问题
I'm having a problem when rendering even simple shapes with partial opacity to QGLFrameBufferObjects in Qt.
I have reduced the problem down to this:
When I render a simple quad to a QGLFrameBufferObject with color set to (1,0,0,.5), and then blit that to the screen, I get a result that is way too light a red for 50% opacity. If I draw the same quad with the same color (same code, in fact) directly to the screen, I get the correct color value. If I render the quad with opacity == 1.0, then the results are the same...I get a full, deep red in both cases. I've confirmed that the color is really wrong in the buffer by dumping the buffer to disk directly with buffer.toImage().save("/tmp/blah.tif").
In both cases, I've cleared the output buffer to (1,1,1,1) before performing the operation.
Why are things I draw that are partially transparent coming out lighter when drawn to an offscreen buffer than if I draw them right to the screen? There must be some state that I have to set on the FBO or something, but I can't figure out what it is.
回答1:
Alpha does not mean "transparent." It doesn't mean anything at all. It only takes up a meaning when you give it one. It only mean "transparent" when you set up a blend mode that uses alpha to control transparency. So if you didn't set up a blend mode that creates the effect of transparency, then alpha is just another color component that will be written exactly as is to the framebuffer.
来源：https://stackoverflow.com/questions/9913957/alpha-compositing-wrong-when-rendering-to-qglframebufferobject-vs-screen