PMG

Reading from the Screen
There may be times when you want to read the pixels already on the screen so that you
can save them to disk as an image file or can manipulate them in memory (i.e., for special
effects). OpenGL allows you to do this by providing you with the glReadPixels() function,
which is defined as:
void glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format,
GLenum type, GLvoid *pixels);
glReadPixels() has essentially the same parameters as glDrawPixels() with the addition of an
(x, y) coordinate. The (x, y) coordinate specifies the lower-left corner of the rectangle with
dimensions defined by width and height that will be read from the screen and stored in the
pixels parameter. The format and type parameters work the same way as glDrawPixels() and
can be the same values as those defined in Tables 6.1 and 6.2.
As an example, if you want to read the top half of your OpenGL window into an RGB
buffer, you might use the glReadPixels() function like this:
void *imageData;
int screenWidth, screenHeight;
...
glReadPixels(0, screenHeight/2, screenWidth, screenHeight/2, GL_RGB, GL_UNSIGNED_BYTE,
imageData);
Note
Since glReadPixels() is reading from the frame buffer and hence across the AGP bus of the video
card, the execution time of the function can be relatively long. Try not to use glReadPixels() often,
if at all, during runtime.