The scenario I'm dealing with is actually as follow: I need to get the screen generated by OpenGL and send it through HDMI to a FPGA component while keeping the alpha channel. But right now the data that is being sent through HDMI is only RGB (24bit without alpha channel) So i need a way to force sending the Alpha bits through this port somehow.
See image: http://i.imgur.com/hhlcbb9.jpg
One solution i could think of is to convert the screen buffer from RGBA mode to RGB while mixing the Alpha channels within the RGB buffer.
For example:
The original buffer: [R G B A][R G B A][R G B A]
The output i want: [R G B][A R G][B A R][G B A]
The point is not having to go through every single pixels. But I'm not sure if it's possible at all using OpenGL or any technology (VideoCore kernel?)
opengl frame buffer
Do you actually mean a framebuffer, or some kind of texture? Because framebuffers cannot be resized, and the size of this resulting image will be larger in the number of pixels by 25%. You can't actually do that.
You could do it with a texture, but only by resizing it. You would have to get the texel data with glGetTexImage into some buffer, then upload the texel data to another texture with glTexImage2D. You would simply change the pixel transfer format and texture width appropriately. The read would use GL_RGBA, and the write would use GL_RGB, with an internal format of GL_RGB8.
The performance of this will almost certainly not be very good. But it should work.
But no, there is no simple call you can make to cause this to happen.
You may be able to send the alpha channel separately in opengl via different rgb or hdmi video output on your video card. So your pc now outputs RGB down one cable, and then the Alpha down the other cable. Your alpha probably needs to be converted to grey scale so that it is still 24 bits. You then select the which signal is the key in your fpga. I'm presuming your fpga supports chroma key.
I have done something similar before but using a program called Brainstorm which uses a specific video card that supports SDI out and it splits the RGB, and the Alpha into separate video channels(cables), and then the vision mixer does the keying.
Today, however, I have created a new solution which mixes the multiple video channels first on the pc, and then outputs the final mixed video directly to either a RTMP streaming server or to a DVI to SDI scan converter.
来源：https://stackoverflow.com/questions/16144540/keep-alpha-transparency-of-a-video-through-hdmi