<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>

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.