[Solved] double setAlphaChannel

[kbatalov]

Hi all,

After second call of setAlphaChannel function image contains wrong alpha channel value.

Please find below small piece of code for more understanding:

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QImage alphaMap(image.width(), image.height(), QImage::Format_Indexed8);
alphaMap.fill(100);

image.setAlphaChannel(alphaMap);

int _tmp = image.alphaChannel().pixelIndex(0,0);  // _tmp == 100 its ok

alphaMap.fill(150);

image.setAlphaChannel(alphaMap);

_tmp = image.alphaChannel().pixelIndex(0,0);  // _tmp == 59! Why?!?!?

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So my question is, what is right way to change existing alpha channel?

[danilocesar]

Since setAlphaChannel definitely changes your pixmap, the value is not wrong. Look:

Your first value is 100, which is in fact 0.3906 of 256.
After that you put a new alpha layer with the value of 150, which is 0.585 of 256.

However, you set an alpha layer to a current transparent area. So your final result will be:
0.585 * 0.3906 * 256 = ~59. Got it?

[kbatalov]

Got it! I had to guess that alpha value is accumulated.
Thanks a lot!

[danilocesar]

Remember that setAlphaChannel is deprecated in 4.6