Change texture opacity in OpenGL

Question

Hello! 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.

Answer 1

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.

Answer 2

The most straightforward way is to change the texture's alpha value on the fly. Since you tell OpenGL about the texture at some point, you will have the bitmap in memory. So just rebind the texture to the same texture id. In case you don't have it in memory, (due to space constraints, since you are on ES), you can retrieve the texture to a buffer again, using glGetTexImage(). That's the clean solution.

Saving/retrieving operations are a bit costly, though, so you might want another solution. Thinking about it, you might be able to work with geometry behind your the geometry displaying your texture or simply work on the material/colour of the geometry that holds the texture. You will probably want to have some additive blending of the back-geometry. Using a glBlendFunc of

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_DST_ALPHA),

you might be able to "easily" and - more important, cheaply - achieve the desired effect.

Answer 3

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.

Answer 4

Most likely you are using cg to get your image into a texture. When you use cg, the alpha is premultiplied, thus why you have to use the alpha for rgba of the color4f func.

Answer 5

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.)

Answer 6

I suspect that you had a black background, and thus by decreasing the amount of every color, you were effectively fading the color to black.