OpenGL sprites - please help this n00b

Posts: 3,591
Joined: 2003.06
Post: #16
Gillissie Wrote:The trick is to multiply each of the color values by the alpha value too, otherwise you'll still see the sprite even at 0 alpha, which seems weird to me.

It's because you're using images with a pre-multiplied alpha channel. There's nothing wrong with it. It's pretty common on iPhone because CoreGraphics pre-multipies alpha on load, and Texture2D uses CoreGraphics for loading images. Plus, as I've been recently reminded, Apple has a "PNG optimization" script that runs during build, which pre-multiplies the alpha among other things (which can be disabled BTW, but still...). If you study up on pre-multiplied alpha and glBlendFunc, you'll see why the math works out that way, and it won't seem quite so weird.
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