libgo/go/image/png/paeth.go

   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 // paeth implements the Paeth filter function, as per the PNG specification.
   8 func paeth(a, b, c uint8) uint8 {
   9         // This is an optimized version of the sample code in the PNG spec.
  10         // For example, the sample code starts with:
  11         //      p := int(a) + int(b) - int(c)
  12         //      pa := abs(p - int(a))
  13         // but the optimized form uses fewer arithmetic operations:
  14         //      pa := int(b) - int(c)
  15         //      pa = abs(pa)
  16         pc := int(c)
  17         pa := int(b) - pc
  18         pb := int(a) - pc
  19         pc = pa + pb
  20         if pa < 0 {
  21                 pa = -pa
  22         }
  23         if pb < 0 {
  24                 pb = -pb
  25         }
  26         if pc < 0 {
  27                 pc = -pc
  28         }
  29         if pa <= pb && pa <= pc {
  30                 return a
  31         } else if pb <= pc {
  32                 return b
  33         }
  34         return c
  35 }
  36
  37 // filterPaeth applies the Paeth filter to the cdat slice.
  38 // cdat is the current row's data, pdat is the previous row's data.
  39 func filterPaeth(cdat, pdat []byte, bytesPerPixel int) {
  40         var a, b, c, pa, pb, pc int
  41         for i := 0; i < bytesPerPixel; i++ {
  42                 a, c = 0, 0
  43                 for j := i; j < len(cdat); j += bytesPerPixel {
  44                         b = int(pdat[j])
  45                         pa = b - c
  46                         pb = a - c
  47                         pc = pa + pb
  48                         if pa < 0 {
  49                                 pa = -pa
  50                         }
  51                         if pb < 0 {
  52                                 pb = -pb
  53                         }
  54                         if pc < 0 {
  55                                 pc = -pc
  56                         }
  57                         if pa <= pb && pa <= pc {
  58                                 // No-op.
  59                         } else if pb <= pc {
  60                                 a = b
  61                         } else {
  62                                 a = c
  63                         }
  64                         a += int(cdat[j])
  65                         a &= 0xff
  66                         cdat[j] = uint8(a)
  67                         c = b
  68                 }
  69         }
  70 }