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