How can I compare two images to detect duplicates and cropped duplicates?

Question

How can I compare two images and determine if they are 100% similar, or only altered in color, or cropping?

Answer 1

Well, abstractly speaking, you need to define a similarity function, that compares two images. To determine if the images are "100% similar" (equal) you can do the following:

• compare the sizes of the images
• if the image sizes are the same simply subtract the pixels from each other
• if ( sum( abs( pixel_1_i - pixel_2_j ) ) / num_pixels < threshold ) return true

For the case that images are differently colored, or cropped

• apply an edge detector to both images
• compute the cross-correlation (in the frequency domain, FFT)
• find the highest peak
• place the (smaller) edge map in the determined position
• calculate the absolute error
• if (error < threshold) return true

BTW: This approach will not work if your images are scaled or rotated.

Further Research:

• cross-correlation: FFT (fast fourier transformation, link1, link2, FFT in C#), zero-padding (needed for the FFT if the input signals have different sizes)
• edge detection: Sobel, Canny (these are very common image processing filters, they should be available in a C# library, just like the FFT)