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Source file src/image/png/paeth_test.go

Documentation: image/png

  // Copyright 2012 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package png
  
  import (
  	"bytes"
  	"math/rand"
  	"testing"
  )
  
  func slowAbs(x int) int {
  	if x < 0 {
  		return -x
  	}
  	return x
  }
  
  // slowPaeth is a slow but simple implementation of the Paeth function.
  // It is a straight port of the sample code in the PNG spec, section 9.4.
  func slowPaeth(a, b, c uint8) uint8 {
  	p := int(a) + int(b) - int(c)
  	pa := slowAbs(p - int(a))
  	pb := slowAbs(p - int(b))
  	pc := slowAbs(p - int(c))
  	if pa <= pb && pa <= pc {
  		return a
  	} else if pb <= pc {
  		return b
  	}
  	return c
  }
  
  // slowFilterPaeth is a slow but simple implementation of func filterPaeth.
  func slowFilterPaeth(cdat, pdat []byte, bytesPerPixel int) {
  	for i := 0; i < bytesPerPixel; i++ {
  		cdat[i] += paeth(0, pdat[i], 0)
  	}
  	for i := bytesPerPixel; i < len(cdat); i++ {
  		cdat[i] += paeth(cdat[i-bytesPerPixel], pdat[i], pdat[i-bytesPerPixel])
  	}
  }
  
  func TestPaeth(t *testing.T) {
  	for a := 0; a < 256; a += 15 {
  		for b := 0; b < 256; b += 15 {
  			for c := 0; c < 256; c += 15 {
  				got := paeth(uint8(a), uint8(b), uint8(c))
  				want := slowPaeth(uint8(a), uint8(b), uint8(c))
  				if got != want {
  					t.Errorf("a, b, c = %d, %d, %d: got %d, want %d", a, b, c, got, want)
  				}
  			}
  		}
  	}
  }
  
  func BenchmarkPaeth(b *testing.B) {
  	for i := 0; i < b.N; i++ {
  		paeth(uint8(i>>16), uint8(i>>8), uint8(i))
  	}
  }
  
  func TestPaethDecode(t *testing.T) {
  	pdat0 := make([]byte, 32)
  	pdat1 := make([]byte, 32)
  	pdat2 := make([]byte, 32)
  	cdat0 := make([]byte, 32)
  	cdat1 := make([]byte, 32)
  	cdat2 := make([]byte, 32)
  	r := rand.New(rand.NewSource(1))
  	for bytesPerPixel := 1; bytesPerPixel <= 8; bytesPerPixel++ {
  		for i := 0; i < 100; i++ {
  			for j := range pdat0 {
  				pdat0[j] = uint8(r.Uint32())
  				cdat0[j] = uint8(r.Uint32())
  			}
  			copy(pdat1, pdat0)
  			copy(pdat2, pdat0)
  			copy(cdat1, cdat0)
  			copy(cdat2, cdat0)
  			filterPaeth(cdat1, pdat1, bytesPerPixel)
  			slowFilterPaeth(cdat2, pdat2, bytesPerPixel)
  			if !bytes.Equal(cdat1, cdat2) {
  				t.Errorf("bytesPerPixel: %d\npdat0: % x\ncdat0: % x\ngot:   % x\nwant:  % x", bytesPerPixel, pdat0, cdat0, cdat1, cdat2)
  				break
  			}
  		}
  	}
  }
  

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