   Scaling: real-world dimensions to pixels
  1) I'm an OpenGL n00b (but the question isn't really OpenGL-specific anyway).2) Assume you have an object, say an F-16 fighter plane, and you know its real-world dimensions. Now you're tasked with rendering this airplane on screen at a distance of 50 meters from the camera. How do you determine the size of the airplane in your 3D world?I suppose screen resolution is one factor, but how do you decide how many pixels in your 3D world should fit in one meter in the real world?   
  First you determine your camera statistics, such as field of view and aspect ratio. Use an appropriate function to load those statistics as a matrix, such as gluPerspective(). Define the units in meters for things like clip distances. Scale your model so that the model unit scale corresponds to meters. Render the model at a point 50 units away from the camera. Let the GL pipeline figure out all the details like pixel positions. Profit.   ;  The basic formula is that the size of the object is inversely proportional to the distance in front (generally Z is used) of the camera.If you have a 45 degree view angle either side of straight ahead then a point that is 5m ahead and 5m to the left will be at the very left hand side of the screen (lets call this -1.0 for now but generally screens are mapped 0.0->1.0). 5m to the right would be at +1.0. At the same horizonatal offset and 10m distance in front of the camera these screen space numbers become -0.5 and 0.5.Easiest to draw this out from a top down view and see how the numbers work.