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it's one of those png's where a silly webdev put ALL of the information into the alpha channel ONLY. the bgr channels are all black, indeed.

what you have to do is:

• read the image with alpha channel
• extract alpha (and throw anything else away...)

Mat m = imread("badalpha.png", IMREAD_UNCHANGED);
Mat bw;
cout << m.type() << " " << m.channels() << endl;
extractChannel(m, bw, 3);
imshow("good", bw);
waitKey();

but, just saying, if you have images like that in your dataset, you have a problem ...