Hostility towards the program terragen has always been present, and most likely, always will be. But let's get down to brass tacks. What is terragen? It's a 3d scenery generator. Right. There's no real Modelling process involved per se, and it looks and works completely different (to all means and purposes of the majority) to a 3d modelling application such as 3d studio max or Maya. It is comprised of a series of mostly numerical controls, and a few random generators based on numerical/slider inputs.
...this means, it's an easy program, and requires little or no effort to pull off good results.
Here's my favourite word of this article. WRONG.
The program is as deep as you want it to be, just as many other art orientated programs are. The quality of the results produced from it are proportional to the artists skill in using it. Just becau