OpenGL-ES: clearing the alpha of the FrameBufferObject

Question

This question is a follow-up to Texture artifacts on iPad

How does one "clear the alpha of the render texture frameBufferObject"? I've searched around here, StackOverflow and various search engines but no luck. I've tried a few things... for example calling GlClear(GL_COLOR_BUFFER_BIT) at the beginning of my render loop... but it doesn't seem to make a difference. Any help is appreciated since I'm still new to OpenGL.

Cheers!

p.s. I read on SO and in Apple's documentation that GlClear should always be called at the beginning of the renderLoop. Agree? Disagree? Here's where I read this: https://stackoverflow.com/questions/2538662/how-does-glclear-improve-performance

Answer 1

Try

glColorMask(FALSE, FALSE, FALSE, TRUE);//This ensures that only alpha will be effected
glClearColor(0, 0, 0, alphaValue);//alphaValue - Value to which you need to clear
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

to clear both the color buffer (with transparent black) and the depth buffer, in case you are using depth testing.

glClear must be called unless you are really sure you are redrawing all the pixels every frame, so parts of previous frames don't show over your new render.

Answer 2

Once have a look at the below link (please check the afterRender method)

http://code.google.com/p/cocos2d-iphone/issues/detail?id=631&q=glClear%28GL_COLOR_BUFFER_BIT&colspec=ID%20Type%20Status%20Priority%20Milestone%20Component%20Owner%20Summary

Here the fix was to clear the RenderTextures's Alpha channel with 1.0 so that it will not have artifact when you draw other nodes on RenderTexture.That clearing should be done once you are done drawing on the renderTexture. Please let me know if you need further clarification.

Btw, for glClear call.... If you are drawing a full screen background, there is no need of calling glClear.The game i worked on recently had a bit of performance gain when i removed glClear call.The link you posted seems interesting.Thanks for the link.