title: Change texture opacity in OpenGL
body: <p>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 <code>glBlendFunc</code>, but with no luck – it seems that I would need something like <code>GL_SRC_ALPHA_MINUS_CONSTANT</code>, which is not available. I am working on iPhone, with OpenGL ES.</p>