = abs(pb)
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if pa <= pb && pa <= pc {
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return a
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} else if pb <= pc {
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return b
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}
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return c
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}
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// filterPaeth applies the Paeth filter to the cdat slice.
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// cdat is the current row's data, pdat is the previous row's data.
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func filterPaeth(cdat, pdat []byte, bytesPerPixel int) {
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var a, b, c, pa, pb, pc int
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for i := 0; i < bytesPerPixel; i++ {
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a, c = 0, 0
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for j := i; j < len(cdat); j += bytesPerPixel {
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b = int(pdat[j])
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pa = b - c
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pb = a - c
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pc = abs(pa + pb)
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pa = abs(pa)
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pb = abs(pb)
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if pa <= pb && pa <= pc {
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// No-op.
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} else if pb <= pc {
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a = b
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} else {
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a = c
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}
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a += int(cdat[j])
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a &= 0xff
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cdat[j] = uint8(a)
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c = b
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}
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}
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}
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