paeth.go

Documentation: image/png

		 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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