63acf5de0980 — Sean Russell 12 years ago
Go version 1
7 files changed, 1100 insertions(+), 0 deletions(-)

A => README.txt
A => compareargs.go
A => gpl.txt
A => lpyramid.go
A => metric.go
A => perceptualdiff/main.go
A => rgbaimage.go
A => README.txt +58 -0
@@ 0,0 1,58 @@ 
+perceptualdiff - a program that compares two images using
+a perceptually based image metric.
+Copyright (C) 2006 Yangli Hector Yee
+	yeehector@users.sourceforge.net
+	http://pdiff.sourceforge.net/
+Go version Copyright (C) 2012 Sean Russell
+
+This program is free software; you can redistribute it and/or modify it
+under the terms of version 2 of the GNU General Public License as published by
+the Free Software Foundation.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+PARTICULAR PURPOSE.  See the GNU General Public License for more details in the
+file gpl.txt.
+
+Build Instructions
+1. Download cross platform make from http://www.cmake.org
+2. Download freeimage from https://sourceforge.net/projects/freeimage
+3. Edit CMakeLists.txt to tell it where to find your free image build
+4. Type cmake .
+5. Type make . (or on Windows systems cmake makes a Visual Studio
+Project file)
+6. To specify the install directory, use make install DESTDIR="/home/me/mydist"
+
+Usage
+
+perceptualdiff image1.(tif | png) image2.(tif | png) [options]
+-verbose : Turns on verbose mode
+-fov deg: field of view, deg, in degrees. Usually between 10.0 to 85.0. 
+This controls how much of the screen the oberserver is seeing. Front row of 
+a theatre has a field of view of around 25 degrees. Back row has a field of
+ view of around 60 degrees.
+-threshold p : Sets the number of pixels, p, to reject. For example if p is
+ 100, then the test fails if 100 or more pixels are perceptably different.
+-gamma g : The gamma to use to convert to RGB linear space. Default is 2.2
+-luminance l: The luminance of the display the observer is seeing. Default
+ is 100 candela per meter squared
+-colorfactor   : How much of color to use, 0.0 to 1.0, 0.0 = ignore color.
+-downsample    : How many powers of two to down sample the image.
+-output foo.ppm : Saves the difference image to foo.ppm
+
+Credits
+
+Hector Yee, project administrator and originator - hectorgon.blogspot.com
+Scott Corley, for png file IO code
+Tobias Sauerwein, for make install, package_source Cmake configuration
+Cairo Team for bugfixes
+Jim Tilander, Rewrote the IO to use FreeImage.
+
+Version History
+
+1.0   - Initial distribution
+1.0.1 - Fixed off by one convolution error and libpng interface to 1.2.8
+1.0.2 - [jt] Converted the loading and saving routines to use FreeImage
+1.1 - Added colorfactor and downsample options. Also always output
+difference file if requested. Always print out differing pixels even if the test passes.
+1.1.1 - Turn off color test in low lighting conditions.

          
A => compareargs.go +74 -0
@@ 0,0 1,74 @@ 
+package perceptualdiff
+
+import (
+	"flag"
+	"fmt"
+)
+
+/*
+Comapre Args
+Copyright (C) 2012 Sean Russell
+Based entirely off the C++ code by Yangli Hector Yee
+
+This program is free softwareyou can redistribute it and/or modify it under
+the terms of version 2 of the GNU General Public License as published by the
+Free Software Foundation.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTYwithout even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+this programif not, write to the Free Software Foundation, Inc., 59 Temple
+Place, Suite 330, Boston, MA 02111-1307 USA
+*/
+
+const copyright = `PerceptualDiff version 1.1.1, Copyright (C) 2006 Yangli Hector Yee
+PerceptualDiff comes with ABSOLUTELY NO WARRANTY
+This is free software, and you are welcome
+to redistribute it under certain conditions
+See the GPL page for details: http://www.gnu.org/copyleft/gpl.html
+
+`
+
+// Args to pass into the comparison function
+type CompareArgs struct {
+	ImgA    *RGBAImage	// Image A
+	ImgB    *RGBAImage	// Image B
+	ImgDiff *RGBAImage	// Diff image
+	Verbose bool        // Print lots of text or not
+	LuminanceOnly bool	// Only consider luminanceignore chroma channels in the comparison.
