I would like to set the background with gradient instead of a single color. Is it possible ?
Draw a colored quad over the entire screen. There is no build in “clear to gradient” function.
How i can draw a quad with screen coords?
You need to draw it before you do the transformation that affects the rest of the scene (so right after you load the identity matrix). Then, the screen space goes from (-1, -1) to (1, 1),