Comment # 13
on bug 92309
from Kamil Páral
acutiator, thanks in advanced for the effort. I have zero knowledge about all of this, but this wikipedia article seems to be comprehensible: https://en.wikipedia.org/wiki/Alpha_compositing The important part: ~~~~~~~ If an alpha channel is used in an image, it is common to also multiply the color by the alpha value, to save on additional multiplications during compositing. This is usually referred to as premultiplied alpha. Assuming that the pixel color is expressed using straight (non-premultiplied) RGBA tuples, a pixel value of (0.0, 0.5, 0.0, 0.5) implies a pixel that has 50% of the maximum green intensity and 50% opacity. If the color were fully green, its RGBA would be (0, 1, 0, 0.5). However, if this pixel uses premultiplied alpha, all of the RGB values (0, 1, 0) are multiplied by 0.5 and then the alpha is appended to the end to yield (0, 0.5, 0, 0.5). In this case, the 0.5 value for the G channel actually indicates 100% green intensity (with 50% opacity). For this reason, knowing whether a file uses premultiplied or straight alpha is essential to correctly process or composite it. ~~~~~~~ I've also found the following howtos when searching for this: http://www.gamedev.net/topic/623481-how-to-create-pre-multiplied-alpha-for-use-with-opengl/ http://blog.qythyx.com/2014/01/premultiplied-alpha.html Hope that helps a bit. You might wait a while whether someone more knowledgeable gives you a better advice.
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