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Thread: copying the alpha channel

  1. #1
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    Default copying the alpha channel

    What is the fastest way to copy the alpha values of pixels between 2 images of the same dimensions? That is:

    Qt Code:
    1. copyAlphaChannel(image1, image2);
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    where image1.size() == image2.size().

  2. #2
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    Default Re: copying the alpha channel

    It heavily depends on the image format. In a general case unfortunately you have to iterate over pixels, read their values, adjust the alpha of the other picture and store the modified value. Or you can probably use composition modes available for QPainter to draw one image on top of the other however I'm quite certain that operation is going to be slower than the former one.
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  3. #3
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    Default Re: copying the alpha channel

    The docs seem to hint that composition modes are the preferred way to do it. Can you supply a sequence of composition modes I could apply to achieve the copy?

  4. #4
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    Default Re: copying the alpha channel

    Never mind, I've helped myself with CompositionMode_SourceIn, seems to work ok and fast.

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