1 // Copyright 2012 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package png 6 7 // intSize is either 32 or 64. 8 const intSize = 32 << (^uint(0) >> 63) 9 10 func abs(x int) int { 11 // m := -1 if x < 0. m := 0 otherwise. 12 m := x >> (intSize - 1) 13 14 // In two's complement representation, the negative number 15 // of any number (except the smallest one) can be computed 16 // by flipping all the bits and add 1. This is faster than 17 // code with a branch. 18 // See Hacker's Delight, section 2-4. 19 return (x ^ m) - m 20 } 21 22 // paeth implements the Paeth filter function, as per the PNG specification. 23 func paeth(a, b, c uint8) uint8 { 24 // This is an optimized version of the sample code in the PNG spec. 25 // For example, the sample code starts with: 26 // p := int(a) + int(b) - int(c) 27 // pa := abs(p - int(a)) 28 // but the optimized form uses fewer arithmetic operations: 29 // pa := int(b) - int(c) 30 // pa = abs(pa) 31 pc := int(c) 32 pa := int(b) - pc 33 pb := int(a) - pc 34 pc = abs(pa + pb) 35 pa = abs(pa) 36 pb = abs(pb) 37 if pa <= pb && pa <= pc { 38 return a 39 } else if pb <= pc { 40 return b 41 } 42 return c 43 } 44 45 // filterPaeth applies the Paeth filter to the cdat slice. 46 // cdat is the current row's data, pdat is the previous row's data. 47 func filterPaeth(cdat, pdat []byte, bytesPerPixel int) { 48 var a, b, c, pa, pb, pc int 49 for i := 0; i < bytesPerPixel; i++ { 50 a, c = 0, 0 51 for j := i; j < len(cdat); j += bytesPerPixel { 52 b = int(pdat[j]) 53 pa = b - c 54 pb = a - c 55 pc = abs(pa + pb) 56 pa = abs(pa) 57 pb = abs(pb) 58 if pa <= pb && pa <= pc { 59 // No-op. 60 } else if pb <= pc { 61 a = b 62 } else { 63 a = c 64 } 65 a += int(cdat[j]) 66 a &= 0xff 67 cdat[j] = uint8(a) 68 c = b 69 } 70 } 71 } 72
