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  // 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
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