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
5package png
6
7// intSize is either 32 or 64.
8const intSize = 32 << (^uint(0) >> 63)
9
10func 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.
23func 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.
47func 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