Find differences between images

Question

Suppose i had two nearly identical images and i wanted to locate and highlight the differences between them and produce the diff image. the routine works but this routine ask to supply color which i do not want. here is my code.

public class ImageTool { public static unsafe Bitmap GetDifferenceImage(Bitmap image1, Bitmap image2, Color matchColor) { if (image1 == null | image2 == null) return null; if (image1.Height != image2.Height || image1.Width != image2.Width) return null; Bitmap diffImage = image2.Clone() as Bitmap; int height = image1.Height; int width = image1.Width; BitmapData data1 = image1.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb); BitmapData data2 = image2.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb); BitmapData diffData = diffImage.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.WriteOnly, PixelFormat.Format24bppRgb); byte* data1Ptr = (byte*)data1.Scan0; byte* data2Ptr = (byte*)data2.Scan0; byte* diffPtr = (byte*)diffData.Scan0; byte[] swapColor = new byte[3]; swapColor[0] = matchColor.B; swapColor[1] = matchColor.G; swapColor[2] = matchColor.R; int rowPadding = data1.Stride - (image1.Width * 3); // iterate over height (rows) for (int i = 0; i < height; i++) { // iterate over width (columns) for (int j = 0; j < width; j++) { int same = 0; byte[] tmp = new byte[3]; // compare pixels and copy new values into temporary array for (int x = 0; x < 3; x++) { tmp[x] = data2Ptr[0]; if (data1Ptr[0] == data2Ptr[0]) { same++; } data1Ptr++; // advance image1 ptr data2Ptr++; // advance image2 ptr } // swap color or add new values for (int x = 0; x < 3; x++) { diffPtr[0] = (same == 3) ? swapColor[x] : tmp[x]; diffPtr++; // advance diff image ptr } } // at the end of each column, skip extra padding if (rowPadding > 0) { data1Ptr += rowPadding; data2Ptr += rowPadding; diffPtr += rowPadding; } } image1.UnlockBits(data1); image2.UnlockBits(data2); diffImage.UnlockBits(diffData); return diffImage; } }

calling like this way:

Bitmap diff = ImageTool.GetDifferenceImage(image1, image2, Color.Pink); diff.MakeTransparent(Color.Pink); diff.Save("C:\\test-diff.png",ImageFormat.Png);

some one just guide me how to change this routine as a result we do not have to pass color when i will call GetDifferenceImage() method.
this way image comparison is best technique if not then guide me how to develop a routine which can be more faster to get the diff image.
after getting the diff image how can i merge the diff image with image1. help me to develop a faster merge routine.

Too many questions but they are not very clear:) Are you going to hard code the color inside the function instead of passing it? — Egor4eg
– Egor4eg, Commented Apr 26, 2013 at 9:24
i do not want to pass color or hard code it rather change routine which will not depend on color for image comparison. am i clear? — Mou
– Mou, Commented Apr 26, 2013 at 10:01
As far as I can see the code compares 2 images and produces the resulting image where all differences are highlighted. The color to highlight is sent to the method. How method would know which color to use if you didn't pass or hardcoded the color? — Egor4eg
– Egor4eg, Commented Apr 26, 2013 at 10:08

denver · Accepted Answer · 2013-04-27 03:46:41Z

The diff image is black if two images are identical and has an increasing brightness for pixels with larger differences. You can just change the algorithm so that instead of assigning the pixel the swapcolor it assigns it the difference between the two colors.

 // iterate over height (rows) for (int i = 0; i < height; i++) { // iterate over width (columns) for (int j = 0; j < width; j++) { // for each channel for (int x=0; x<3; x++) { diffPtr[0] = Abs(data1Ptr[0]-data2Ptr[0]); data1Ptr++; // advance image1 ptr data2Ptr++; // advance image2 ptr diffPtr++; // advance diff image ptr } } // at the end of each column, skip extra padding if (rowPadding > 0) { data1Ptr += rowPadding; data2Ptr += rowPadding; diffPtr += rowPadding; } }

How you show/merge the diff will depend on what you are going to do with it.

which u want me to replace with ur code. can u plzz write the full code for my routine called GetDifferenceImage() according to your way. thanks
@Mou Just replace the for loop in your code with the one above and drop the matchColor argument and no longer needed swapColor variable.
very sorry ur code does not help me. better u run your code. just take two very close identical png picture and compare that two images with ur code and see what resulting image is coming.
@Mou I know what the result is. You need to decide how you want to show the difference and do it that way.

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