OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Reading and Copying Pixel Data

Once your rendering is complete, you may want to retrieve the rendered image for posterity. In that case, you can use the glReadPixels() function to read pixels from the read framebuffer and return the pixels to your application. You can return the pixels into memory allocated by the application, or into a pixel pack buffer, if one’s currently bound.

void glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height,                         GLenum format, GLenum type, void *pixels);

Reads pixel data from the read framebuffer rectangle whose lower-left corner is at (x, y) in window coordinates and whose dimensions are width and height, and then stores the data in the array pointed to by pixels. format indicates ...