+	FieldOfView   float64		// Field of view in degrees
+	Gamma         float64		// The gamma to convert to linear color space
+	Luminance     float64		// the display's luminance
+	ThresholdPixels uint	  // How many pixels different to ignore
+  // How much color to use in the metric.
+  // 0.0 is the same as LuminanceOnly = true,
+  // 1.0 means full strength.
+  ColorFactor float64
+  // How much to down sample image before comparing, in powers of 2.
+  DownSample  int
+}
+
+
+func NewCompareArgs() *CompareArgs {
+	c := new(CompareArgs)
+
+	c.ImgA = nil
+	c.ImgB = nil
+	c.ImgDiff = nil
+	c.FieldOfView = 45.0
+	c.ThresholdPixels = 100
+	c.Gamma = 2.2
+	c.Luminance = 100.0
+	c.ColorFactor = 1.0
+
+  return c
+}
+
+func (c *CompareArgs) Print_Args() {
+	fmt.Printf("Field of view is %f degrees\n", c.FieldOfView)
+	fmt.Printf("Threshold pixels is %d pixels\n", c.ThresholdPixels)
+	fmt.Printf("The Gamma is %f\n", c.Gamma)
+	fmt.Printf("The Display's luminance is %f candela per meter squared\n", c.Luminance)
+}

          
A => gpl.txt +340 -0
@@ 0,0 1,340 @@ 
+		    GNU GENERAL PUBLIC LICENSE
+		       Version 2, June 1991
+
+ Copyright (C) 1989, 1991 Free Software Foundation, Inc.
+ 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+			    Preamble
+
+  The licenses for most software are designed to take away your
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+General Public License applies to most of the Free Software
+Foundation's software and to any other program whose authors commit to
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+the GNU Lesser General Public License instead.)  You can apply it to
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+  When we speak of free software, we are referring to freedom, not
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+    (at your option) any later version.
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+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
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+    This is free software, and you are welcome to redistribute it
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+
+  <signature of Ty Coon>, 1 April 1989
+  Ty Coon, President of Vice
+
+This General Public License does not permit incorporating your program into
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+library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.

          
A => lpyramid.go +86 -0
@@ 0,0 1,86 @@ 
+package perceptualdiff
+
+/*
+Laplacian Pyramid
+Copyright (C) 2012 Sean Russell
+Based heavily on C++ code by Yangli Hector Yee
+
+This program is free software; you can redistribute it and/or modify it under
+the terms of version 2 of the GNU General Public License as published by the
+Free Software Foundation.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY
+without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+this program; if not, write to the Free Software Foundation, Inc., 59 Temple
+Place, Suite 330, Boston, MA 02111-1307 USA
+*/
+
+const MAX_PYR_LEVELS = 8
+
+//////////////////////////////////////////////////////////////////////
+// Construction/Destruction
+//////////////////////////////////////////////////////////////////////
+
+type LPyramid struct {
+	Levels [][]float64
+	Width, Height int
+}
+
