View Full Version : GI animation render time increasing

04 April 2006, 10:24 AM
Hi, I'm rendering out an architectural flythrough of an apartment. I'm using a generated sky as a GI light source and just using the light coming in through the windows with a couple of area lights to boost a few places. Things look great on my still renders.

BUT, when I get to rendering out camera moves, things get tricky. For some reason with each frame, the render time seems to be going up. For instance frame 1 is taking 2 minutes, frame 30 is at 5 mins and frame 100 is at 12 minutes. If I render frame 100 only, it takes roughly 4 minutes. I have my GI settings set for camera animation, my recompute set to first time and save solution turned on. I was wondering if this was simply bad ram management, and do I need to break the renders down a bit as I go?



C4D 9.5, AR 2.5, Mac OSX 10.4 , G5 Dual 2.7GHZ

04 April 2006, 12:03 PM
This has been discussed in this thread


04 April 2006, 03:46 PM
unfortunately it's a bug (some peeps prefer not to call it this) that occures within cinema.

as you say, when rendering out a gi animation the frame render times drastically increase.
there's not much solution to this unfortunately unless you use other render methods like AO or baking.

1 tip though is to completely optomise or completely delete all your phong tags if you can afford to do this. this will sometimes eliminate the problem.

Ernest Burden
04 April 2006, 08:44 PM
unfortunately it's a bug (some peeps prefer not to call it this) that occures within cinema.

Maxon officially calls it a "limitation".

STRAT, I haven't had a chance to test your idea/observation about the phong tag work helping. I am pretty careful with them already, so with a complex scene I doubt its the answer. But I should look at the question more closely.

The logic is that the phong tag creates a lot more samples than needed. That makes perfect sense. However, once you have those samples, you've got them. The rendertime should not continue to get longer as it goes along. There is undoubtedly an accumulation problem. fr2 doesn't seem to have it.

04 April 2006, 08:47 PM
Thanks Strat, I just spent an hour reading your postings about this from last summer/autumn. That looked like a very frustrating experience!

I'm anxious to try baking my textures, but I'm not really clear on where to start. Could you (or anyone) point me to a tut? Not just a baking one, I've done those off the 9.5 cd. I think I really need one that looks at baking GI.

And thanks for pointing me to that thread, Srek. It took a while to find, but it was spot on.


04 April 2006, 11:39 PM
The latest 3d fluff disk covers GI baking in depth.

Ernest Burden
04 April 2006, 02:23 AM
The short answer is: Bake the big stuff.

Baking little fiddly objects will not do much for you but waste lots of time. You would have to bake everything in your model to be able to make good use of the technique, and you have to set up the object hierarchy differently to do that. It can be done, but you front-load your time. However, you would be able to render without GI, or even lights. That's fast! But you have to lock down everything first, no changing lights, etc.

The compromise, and its posted in one of the threads on this subject from last summer, is to bake big stuff and then have those surfaces only contribute GI from their material's lum. channels. You have to do that since if you don't they will not contribute lighting back into the scene. As a tradeoff it works very well. You get faster renders and still have full raytrace effects.

04 April 2006, 09:53 PM
OK, I've got the 3D Fluff DVD - good suggestion Loop Corp - and I've been trying to bake my GI. Something's coming up which I think might be unique to GI baking. I've got a model of an apartment. Windows are on all sides. The floor and ceiling are both planes, everything else is built from an extruded cube. All objects are editable. I've connected several of the walls and the ceiling into a single object to speed up the baking process. My Lightsource for the GI pass is from the AR sky, set to the appropriate time and place. I've experimented with adding ambient light (omni) and area lights to boost the interior brightness.

Anyway, the problem is that the ceiling is always baking as completely white. As well, I seem to be getting some bleed through onto the walls below. It's as though the light is leaking in around the edges of the ceiling. Funnily enough, when I used an onmi light to flood into a skylight it too bled into the walls below.

At first I thought this must have something to do with UV's - as in they weren't setup properly on the object after I connected the various objects together. This may be a problem, but I don't think it explains the bleed through. I'm now wondering if it's got anything to do with planes. Because they have no width, is the pixel blending or supersampling just not seeing them. I'll experiment with placing some other geometry there. But in the meantime, can anyone suggest anything? I'm going a little crazy watching this thing bake over and over!



04 April 2006, 11:10 AM
i often employ a hybrid technique -

i allways use a diffuse depth of 1 to save on render times. this means you MUST add infill lights to compensate for lack of bounced light. these are normally attenuated low light emmitting omnis. not only good for gi, but good for radiosity too.

i might add a shed load of infills with a gi accuracy of 5-10%. this 5-10% accuracy is enough to give my scene that gi atmosphere, and the infills do the rest. but to get the same effect of using minimal lights with a high accuracy you must practice with your light placing. an excellent experience and understanding of gi rendering can make this method as good as any rendering you can ever acheive. good for anims too.

another tip - to help compensate for lack of bounced lighting, try adding a single omni light with specular and diffuse properties disabled and only casting an ambient light, set at maybe 10%. works wonders if performed correctly.

04 April 2006, 01:21 PM
Thanks Strat. For the time being I'm focusing on baking. I realised the problem I mentioned earlier was due to planes. A poly can only have one UV poly, so the baker was rendering the side of the plane facing the sky, not the one inside the apartment.

I seem to still have an issue though. GI light seems to be leaking in at the corners when I bake the texture. So when I look at my baked GI map, I've got little splotches of super bright light on the edges of the UV polys that are near the outside of the building.

I've read that low poly meshes can have problems with baking, but subdividing doesn't seem to solve the issue.

Any thoughts?

btw - Strat, I really like your website gallery. That's some gorgeous stuff.


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