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Editorial Team
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Asked: 2026-05-14T04:06:16+00:00

This is hopefully a simple question: I have an OpenGL texture and would like

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This is hopefully a simple question: I have an OpenGL texture and would like to be able to change its opacity, how do I do that? The texture already has an alpha channel and blending works fine, but I want to be able to decrease the opacity of the whole texture, to fade it into the background. I have fiddled with glBlendFunc, but with no luck – it seems that I would need something like GL_SRC_ALPHA_MINUS_CONSTANT, which is not available. I am working on iPhone, with OpenGL ES.

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  1. Editorial Team

    Thank You all for the ideas. I’ve played both with glColor4f and glTexEnv and at last forced myself to read the glTexEnv manpage carefully. The manpage says that in the GL_MODULATE texturing mode, the resulting color is computed by multiplying the incoming fragment by the texturing color (C=Cf×Ct), same goes for the alpha. I tried glColor4f(1, 1, 1, opacity) and that did not work, but passing the desired opacity into all four arguments of the call did the trick. (Still not sure why though.)

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