Colorbuffer alpha value doesnt get cleared

[Andreas K]

Hi,

when I do

glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

glClear(GL_COLOR_BUFFER_BIT);

and I read the buffer afterwards with glReadPixels() I get 0xFF as alpha value. The value I get is:

00 00 00 FF .... for each pixel.

How can I clear the FF also?

Andreas

[Dan Bartlett]

Does your buffer have an alpha channel? If it doesn't, then the specification states that the alpha value obtained should 1.0. Alpha bits can be queried with

glGetIntegerv(GL_ALPHA_BITS, &AlphaBits); //deprecated
or by querying an FBO with

glGetFramebufferAttachmentParameteriv( GL_READ_FRAMEBUFFER,
GL_COLOR_ATTACHMENT0, GL_FRAMEBUFFER_ATTACHMENT_ALPHA_SIZE, &AlphaSize);
// or when default framebuffer is bound:
//glGetFramebufferAttachmentParameteriv( GL_READ_FRAMEBUFFER, GL_BACK_LEFT,
//GL_FRAMEBUFFER_ATTACHMENT_ALPHA_SIZE, &AlphaSize);
If you have 0 alpha bits returned, then you'd need to make sure that you create a buffer/attachment that contains an alpha channel.

Otherwise, have you changed the default color write mask anywhere? This would disable drawing to the alpha channel at all, including clearing

glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_FALSE);

[Andreas K]

Thanks a lot for your quick reply!

glGetIntegerv(GL_ALPHA_BITS, &AlphaBits); //deprecated

gives me 0. This means I do not have Alpha enabled. Also have no glColorMask.

So how do I enable Alpha for my buffer?

Thx, Andreas

[overlay]

You enable alpha with the library you use to initialize your window.

For example, if you use glut, you have to pass GLUT_ALPHA to glutInitDisplayMode().

[Andreas K]

Thanks u very much! I am using SDL and SDL_GL_SetAttribute() helped!!!

andreas