How to calculate standard deviation on image with transparency

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honestly, try to avoid images with transparency for computer-vision. those were made, to look nice in games or on webpages, but here, you have a problem. (even, if you read it in correctly, using the IMREAD_ANY_COLOR flag)

"I want to calulate this only on the red star." -- not possible. you will always have some border pixels around it

Thanks Berak! I will try to find another way then.

it's a bit unclear, what you're trying to achieve here. even if you could avoid the border pixels, using a mask or such, -- the stddev of a monochrome surface is just 0, right ? (remember your last experiment)

so, what is it about ?

well, i want to know the mean color of the object. This is just 1 color, but I have other images which have a lot of different colors in it. For example an image that contains a rose. I segmented the rose out of the image and so I have my blob ready. Now I need stddev and mean calculations which I can use for other things. Now I know that stddev of a monochrome surface is 0. That's why I don't use a mask.

maybe you can sample a small roi inside the star, like:

Mat roi = image(Rect(200,200,10,10)); // i made up the numbers, just to show !
Scalar m,d;
meanStdDev(roi, m, d);

Yes, I was thinking the same thing. Maybe, because I can find the contour of the star and find the center of that contour and after that create a ROI around the center. I will try! Thanks for your help! :)