Hello all, I've developed a sprite engine using OpenGL, and one of the features I'd like to have is the ability to render sprites' shadows. Each sprite in my engine uses a 32-bit RGBA texture. Is there a method of rendering a textured (unlit) polygon with alpha testing (for color-key discarding of pixels) to the color buffer such that I may use only the alpha channel to define a "shadow-texture"? For example, using the stone on the left, which uses its alpha channel to define color key (e.g. the green area), whose pixels I reject using alpha testing. I'd like to somehow use that alpha channel to define a texture similar to the one on the right, such that I might blend it with the color buffer to create a sprite "shadow". I can pre-generate such textures at texture load-time, but I'd like to know if there is a method of doing this realtime? Thanks in advance,

Thanks for pointing out the blatantly obvious I was searching for a much more complex solution, without realizing the simplicity of the question!