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it's one of those silly PNG's where ALL of the information is in the alpha channel ONLY (R,G,B are all black !).
so, when you try to convert it to grayscale, the alpha will be thrown away first, and you're left with a black image.
if you need a grayscale image for findContours(), rather:
Mat gray;
extractChannel( baseImage, gray, 3 ); // get the alpha channel
it will look like this:
