I need a raytracer, but I have some trouble with povray. is there a free, cross platform alternative which accepts a simple subset of povray syntax ? need to run it from command line. no graphics.
MegaPOV is a version of POV-Ray with unofficial patches and improvements. It is free and runs under Snow Leopard (just tested under OS X 10.6.7).
Good luck finding anything substantial that reads POV-Ray Scene Description Language. There's nothing but POV-Ray and maybe some handy little utilities and maybe some translators out there that can do that, as far as I've seen. (But I'm often missing out on something...)
But, loosening up on that requirement, there's Art of Illusion, LuxRender, Pixie, Kerkythea, Radiance (primarily a global illumination tool, but does ray tracing too), and more... try browsing the Wikipedia article on ray-tracing.
I didn't check if all these can be run from a command line or from within a program, but I think so.
While POV-Ray's SDL hasn't caught on outside its original turf, Renderman is much more widely accepted. Since that may be outside your interest, I'll say no more.
Another thing - pure raytracing seems to have faded away, mostly. Many renderers combine different algorithms for their total effect. POV-Ray, even, has photon mapping where light is thrown forward from source to surface, as in physical reality. Just how to handle interacting lighting between surfaces (global illumination), and rays passing through transparent/translucent materials that don't originate from the camera, is an active area of research, with the current state of the art being multiple choices depending on the type of scene to render. Thanks to super-duper hardware, compilers and operating systems today, creators of 3D scenes have choices among algorithms, including some previously thought hopelessly inefficient. We live in marvelous times!
So, instead of asking for just an alternative to POV-Ray, explain what kinds of scenes you want to make, with what final visual effects. Would you like HDRI output? Programming in Python for precise complex geometry? There's almost certainly a tool for the job.
Luxrender is very nice. Syntax is different from Povray but similar. GUI & command line versions.
There are a few java ray tracers out there which are open source. this is one of them.
You can probably run them with inputs from the command line. I wrote a very simple one in java, and I have a friend who has a better one which can be easily modified to do what you need.