Hi,

you can do that with a fragment shader. You set the projection mode to orthogonal and render a quad with the texture on it. Your vertex shader may look like this:

Your fragment shader now is important:

Now you can either copy the result to the main program using glReadPixels or into a texture using glCopyTexImage2D. To get the result in a texture, it is also possible/better to use Framebuffers.

Hi,

if that's the case, I think it's easiest to load the whole picture into the main program using glReadPixels, then create an array with only the alpha data and then write this array back into the graphics memory using glDrawPixels with format parameter =GL_ALPHA. However, this needs a lot of bandwidth. Still, if you can use framebufferobjects and need the data in a texture, I suggest you to use one as it saves you another time-intensive copying procedure.

As far as I know you can either draw the texture into the framebuffer from where you can read it using glReadPixels or use a framebuffer object, which is a framebuffer linked to a texture. I posted a link to a tutorial for framebufferobjects before, but if you can't read german, I suggest you to take a look at http://www.opengl.org/registry/specs/ARB/framebuffer_object.txt The only reason against FBOs is that some older graphics cards don't support this extension. In this case, you have to render the texture to the main frame buffer, read it via glReadPixels, create the alpha data, write the alpha data back to the frame buffer and copy the contents of the framebuffer back into the texture. This procedure is definitely slower than using FBOs. As far as I know there is no other method to read and write into textures, but maybe I'm wrong.