How compare function marks differences on resulting picture?

[yaci]

I am trying to write a function in Java that would perform basic image comparison and produce resulting image similar to the one produced by compare.exe. I want my function to be simple, covering the functionality of "compare.exe 1.png 2.png result.png". So nothing complicated. Actually I have the code already but I cannot figure out how the color of red pixels (difference marks) is calculated. I tried going through IM source code but I couldn't understand it. :/ Here's what I've got so far:

Code: Select all

	public void compare(String path1, String path2, String resultPath) throws Exception
	{
		File file1 = new File(path1);
		File file2 = new File(path2);
		File file3 = new File(resultPath);		
		BufferedImage image1 = ImageIO.read(file1);
		BufferedImage image2 = ImageIO.read(file2);
		
		int columns = image1.getWidth();
		int rows = image1.getHeight();
		
		BufferedImage image3 = new BufferedImage(columns, rows, BufferedImage.TYPE_INT_RGB);
			       
	    int rgb; int rgb2; int newrgb; int r; int g; int b;
	    	    
		for (int row=0; row<rows; row++)
		{
		    for (int col=0; col<columns; col++)
		    {
		       rgb = image1.getRGB(col, row);
		       rgb2= image2.getRGB(col, row);
		       		       
		       r =  (rgb >> 16) & 0x000000FF; 
		       g =  (rgb >> 8) & 0x000000FF;
		       b =  rgb & 0x000000FF;
		       
		       if (rgb == rgb2)
		       {		    	   		    	   		    	   
			       r += (int)((255 - r) * 0.8);
			       g += (int)((255 - g) * 0.8);
			       b += (int)((255 - b) * 0.8);
			       
		    	   newrgb = (r << 16) | (g << 8) | b;	
		       }
		       else
		       {
		    	   newrgb = 0x00FF0000 | rgb;	//THE PROBLEM IS HERE. HOW SHOULD I CALCULATE IT?
		       }
		       
		       image3.setRGB(col, row, newrgb);				
			}		       		    
		}		
		ImageIO.write(image3, "png", file3);
	}

Can anyone point me how to calculate newrgb value when a difference is found? I would like to achieve similar (doesn't have to be identical) to original ImageMagick's compare.exe. If possible I would like to avoid RGB->HSL->RGB conversions.

Also I would appreciate any comments on how to optimize the above code. It's already faster than original compare but maybe there is some room for additional tweaks.

[fmw42]

compare allow any of several formulae for the comparison statistic. see http://www.imagemagick.org/Usage/compare/#statistics you can look up any or all from a google search for the formula or from Wikipedia. My personal favorite is rmse, which I believe is

rmse = sqrt (( rdiff^2 + gdiff^2 +bdiff^2 )/3))

[anthony]

Essentially the compare function takes the first image and tints it white or red depending on the -fuzz value of the resulting difference. however there is also some modification options... -highlight-color and -lowlight-color (options only used by compare at this time).

See IM Examples, Compare, Compare Program
http://www.imagemagick.org/Usage/compare/#difference