+var Kernel []float64
+
+func init() {
+	Kernel = []float64{0.05, 0.25, 0.4, 0.25, 0.05}
+}
+
+func NewLPyramid(image []float64, width, height int) *LPyramid {
+	rv := new(LPyramid)
+	rv.Levels = make([][]float64, MAX_PYR_LEVELS)
+	rv.Width, rv.Height = width, height
+	// Make the Laplacian pyramid by successively
+	// copying the earlier levels and blurring them
+	for i:=0; i<MAX_PYR_LEVELS; i++ {
+		if i == 0 {
+			rv.Levels[i] = make([]float64, width, height)
+			copy(rv.Levels[i], image)
+		} else {
+			rv.Levels[i] = make([]float64, width * height)
+			rv.convolve(rv.Levels[i], rv.Levels[i - 1])
+		}
+	}
+	return rv
+}
+
+func (l *LPyramid) convolve(a, b []float64) {
+	// convolves image b with the filter kernel and stores it in a
+	var y,x,i,j,nx,ny int
+
+	for y=0; y<l.Height; y++ {
+		for x=0; x<l.Width; x++ {
+			index := y * l.Width + x
+			a[index] = 0.0
+			for i=-2; i<=2; i++ {
+				for j=-2; j<=2; j++ {
+					nx=x+i
+					ny=y+j
+					if nx<0 { nx=-nx }
+					if ny<0 { ny=-ny }
+					if nx>=l.Width { nx=2*l.Width-nx-1 }
+					if ny>=l.Height { ny=2*l.Height-ny-1 }
+					a[index] += Kernel[i+2] * Kernel[j+2] * b[ny * l.Width + nx]
+				}
+			}
+		}
+	}
+}
+
+func (p *LPyramid) Get_Value(x, y int, level uint) float64 {
+	index := x + y * p.Width
+	l := level
+	if l > MAX_PYR_LEVELS {
+		l = MAX_PYR_LEVELS
+	}
+	return p.Levels[level][index]
+}

          
A => metric.go +325 -0
@@ 0,0 1,325 @@ 
+package perceptualdiff
+
+/*
+Metric
+Copyright (C) 2012 Sean Russell
+Based heavily on C++ code by Yangli Hector Yee
+
+This program is free software; you can redistribute it and/or modify it under
+the terms of version 2 of the GNU General Public License as published by the
+Free Software Foundation.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY
+without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+this program; if not, write to the Free Software Foundation, Inc., 59 Temple
+Place, Suite 330, Boston, MA 02111-1307 USA
+*/
+
+import (
+	. "math"
+	"fmt"
+	"image/color"
+)
+
+/*
+* Given the adaptation luminance, this function returns the
+* threshold of visibility in cd per m^2
+* TVI means Threshold vs Intensity function
+* This version comes from Ward Larson Siggraph 1997
+*/
+func tvi(adaption_luminance float64) float64 {
+      // returns the threshold luminance given the adaptation luminance
+      // units are candelas per meter squared
+
+      var r float64
+			log_a := Log10(adaption_luminance)
+
+      if log_a < -3.94 {
+            r = -2.86
+      } else if log_a < -1.44 {
+            r = Pow(0.405 * log_a + 1.6 , 2.18) - 2.86
+      } else if log_a < -0.0184 {
+            r = log_a - 0.395
+      } else if log_a < 1.9 {
+            r = Pow(0.249 * log_a + 0.65, 2.7) - 0.72
+      } else {
+            r = log_a - 1.255
+      }
+
+      return Pow(10.0 , r)
+}
+
+
+// computes the contrast sensitivity function (Barten SPIE 1989)
+// given the cycles per degree (cpd) and luminance (lum)
+func csf(cpd, lum float64) (result float64) {
+	a := 440.0 * Pow((1.0 + 0.7 / lum), -0.2)
+	b := 0.3 * Pow((1.0 + 100.0 / lum), 0.15)
+
+	result = a * cpd * Exp(-b * cpd) * Sqrt(1.0 + 0.06 * Exp(b * cpd))
+
+	return result
+}
+
+/*
+* Visual Masking Function
+* from Daly 1993
+*/
+func mask(contrast float64) (result float64) {
