View Full Version : Advice on # recursions with glass scene?

07-22-2004, 05:24 AM
I'm asking this question to save time, basically...

I have a scene with some glass, reflections, etc..only about 4 objects total.

with RT reflections, refraction and transparency, I get decent render times with recursions set to 8. at 24, the render times get ridiculous; in fact, at Extreme Enhanced on a 640x480 image, the first pass wasn't finished after an hour, with 16 passes to go, so I bailed!

RT reflections are key to this scene; really makes the chrome and glass pop and look real, and makes them look like they're in the environment they're reflecting.

my question is: whether the extra recursions will truly provide a visible benefit...enough to warrant the days' worth of render time when I want to produce a printable size, say 6" x 4"?

Has anyone here gone down this road, and can you say whether the extra recursions were worth the glacial rendering times?

thank you


07-22-2004, 11:50 AM
I very rarely render anything over 4 recursions.

As you said, it jacks the render time way up, and only hypersnobs are going to notice/care that there isn't a reflection of object A in the reflection of object B on the surface of object A. Also, it only matters when reflective objects are facing each other. Furthermore, if your camera is moving much, than even those hypersnobs aren't going to notice unless they're watching it frame by frame.

I don't know why the default is 16 (or whatever it is) way higher that it needs to be.

07-22-2004, 03:07 PM
I am even stingier- using 3...

I think... out--->floor bounce--->surface bounce--->one more bounce for the hypersnobs... LOL


07-23-2004, 02:01 AM
For my architectural renderings i use between 3 and 6, works fine for everything. Just curious, are hypersnobs the new version of hypervoxel sprites ??? :D

07-23-2004, 06:02 AM
I tried doing a render of glass with recursions set at 4, and could tell right away that parts of the glass would be black; other areas of the same glass didn't have that snappy crispness that I get with 8+ recursions...

maybe it all depends on *where* the glass is in a scene and how prominent? what kind of light it's catching and whether you're intended to see through it to something else?


07-26-2004, 07:08 AM
If I may be so bold?


In 3d there is nothing cast in stone... Everything you hear and read (minus tech manuals) is just a loose guideline...


07-26-2004, 07:10 AM
that's a good notion to relate...

I'm still pretty green at this, but I think that I was fooled by the precision of some images into thinking that the creation of those images was always this exact science.

t'aint so!


07-26-2004, 08:41 AM
Originally posted by Johnny
I tried doing a render of glass with recursions set at 4, and could tell right away that parts of the glass would be black

I also got black surfaces in glasses surfaces with low recursion...maybe its just a kind of test and error...increase recursions till ure happy to the image!

07-26-2004, 05:45 PM

working out the correct number of bounces when rendering a scene is a common problem. With most scenes I tend to use 4 though I'll often experiment a bit with higher numbers if the scene needs it.

The *most* important thing though is to set up the scene so that when the number of recursions is exceeded you get something better than plain black.

You can set reflection/refraction to use raytracing + either backdrop or a spherical map. As backdrop is the most commonly used, and the default backdrop is black you get black artifacts.

If you set the backdrop colour to a fairly neutral colour from your scene the artifacts should be much less obvious.