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    <p>It doesn't seem like OpenCV handles alpha the way you want it to.</p> <p>You have two options:</p> <ol> <li>Write your own cross-correlation method that will use the alpha channel</li> <li>Transform your images so your alpha channel becomes irrelevant</li> </ol> <p>Since the first option is straightforward, I will explore the second option here. I'm going to re-use the sample code I provided to <a href="https://stackoverflow.com/questions/4720168/image-in-image-algorithm/4720228#4720228">a similar question</a> earlier. If you apply cross-correlation directly to your images, the background interferes with the template matching (in particular, light background parts). If you play around with color channels, you will find that matching in the blue channel gives the correct result. This depends on the image content and isn't a consistent way to solve the problem.</p> <p>Another option is to perform edge detection (e.g. <a href="http://en.wikipedia.org/wiki/Sobel_operator" rel="nofollow noreferrer">Sobel</a>) on the image and template, and perform cross-correlation then. Here are the edge detected images (I used the Sobel edge detector on the Luma channel in GIMP, and then some intensity stretching).</p> <p><img src="https://i.stack.imgur.com/P5uDs.png" alt="map"></p> <p><img src="https://i.stack.imgur.com/IVouP.png" alt="building"></p> <p>As you can see, the alpha channel here has become irrelevant, as most of the terrain has become zero intensity and will not contribute to the cross-correlation calculation. So now cross-correlation can be directly applied, giving the desired result:</p> <pre><code>misha@misha-desktop:~/Desktop/stackoverflow$ python cross-correlation.py map-blue.png building-maskz-blue.png (163, 244) </code></pre> <p>Finally, here's <a href="https://stackoverflow.com/questions/4569993/2d-subimage-detection-in-open-cv">another related question</a>.</p> <p>PS. What game is this?</p>
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    1. COThanks, but I didn't get the same results using Sobel on grayscaled image & template (see question). The image is from is an old DOS game - Earth 2140.
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    2. COThe reason your images didn't work is because areas where there are no edges are **not black** (they are 127 neutral gray). Compare them to my images. You need the non-edges areas to be **zero** so they don't interfere with the cross-correlation calculation.
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    3. COOK, I used cvConvertAbsScale to get non-edges areas to be zero. (see question) However, still my Sobel isn't the same as yours (especially the template). Could it be the fact that I used OpenCV and you used GIMP for Sobel?
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