+	a := Pow(392.498 * contrast,  0.7)
+	b := Pow(0.0153 * a, 4.0)
+	result = Pow(1.0 + b, 0.25)
+
+	return result
+}
+
+// convert Adobe RGB (1998) with reference white D65 to XYZ
+func AdobeRGBToXYZ(r, g, b float64) (x, y, z float64) {
+	// matrix is from http://www.brucelindbloom.com/
+	x = r * 0.576700 + g * 0.185556 + b * 0.188212
+	y = r * 0.297361 + g * 0.627355 + b * 0.0752847
+	z = r * 0.0270328 + g * 0.0706879 + b * 0.991248
+	return
+}
+
+type XYZ struct {
+	x, y, z float64
+}
+func NewXYZ() XYZ {
+	return XYZ{-1,0,0}
+}
+const epsilon  = 216.0 / 24389.0
+const kappa = 24389.0 / 27.0
+
+func (xyz *XYZ) XYZToLAB(x, y, z float64) (L, A, B float64) {
+	// reference white
+	if xyz.x < 0 {
+		xyz.x, xyz.y, xyz.z = AdobeRGBToXYZ(1, 1, 1)
+	}
+	f := make([]float64, 3)
+	r := make([]float64, 3)
+	r[0] = x / xyz.x
+	r[1] = y / xyz.y
+	r[2] = z / xyz.z
+	for i := 0; i < 3; i++ {
+		if r[i] > epsilon {
+			f[i] = Pow(r[i], 1.0 / 3.0)
+		} else {
+			f[i] = (kappa * r[i] + 16.0) / 116.0
+		}
+	}
+	L = 116.0 * f[1] - 16.0
+	A = 500.0 * (f[0] - f[1])
+	B = 200.0 * (f[1] - f[2])
+	return
+}
+
+func Yee_Compare(args *CompareArgs) bool {
+	for i := 0; i < args.DownSample; i++ {
+		if args.Verbose {
+			fmt.Printf("Downsampling by %d\n", 1 << uint(i+1))
+		}
+		tmp, err := args.ImgA.DownSample()
+		if err == nil {
+			args.ImgA = tmp
+		}
+		tmp, err = args.ImgB.DownSample()
+		if err == nil {
+			args.ImgB = tmp
+		}
+	}
+
+	bounds := args.ImgA.Bounds()
+	w, h := bounds.Dx(), bounds.Dy()
+	if ((w != args.ImgB.Bounds().Dx()) || (h != args.ImgB.Bounds().Dy())) {
+		fmt.Printf("Image dimensions do not match\n")
+		return false
+	}
+
+	dim := w * h
+	identical := true
+	for ix := 0; ix < w; ix++ {
+		for iy := 0; iy < h; iy++ {
+			ca := args.ImgA.At(ix, iy).(color.RGBA)
+			cb := args.ImgB.At(ix, iy).(color.RGBA)
+			if ca.R != cb.R && ca.G != cb.G && ca.B != cb.B && ca.A != cb.A {
+				identical = false
+				break
+			}
+		}
+	}
+	if identical {
+		fmt.Printf("Images are binary identical\n")
+		return true
+	}
+
+	// assuming colorspaces are in Adobe RGB (1998) convert to XYZ
+	aX := make([]float64, dim)
+	aY := make([]float64, dim)
+	aZ := make([]float64, dim)
+	bX := make([]float64, dim)
+	bY := make([]float64, dim)
+	bZ := make([]float64, dim)
+	aLum := make([]float64, dim)
+	bLum := make([]float64, dim)
+
+	aA := make([]float64, dim)
+	bA := make([]float64, dim)
+	aB := make([]float64, dim)
+	bB := make([]float64, dim)
+
+	if args.Verbose {
+		fmt.Printf("Converting RGB to XYZ\n")
+	}
+
+	var x, y int
+	xyz := NewXYZ()
+	for y = 0; y < h; y++ {
+		for x = 0; x < w; x++ {
+			ra, ga, ba, _ := args.ImgA.At(x, y).RGBA()
+			// l is never actually used, AFAICT.  Bug in original algo?
+			var r, g, b float64
+			i := x + y * w
+			r = Pow(float64(ra) / 255.0, args.Gamma)
+			g = Pow(float64(ga) / 255.0, args.Gamma)
+			b = Pow(float64(ba) / 255.0, args.Gamma)
+			aX[i], aY[i], aZ[i] = AdobeRGBToXYZ(r,g,b)
+			_, aA[i], aB[i] = xyz.XYZToLAB(aX[i], aY[i], aZ[i])
+			rb, gb, bb, _ := args.ImgA.At(x, y).RGBA()
+			r = Pow(float64(rb) / 255.0, args.Gamma)
+			g = Pow(float64(gb) / 255.0, args.Gamma)
+			b = Pow(float64(bb) / 255.0, args.Gamma)
+			bX[i],bY[i],bZ[i] = AdobeRGBToXYZ(r,g,b)
+			_, bA[i], bB[i] = xyz.XYZToLAB(bX[i], bY[i], bZ[i])
