Starting by the obvious, what is a voxel ? A voxel is a short for volume element in the same way as pixel is a contraction of picture element. OK that doesn't really make sense but they just wanted to make it sound the same. There are different kind of voxels, one that we won't consider here is to see voxel as a way to represent volumetric object as three dimensional bitmaps instead of vectors (by opposition, polygonal objects are made of vectors). In this situation voxels are like Lego. Small bricks that constitute a bigger object, similar to the relation pixels have with the bigger image.

But here we'll only discuss the voxel terrain as featured in games like Outcast or Comanche. The voxel terrain engine is a 2.5D engine, it doesn't have all the levels of freedom that a regular 3D engine offers. It has the same degree of freedom as ray casting engines (popularized by Doom) and they are so close than in fact you could probably implement a voxel terrain engine as a ray caster. We won't do that, instead we'll use rasterization with a few tricks. Why is it called voxel if this isn't the same as the first definition ? Because our terrain is constituted by small 2D bricks that expand vertically. So this is a voxel, only a more restrictive definition of a voxel.

Before drawing every single terrain element you have to determine what is visible and what is not. This is both a performance and a correctness problem. If a terrain element is partially visible or totally occluded and you draw it anyway, then you have a correctness issue. If you draw terrain elements that are situated behind your camera or outside of your view then you have spent numerous cpu cycles for basically nothing.  

This is the second part of our series of articles and it follows Voxel terrain engine - Building the terrain.

Our render is now correct thanks to our visibility algorithm, but it definitely lacks polish. How can we improve the quality ? First we should have one terrain element covering one pixel or less horizontally (we don't have that fine grain for vertical resolution because it's hard to determine beforehand how it will be expanded on the screen). Then we can use supersampling and use a better sky model, I'll detail those below.

This is the third part of the Voxel terrain engine article. It follows the second part called Voxel terrain - Visibility, occlusion.

So we reached the final part of our series of tutorials and as we exposed those algorithms to you, we need to remind you one thing : we barely scratched the surface of the terrain engine.

This is the final part of the Voxel terrain engine article. It follows the third part called Voxel terrain - Beautification features.