hello i''m just starting to learn opengl and 3d so i figured i would make a simple pong game just to get used to moving objects in 3d the way i''m doing it is i set up a 2d ortho viewing projection using gluOrtho2d(-90, 90, -90, 90) set it MatrixMode to GL_MODELVIEW and draw my paddles and ball around the origin using GL_QUADS paddle 1 & 2 = -5.0f, 20.0f, -5.0f, -20.0f, 5.0f, -20.0f, 5.0, 20.0f so the width is 10 and the height should be 40? and the ball is draw with -5.0f, 5.0f, 5.0f, -5.0f, 5.0f, -5.0f 5.0, 5.0f so it should have a width and height of 10? i then translte them into there places now on to that actual question when i got to do my collsion dection at the top of the screen this happens i want to use this for the top of the screen pad.height i''m hoping is 40 buy the way i set up the model if( (pad.y + (pad.height / 2)) >= 90) pad.y = (90 - (pad.height / 2)); but i have to use this if((pad.y + 45) >= 90) pad.y = (90 - 45); now for at the bottom of the screen if((pad.y - (pad.height / 2)) <= -90) pad.y = (-90 + (pad.height / 2)); works fine so i''m wondering if anybody can tell me how come this happens? sorry if i put to much info but i think its better having a bunch of usles stuff then not enough...thx for any help