+			aLum[i] = aY[i] * args.Luminance
+			bLum[i] = bY[i] * args.Luminance
+		}
+	}
+
+	if args.Verbose {
+		fmt.Printf("Constructing Laplacian Pyramids\n")
+	}
+
+	la := LPyramid{[][]float64{aLum}, w, h}
+	lb := LPyramid{[][]float64{bLum}, w, h}
+
+	num_one_degree_pixels := (2.0 * Tan(args.FieldOfView * 0.5 * Pi / 180.0) * 180.0 / Pi)
+	pixels_per_degree := float64(w) / num_one_degree_pixels
+
+	if (args.Verbose) {
+		fmt.Printf("Performing test\n")
+	}
+
+	num_pixels := 1.0
+	var adaptation_level uint
+	for i := uint(0); i < MAX_PYR_LEVELS; i++ {
+		adaptation_level = i
+		if num_pixels > num_one_degree_pixels {
+			break
+		}
+		num_pixels *= 2
+	}
+
+	cpd := make([]float64, MAX_PYR_LEVELS)
+	cpd[0] = 0.5 * pixels_per_degree
+	for i := 1; i < MAX_PYR_LEVELS; i++ {
+		cpd[i] = 0.5 * cpd[i - 1]
+	}
+	csf_max := csf(3.248, 100.0)
+
+	F_freq := make([]float64, MAX_PYR_LEVELS - 2)
+	for i := 0; i < MAX_PYR_LEVELS - 2; i++ {
+		F_freq[i] = csf_max / csf(cpd[i], 100.0)
+	}
+
+	var pixels_failed uint
+	for y = 0; y < h; y++ {
+	  for x = 0; x < w; x++ {
+		  index := x + y * w
+			contrast := make([]float64, MAX_PYR_LEVELS - 2)
+		  var sum_contrast float64
+			for i := uint(0); i < MAX_PYR_LEVELS - 2; i++ {
+				n1 := Abs(la.Get_Value(x,y,i) - la.Get_Value(x,y,i + 1))
+				n2 := Abs(lb.Get_Value(x,y,i) - lb.Get_Value(x,y,i + 1))
+				var numerator float64
+				if n1 > n2 { numerator = n1 } else { numerator = n2 }
+				d1 := Abs(la.Get_Value(x,y,i+2))
+				d2 := Abs(lb.Get_Value(x,y,i+2))
+				denominator := d2
+				if d1 > d2 { denominator = d1 }
+			  if denominator < 1e-5 { denominator = 1e-5 }
+			  contrast[i] = numerator / denominator
+			  sum_contrast += contrast[i]
+		  }
+		  if sum_contrast < 1e-5 { sum_contrast = 1e-5 }
+			F_mask := make([]float64, MAX_PYR_LEVELS - 2)
+			adapt := la.Get_Value(x,y,adaptation_level) + lb.Get_Value(x,y,adaptation_level)
+		  adapt *= 0.5
+		  if adapt < 1e-5 { adapt = 1e-5 }
+			for i := 0; i < MAX_PYR_LEVELS - 2; i++ {
+			  F_mask[i] = mask(contrast[i] * csf(cpd[i], adapt))
+		  }
+		  var factor float64
+			for i := 0; i < MAX_PYR_LEVELS - 2; i++ {
+			  factor += contrast[i] * F_freq[i] * F_mask[i] / sum_contrast
+		  }
+		  if factor < 1 { factor = 1 }
+		  if factor > 10 { factor = 10 }
+			delta := Abs(la.Get_Value(x,y,0) - lb.Get_Value(x,y,0))
+			pass := true
+		  // pure luminance test
+		  if delta > factor * tvi(adapt) {
+			  pass = false
+		  } else if !args.LuminanceOnly {
+			  // CIE delta E test with modifications
+				color_scale := args.ColorFactor
+			  // ramp down the color test in scotopic regions
+			  if adapt < 10.0 {
+				  // Don't do color test at all.
+				  color_scale = 0.0
+			  }
+				da := aA[index] - bA[index]
+			  db := aB[index] - bB[index]
+			  da = da * da
+			  db = db * db
+				delta_e := (da + db) * color_scale
+			  if delta_e > factor {
+				  pass = false
+			  }
+		  }
+		  if !pass {
+			  pixels_failed++
+			  if args.ImgDiff != nil {
+				  args.ImgDiff.Set(x, y, color.RGBA{255, 0, 0, 255})
+			  }
+		  } else {
+			  if args.ImgDiff != nil {
+				  args.ImgDiff.Set(x, y, color.RGBA{0, 0, 0, 255})
+			  }
+		  }
+	  }
+  }
+
+	different := fmt.Sprintf("%d pixels are different\n", pixels_failed)
+
+  // Always output image difference if requested.
