Question

Let’s say I want to draw a simple quad like this:

glBegin(GL_QUADS); glVertex2f(-1.0,-1.0); glVertex2f(1.0,-1.0); glVertex2f(1.0, 1.0); glVertex2f(-1.0, 1.0); glEnd(); 

Is there a way to draw this such that it appears behind all 3D objects and fills up the whole screen? My camera moves with the mouse, so this quad also has to appear stationary with camera movement. The purpose is to make a simple background.

Do I have to do some crazy transformations based on eye position/rotation, or is there a simpler way using glMatrixMode()?

Answer 1

It sounds like what you want to do is similar to what you would when drawing a 2D HUD in a simple game, or just a background that is persistent. I’m no expert, but I did just that relatively recently. What you want to do is change the projection matrix to an orthographic projection, render your quad, and then switch back to whatever projection you had before. You can do this with the matrix stack, and it is completely independent of any camera.

So, first:

glMatrixMode(GL_PROJECTION); glPushMatrix(); glLoadIdentity(); int w = glutGet(GLUT_WINDOW_WIDTH); int h = glutGet(GLUT_WINDOW_HEIGHT); gluOrtho2D(0, w, h, 0); 

This pushes a new projection matrix onto the projection stack (I was using glut for this project, but it should translate to just normal OpenGL). Then, I get the window width and height, and use gluOrtho2D to set up my orthographic projection.

Next:

glMatrixMode(GL_MODELVIEW); glPushMatrix(); glLoadIdentity(); // Draw your quad here in screen coordinates 

Here I just push a new, clean matrix onto the modelview. You may not need to do this.

Lastly:

glPopMatrix() // Pops the matrix that we used to draw the quad glMatrixMode(GL_PROJECTION); glPopMatrix(); // Pops our orthographic projection matrix, which restores the old one glMatrixMode(GL_MODELVIEW); // Puts us back into GL_MODELVIEW since this is probably what you want 

I hope this helps. As you can tell, it requires no use of the eye position. This is mainly because when we use a new orthographic projection that perfectly fits our window, this is irrelevant. This may not put your quad in the back however, if this is executed after the other draw calls.

EDIT: Turning off depth testing and clearing the depth buffer could help as well, as Boojum suggests.