Normally if you paint a background image you simply draw a textured full-screen square while depth testing and writing is disabled before drawing the actual scene (where you re-enable depth)

Normally if you paint a background image you simply draw a textured full-screen square while depth testing and writing is disabled before drawing the actual scene (where you re-enable depth)

In the code you posted you draw the quad but then clear the screen again. Don't do that. Move the drawing of the quad from main to right after the glClear call.