+  if args.ImgDiff != nil {
+	  if args.ImgDiff.WriteToFile(args.ImgDiff.Name) {
+		  fmt.Printf("Wrote difference image to %s\n", args.ImgDiff.Name)
+	  } else {
+		  fmt.Printf("Could not write difference image to %s\n", args.ImgDiff.Name)
+	  }
+  }
+
+  if pixels_failed < args.ThresholdPixels {
+	  fmt.Printf("Images are perceptually indistinguishable\n%s\n", different)
+	  return true
+  }
+
+  fmt.Printf("Images are visibly different\n%s\n", different)
+
+  return false
+}

          
A => perceptualdiff/main.go +88 -0
@@ 0,0 1,88 @@ 
+package main
+
+/*
+PerceptualDiff - a program that compares two images using a perceptual metric
+based on the paper :
+	A perceptual metric for production testing. 
+	Journal of graphics tools, 9(4):33-40, 2004, Hector Yee
+Copyright (C) 2012 Sean Russell
+Based heavily on C++ code by Yangli Hector Yee
+
+This program is free software; you can redistribute it and/or modify it under
+the terms of version 2 of the GNU General Public License as published by the
+Free Software Foundation.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+this program; if not, write to the Free Software Foundation, Inc., 59 Temple
+Place, Suite 330, Boston, MA 02111-1307 USA
+*/
+
+import (
+	"perceptualdiff"
+	"os"
+)
+
+func main() {
+	args := perceptualdiff.NewCompareArgs()
+
+	if !parse_Args(args) {
+		os.Exit(-1)
+	}
+
+	passed := perceptualdiff.Yee_Compare(args)
+
+	if passed {
+		os.Exit(0)
+	} else {
+		os.Exit(1)
+	}
+}
+
+
+func parse_Args(c *CompareArgs) bool {
+	flag.BoolVar(&c.Verbose, "verbose", false, "Turns on verbose mode")
+	flag.Float64Var(&c.FieldOfView, "fov", 45.0, "Field of view in degrees (0.1 to 89.9)")
+	flag.UintVar(&c.ThresholdPixels, "threshold", 100, "#pixels p below which differences are ignored")
+	flag.Float64Var(&c.Gamma, "gamma", 2.2, "Value to convert rgb into linear space")
+	flag.Float64Var(&c.Luminance, "luminance", 100.0, "White luminance")
+	flag.BoolVar(&c.LuminanceOnly, "luminanceonly", false, "Only consider luminanceignore chroma (color) in the comparison")
+	flag.Float64Var(&c.ColorFactor, "colorfactor", 1.0, "How much of color to use, 0.0 to 1.0, 0.0 = ignore color.")
+	flag.IntVar(&c.DownSample, "downsample", 0, "How many powers of two to down sample the image.")
+	output_file_name := flag.String("output", "", "Write difference to a file")
+	flag.Usage = func() {
+		fmt.Printf(copyright)
+		fmt.Printf("PeceptualDiff <image1> <image2>\n\n")
+		fmt.Printf("Compares image1 and image2 using a perceptually based image metric\n")
+		flag.PrintDefaults()
+		fmt.Printf("Note: Input or Output files can also be in the PNG or JPG format ")
+		fmt.Printf("or any format\n")
+		fmt.Printf("that FreeImage supports.\n")
+	}
+	flag.Parse()
+
+	if flag.NArg() < 2 {
+		flag.Usage()
+		return false
+	}
+
+	var err error
+	c.ImgA, err = perceptualdiff.ReadFromFile(flag.Arg(0))
+	if err != nil {
+		fmt.Printf("FAIL: Cannot open %s\n", flag.Arg(0))
+		return false
+	}
+	c.ImgB, err = perceptualdiff.ReadFromFile(flag.Arg(1))
+	if err != nil {
+		fmt.Printf("FAIL: Cannot open %s\n", flag.Arg(1))
+		return false
+	}
+
+	if *output_file_name != "" {
+		c.ImgDiff = NewRGBAImage(c.ImgA.Bounds().Dx(), c.ImgA.Bounds().Dy(), *output_file_name)
+	}
+	return true
+}

          
A => rgbaimage.go +129 -0
@@ 0,0 1,129 @@ 
+package perceptualdiff
+
+import (
+	"fmt"
+	"image"
+	"image/color"
+	"image/draw"
+	"image/jpeg"
+	"image/png"
+	"os"
+	"path"
+	_ "image/gif"
+)
+
+/*
+rgbaimage.go
+Copyright (C) 2012 Sean E. Russell
+Based heavily on C++ code by Yangli Hector Yee, which was rewritten by Jim Tilander
+
+This program is free software; you can redistribute it and/or modify it under
+the terms of version 2 of the GNU General Public License as published by the
+Free Software Foundation.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+this program; if not, write to the Free Software Foundation, Inc., 59 Temple
+Place, Suite 330, Boston, MA 02111-1307 USA 
+*/
+
+/** Class encapsulating an image containing R,G,B,A channels.
