blob: 9ed6300c865470238eb82ed7efebf3ba5953a6ac [file] [log] [blame]
 // 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 } } }