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I have a simple one channel (8bit) bitmap with luminance data only, and I want to blend it with the existing framebufer like Screen blending mode does it in Photoshop.
So the source’s white pixels (255) should result white, source’s 50% gray pixels (128) should result the framebuffer pixel enlighted by 50%, and source’s black pixels should leave the result alone. Do I have to set glColor4f as well?
Can some glBlendFunc expert of you help me here?
Screen blending is C = S + (1 – S) × D, so what you want is
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_COLOR). If you ever introduce an alpha channel, you should still be able to get correct results if you keep your image data in premultiplied format.