+ *
+ * Internal representation assumes data is in the ABGR format, with the RGB
+ * color channels premultiplied by the alpha value.  Premultiplied alpha is
+ * often also called "associated alpha" - see the tiff 6 specification for some
+ * discussion - http://partners.adobe.com/asn/developer/PDFS/TN/TIFF6.pdf
+ *
+ */
+
+type RGBAImage struct {
+	*image.RGBA
+	Name string
+}
+
+func NewRGBAImage(w, h int, name string) *RGBAImage {
+	rv := new(RGBAImage)
+	rv.RGBA = image.NewRGBA(image.Rect(0, 0, w, h))
+	if name != "" {
+		rv.Name = name
+	}
+	return rv
+}
+
+type ImgError struct {
+	mesg string
+}
+func (i ImgError) Error() string {
+	return i.mesg
+}
+
+
+func (r *RGBAImage) DownSample() (*RGBAImage, error) {
+	b := r.Bounds()
+	if b.Dx() <= 1 || b.Dy() <= 1 {
+		return nil, ImgError{"bad boundaries"}
+	}
+	nw := b.Dx() / 2
+	nh := b.Dy() / 2
+	img := NewRGBAImage(nw, nh, r.Name)
+	for y := 0; y < nh; y++ {
+		for x := 0; x < nw; x++ {
+			d := make([]color.RGBA, 4)
+			// Sample a 2x2 patch from the parent image.
+			d[0] = r.At(2*x+0, 2*y+0).(color.RGBA)
+			d[1] = r.At(2*x+1, 2*y+0).(color.RGBA)
+			d[2] = r.At(2*x+0, 2*y+1).(color.RGBA)
+			d[3] = r.At(2*x+1, 2*y+1).(color.RGBA)
+			// Find the average color.
+			ra := (d[0].R + d[1].R + d[2].R + d[3].R) / 4
+			rg := (d[0].G + d[1].G + d[2].G + d[3].G) / 4
+			rb := (d[0].B + d[1].B + d[2].B + d[3].B) / 4
+			img.Set(x, y, color.RGBA{ra, rg, rb, 255})
+		}
+	}
+	return img, nil
+}
+
+func (r *RGBAImage) WriteToFile(filename string) bool {
+	fout, err := os.Create(filename)
+	if err != nil {
+		fmt.Print(err)
+		return false
+	}
+	defer fout.Close()
+	switch path.Ext(filename) {
+	default:
+		fmt.Printf("Can't save to unknown filetype %s\n", filename)
+		fmt.Printf("Supported extensions are png, jpg, and jpeg\n")
+		return false
+	case "png", "PNG":
+		err = png.Encode(fout, r)
+	case "jpg", "jpeg", "JPG", "JPEG":
+		err = jpeg.Encode(fout, r, &jpeg.Options{100})
+	}
+	if err != nil {
+		fmt.Print(err)
+		return false
+	}
+	return true
+}
+
+func ReadFromFile(filename string) (*RGBAImage, error) {
+	fin, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer fin.Close()
+
+	fimg, _, err := image.Decode(fin)
+	if err != nil {
+		return nil, err
+	}
+	b := fimg.Bounds()
+	rgba := image.NewRGBA(image.Rect(0, 0, b.Dx(), b.Dy()))
+	draw.Draw(rgba, rgba.Bounds(), fimg, b.Min, draw.Src)
+	return &RGBAImage{rgba, filename}, nil
+}