I am rendering a scene with option to use Orthographic projection as well as perspective projection.
I am able to draw full screen background image using orthographic projection, but I don’t know how to do it in perspective mode. In perspective mode the background image appears very small as I am rendering on a screen size rectangle with background texture mapped to it, near far clipping plane
An image can be generated with some elements in ortho mode and other elements in persp mode. I suggest you start in ortho mode (with Z-buffer off), draw the background image, then switch to persp or ortho to draw the rest of the scene.
I do something similar in my texture loading demo, where I draw the OpenGL logo in the upper left hand corner of the screen in ortho mode, then draw an earth and floor in perspective mode. See the routine ‘Render_Scene’ in the file ‘source.cpp’ at: