How to find one image inside of another?

Question

I have 2 bmp images. ImageA is a screenshot (example) ImageB is a subset of that. Say for example, an icon.

I want to find the X,Y coordinates of ImageB within ImageA (if it exists).

Any idea how I would do that?

Answer 1

Finding sub images in an image
Find an image in an Image
Check if an image exists within another image

Answer 2

1. So is there any warping of ImageB in ImageA?
2. How "exact" are the images, as in, pixel-for-pixel they will be the same?
3. How much computational power do you have for this?

If the answers to the first two questions are No and Yes, then you have a simple problem. It also helps to know the answer to Q3.

Update:

The basic idea's this: instead of matching a window around every pixel in imageB with every pixel in imageA and checking the correlation, let's identify points of interest (or features) in both images which will be trackable. So it looks like corners are really trackable since the area around it is kinda similar (not going into details) - hence, let's find some really strong corners in both images and search for corners which look most similar.

This reduces the problem of searching every pixel in B with A to searching for, say, 500 corners in B with a 1000 corners in A (or something like that) - much faster.

And the awesome thing is you have several such corner detectors at your disposal in OpenCV. If you don't feel using emguCV (C# varriant), then use the FAST detector to find matching corners and thus locate multiple features between your images. Once you have that, you can find the location of the top-left corner of the image.