I would like to fade out a VBO object in OpenGL ES. The VBO is drawn using RGBA format GL_UNSIGNED_BYTE like this:
glEnableClientState(GL_COLOR_ARRAY);
glColorPointer( 4, GL_UNSIGNED_BYTE, ....
I am fairly certain I should set up blending like this:
glEnable( GL_BLEND );
glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );
glColor4f( 1, 1, 1, alpha ); // I will be changing alpha from 1 to 0
But it doesn't fade, which makes sense, since I think the src alpha is coming from my VBO which is fixed.
So I thought maybe I should just pass 3 bytes to the driver keeping my VBO colors and then alpha will come from the glColor4f command.
glColorPointer( 3, GL_UNSIGNED_BYTE, ....
But this crashes for unclear reasons (iPad development with Xcode) which I am still trying to decipher. I would think all of my glColorPointer offsets would still be fine - I still have all 4 bytes (RGBA) in MyVertexObject so I don't think it's a padding issue - I do not change any offset values in the glColorPointer command, just changed the 4 to a 3.
If I disable GL_COLOR_ARRAY the fade works perfectly but now I've lost the colors and it's using the white color I set above.
So I'm stuck as it seems I can't control the alpha channel separately from the RGB colors. Is there another way to fade a VBO with colors? Thanks.
You can specify blend factors independently of the colors in your VBOs by using GL_CONSTANT_ALPHA as the blend function:
glBlendColor(0.0f, 0.0f, 0.0f, alpha);
glBlendFunc(GL_CONSTANT_ALPHA, GL_ONE_MINUS_CONSTANT_ALPHA);
This will use the alpha component specified in glBlendColor() for the blend function.