OpenGL - selective world rendering

Question

I'm building a miniature city with the basic minimum looks of a city (roads,buildings,trees etc) where u can move around. I know that rendering the whole model set in each frame doesn't work...

So can anyone give me an insight on the standard (but easiest) procedure used in selectively rendering only the visible parts of the system? I mean, just displaying only the visible stuff (with respect to the camera position) and not rendering the unseen part.. Im using VC++ and GLUT API.

Answer 1

Maybe this Wikipedia article provides a very basic introduction to the field of culling techniques.

A good starting point and one of the easiest techniques is view frustum culling. With this method you check for each object in your scene if it is inside the viewing volume (viewing frustum). This basically amounts to checking for some simplified bounding volume of the geometry (like a box or a sphere, that completely contain the geometry) if it lies inside the viewing frustum, defined by six planes.

This can further be optimized by grouping objects by their position and create a so-called bounding volume hierarchy, this way you e.g. first check if a whole city block is inside the viewing volume (by using a bounding volume that contains the whole block) and only if it is, you further check the individual houses.

A more complicated technique is occlusion culling, which means checking if an object is completely hidden behind another object. Because these techniques can get substantially more complicated it should (if done) actually be done after the view frustum culling. OpenGL has hardware occlusion queries that can aid you in determining if an object is actually visible, but they require some additional work to work well. Especially for cities there may be special two-dimensional occlusion culling techniques (long time ago I heard about that, don't know).

This is just a very broad overview, feel free to google for individual keywords. It is always a good idea to carefully weight if the additional CPU-overhead is worth it (especially with complicated occlusion culling techniques), considering that nowadays the trend is to batch as many geometry as possible into a single draw call (by the way, I hope you don't use immediate mode glBegin/glEnd, otherwise changing this to vertex arrays or better VBOs is the first point on your agenda). But view frustum culling might be a nice and easy starting point, especially if the city gets rather large.