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Reading alpha with glReadPixels

Started by andy_fish Aug 8, 2003 at 1:58 PM 2 replies 4.3k views

Original Post

Aug 8, 2003 at 1:58 PM

Is there any way to read the alpha component of the framebuffer with glReadPixels? When I try, everything I read has an alpha of 1. What I want to do is be able to clear a certain area to have an alpha of 0, then draw to that area, then use glReadPixels to copy that area to a buffer, in such a way so that the pixels that I didn''t draw to still have an alpha of 0. Is this possible? Thanks.

Corrail

Aug 8, 2003 at 2:19 PM

Do you have defined an alhpa channel in your pixel format??

--------------------------------------------------------

"If it looks good, it is good computer graphics"
"If it looks like computer graphics, it is bad computer graphics"

Corrail
corrail@gmx.at
ICQ#59184081

andy_fish

Aug 8, 2003 at 3:54 PM

Yeah both the framebuffer and the call to glReadPixels use GL_RGBA

Corrail

Aug 9, 2003 at 1:33 AM

Yes, but did you set up an alpha channel (and the color channels) in your PIXELFORMATDESCRIPTOR like this:

PIXELFORMATDESCRIPTOR pfd = {
sizeof(PIXELFORMATDESCRIPTOR),
1,
PFD_DRAW_TO_WINDOW |
PFD_SUPPORT_OPENGL |
PFD_DOUBLEBUFFER ,
PFD_TYPE_RGBA,
32, //Color Bits
8, // Bits for Red Channel
0, // ignored
8, // Bits for Green Channel
0, // ignored
8, // Bits for Blue Channel
0, // ignored
8, // Bits for Alpha Channel
0, // ignored
0,0,0,0,0,
32,
0,
0,
PFD_MAIN_PLANE,
0,
0,0,
};

But be careful! If you use this with Descriptor with 32 Bits and run you application in windowed mode on a 16 Bit Desktop you recieve a software OpenGL context!! If Desktop has 16 Bits you have to design the Descriptor for 16 Bits too (16 Bit Color Bits, 4 Bits for each channel)

--------------------------------------------------------

"If it looks good, it is good computer graphics"
"If it looks like computer graphics, it is bad computer graphics"

Corrail
corrail@gmx.at
ICQ#59184081

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