Change texture opacity in OpenGL

Basically you have two options: use glTexEnv for your texture with GL_MODULATE and specify the color using glColor4* and use a non-opaque level for the alpha channel. Note that glTexEnv should be issued only once, when you first load your texture. This scenario will not work if you specify colors in your vertex-attributes though. Those will namely override any glColor4* color you may set. In that case, you may want to resort to either of 2 options: use texture combiners (advanced topic, not nice to use in fixed-pipeline), or "manually" change the vertex color attribute of each individual vertex (can be undesirable for larger meshes).

Use a texture combiner. Set the texture stage to do a GL_MODULATE operation between a texture and constant color. Then change the constant color from your code (glTexEnv, GL_TEXTURE_ENV_COLOR).

This should come as "free" in terms of performance. On most (if not all) graphics chips combiner operations take the same number of GPU cycles (usually 1), so just using a texture versus doing a modulate operation (or any other operation) is exactly the same cost.

I have no idea about OpenGL ES, but in standard OpenGL you would set the opacity by declaring a colour for the texture before you use it:

//          R,   G,   B,   A
glColor4f(1.0, 1.0, 1.0, 0.5);

The example would give you 50% alpha without affecting the colour of your texture. By adjusting the other values you can shift the texture colour too.