github.com/pdfcpu/pdfcpu@v0.11.1/pkg/filter/paeth.go (about)

     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  // The code to compute Paeth is borrowed from image/png in the stdlib  because it is internal over there to the png reader.
     6  // The PNG Paeth filter is documented here: www.w3.org/TR/PNG-Filters.html
     7  
     8  package filter
     9  
    10  // intSize is either 32 or 64.
    11  // Disabled intSize 64 for govet.
    12  const intSize = 32 //<< (^uint(0) >> 63)
    13  
    14  func abs(x int) int {
    15  	// m := -1 if x < 0. m := 0 otherwise.
    16  	m := x >> (intSize - 1)
    17  
    18  	// In two's complement representation, the negative number
    19  	// of any number (except the smallest one) can be computed
    20  	// by flipping all the bits and add 1. This is faster than
    21  	// code with a branch.
    22  	// See Hacker's Delight, section 2-4.
    23  	return (x ^ m) - m
    24  }
    25  
    26  // paeth implements the Paeth filter function, as per the PNG specification.
    27  func paeth(a, b, c uint8) uint8 {
    28  	// This is an optimized version of the sample code in the PNG spec.
    29  	// For example, the sample code starts with:
    30  	//	p := int(a) + int(b) - int(c)
    31  	//	pa := abs(p - int(a))
    32  	// but the optimized form uses fewer arithmetic operations:
    33  	//	pa := int(b) - int(c)
    34  	//	pa = abs(pa)
    35  	pc := int(c)
    36  	pa := int(b) - pc
    37  	pb := int(a) - pc
    38  	pc = abs(pa + pb)
    39  	pa = abs(pa)
    40  	pb = abs(pb)
    41  	if pa <= pb && pa <= pc {
    42  		return a
    43  	} else if pb <= pc {
    44  		return b
    45  	}
    46  	return c
    47  }
    48  
    49  // filterPaeth applies the Paeth filter to the cdat slice.
    50  // cdat is the current row's data, pdat is the previous row's data.
    51  func filterPaeth(cdat, pdat []byte, bytesPerPixel int) {
    52  	var a, b, c, pa, pb, pc int
    53  	for i := 0; i < bytesPerPixel; i++ {
    54  		a, c = 0, 0
    55  		for j := i; j < len(cdat); j += bytesPerPixel {
    56  			b = int(pdat[j])
    57  			pa = b - c
    58  			pb = a - c
    59  			pc = abs(pa + pb)
    60  			pa = abs(pa)
    61  			pb = abs(pb)
    62  			if pa <= pb && pa <= pc {
    63  				// No-op.
    64  			} else if pb <= pc {
    65  				a = b
    66  			} else {
    67  				a = c
    68  			}
    69  			a += int(cdat[j])
    70  			a &= 0xff
    71  			cdat[j] = uint8(a)
    72  			c = b
    73  		}
    74  	}
    75  }