How to programmatically alpha fade a textured object in OpenGL ES 1.1 (iPhone)

Question

I've been using OpenGL ES 1.1 on the iPhone for 10 months, and in that time there is one seemingly simple task I have been unable to do: programmatically fade a textured object. To keep it simple: how can I alpha fade, under code control, a simple 2D triangle that has a texture (with alpha) applied to it. I would like to fade it in/out while it is over a scene, not a simple colored background. So far the only technique I have to do this is to create a texture with multiple pre-faded copies of the texture on it. (Yuck)

As an example, I am unable to do this using Apple's GLSprite sample code as a starting point. It already textures a quad with a texture that has its own alpha. I would like to fade that object in and out.

Answer 1

Maybe I'm not getting you right, but to me it seems trivial and I've been doing it my apps successfully. The way to go is:

1. enable texturing and everything you need
2. enable blending: glEnable(GL_BLEND)
3. select a blend mode glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA)
4. set a color to blend with: glColor4f(r * a, g * a , b * a, a)
5. draw your geometry

The blend function is for porter-duff over using premultiplied colors/textures. The GL_TEXTURE_ENV_MODE must be set to GL_MODULATE, but that's the default.

Answer 2

Thanks to some help on the Apple dev forums, I got this working using a different variation thank Nikolai, also with GL_MODULATE:

glColor4f(1., 1., 1., myDesiredAlpha);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

When I tried using "GL_ONE" instead of "GL_SRC_ALPHA" it would not fade for me.

Answer 3

Nikolai's solution is correct; please ignore what I said on the Apple forums. Since the texture is pre-multiplied, the per-vertex color should be too. You should use GL_ONE rather than GL_SRC_ALPHA, and do this:

glColor4f(1., 1., 1., myDesiredAlpha);
glColor4f(myDesiredAlpha, myDesiredAlpha, myDesiredAlpha, myDesiredAlpha);

Answer 4

Hello,

Could you tell me how to do this using a vertex array? I'm sure I'm missing something simple, but I want to change the alpha on some vertices within an array on the fly and can't come up with a solution.

So for example, if I had a vertex array containing data for 10 textured quads and I wanted to change the alpha of one of those quads during runtime, how would I do it?

At the moment I can only change the alpha on the whole vertex array.

Thanks for your help.