View Full Version : CG lighting techniques used in compositing CG to live action

12-21-04, 05:05 AM
Hi there! This is my first time posting here but I've been visiting this site for a long time now. I find the content of this site very heplful and informative. Anyway, enough of these. I've got a few questions to ask and anyone is free to answer. Right now, we're in the midst of finishing a 3d animated film. A bunch of us here, being kinda bored right now at work have decided to do a some sort of spoof of our film. We're gonna use some of the film's characters and put them in a live action setting like our workplace and make them do some skits for our amusement. This will also serve as an rnd for all of us. Now on to the questions! Basically, I just want to ask what are the 3D lighting techniques that are being used in the vfx industry when compositing CG characters to live action bg plate. I'm already familiar with HDRI and have done some experiments with. But I also want to learn the tricks that the vfx guys have been using through the years. Like, I've seen guys from ILM shoot a grey sphere whenever a shot will require CG elements. Anybody has any idea what this is for and how this is used for light the CG scenes. From what I understand, this kinda help them determine the diffuse lighting in the live action set. Is this true? Im also thinking of shooting an object with a known color, kind of like those white cards used for white balancing, and using this to determine the color of the lights in the live action set. Anybody here done this before? And also where in the web can I find info on this? Any ideas or suggestions will be highly appreciated! Thanks!


12-21-04, 08:55 PM
Here at Cluster (www.clusteranimation.com) we use the chromatic sphere to get a HDR Image, and we use it to light our character, I send lots of passes (diffuse, specular, occlusion, normals, facing forward, IBI, etc), and then in after effects, or shake, or in combustion you join all of them with the plate, and you tweak all the parameters.

I think there are some siggraph papers for all of this techniques.


You could also find very good info from cinefex:


You should buy some magazines.

12-22-04, 08:38 AM
Thanks a bunch for all the links! :) I havent read a single one of them though i've downloaded those that i think i might need. Anyway, another question. You mentioned something about generating an occlusion pass. I've heard about ambient occlusion mapping though i havent messed around with that stuff. Now that you've mentioned it, im thinking of doing some experiments on it. Could you give a general idea on how you generate your occlusion pass? Does this require global illumination? There's this tutorial by andrew whitehurst about ambient occlusion. Unfortunately I dont have a copy of prman. So do you know of other sites that have info on this. Thanks man for the help :)


12-22-04, 04:38 PM
Here is a plugin for Mental Ray:


Under stuff, its called dirtmap.

If you read this article: http://www.deathfall.com/article.php?sid=4383 You'll understand how it works.

It would be nice if you post your WIP here, so that we can give you some feedback.

Good luck!

12-24-04, 07:40 PM
hi sunstar i was just curiouswhich of those papers are for renderman can you tell me plz cause the names are strange

12-24-04, 07:49 PM
and about the thread an advice for cg in live action which i am making for fun right now I am using bent not normals pass ambient occlusion pass direction light+ hdr diffuse and reflection pass + additional speculars and depth of field , motion blur passes if you don't know how to composite ambirnt occlusion and bent normals and hdr you can use a script for shake or built in nuke features this is not prman only technic you can render bent normals with the shader pack from mental images misss and about the probe from ILM yes they are testing colors i am using diffrent probes for capturing the probe and use it as a hdr environment ...

01-06-05, 01:38 PM
It would be nice if you post your WIP here, so that we can give you some feedback.

Yeah i would definitely post my WIP here but I've been recently assigned some workload and I'll be doin' this on whatever free time i can get. So it may take a while. Right now i've gone over the tutorial and i cant wait to do some tests :D

I am using bent not normals pass ambient occlusion pass direction light+ hdr diffuse and reflection pass + additional speculars and depth of field , motion blur passes

Where i can get info on bent normals. I have no idea what it is :(

01-06-05, 01:48 PM
where can i get the linux version? Im using linux. I read in the readme.txt that the TheMill has compiled a linux version. Where can i get a copy of that?

01-06-05, 03:31 PM
You can find an example about bent normals here:


Let me konw if you understand what the bent normal pass is, if you don't I'll try to explain.

01-06-05, 07:23 PM
well, i now have a fairly good handle on the concept of ambient occlusion after reading some stuff about it on the web. Thanks guys!!! Thumbs up! You've been a HUGE help with all the links and ideas you've posted!!! :D With reagrd to the bent normal thing, actually i've known about andrew's ambient occlusion tutorial for a long time but it's only right now that i took the time to really undestand it. And plus reading the ILM paper has given me a fairly good idea on what is a "bent normal". Just a question lingering on my mind is why do we have to have a bent normal to use in accessing the environment texture? What's the rationale behind this? I'd just like to know. :)

01-06-05, 09:36 PM
Well.... When you do renders with IBI (Image Base Lighting) what it really does, is that each point of your model send rays and it looks for the color and intesity value from a virtual enviroment of the Image you are using. If you don't use bent normals, a ray is send and it wont consider the ray hitting another object, and you will get the wrong value of color/intesity, so the bent normal corrects this, it will tell the new normal of that point, so that it will consider the occlusion of another object, and in this way it will get the correct value.

I just added an image, where you can see in Red a ray that shouldn't hit the enviroment, but this is something IBI doesn't consider, and you'll get wrong values, so the bent normal will get a better value.

Hope I could make myself clear.

01-07-05, 11:26 AM
Thanks man!! The diagram has been very helpful. Now this is how I understand the process so far. Feel free to comment. I havent done anything on maya yet since Im using a linux box right now. All ive done so far in my spare time is read, read, and read more. For each frame I need to generate an ambient occlusion pass, a bent normals pass and ambient light pass. I can use HDRI for the ambient light pass. These three are combined to generate an ambient environment lighting. In case of a live action background, the ambient environment lighting generated will approximate the environment light present during the shoot. Is this correct?

01-07-05, 09:49 PM
Yeah, you are right, but for example, I don't use Bent Normals, in the projects I have work, you won't notice the diference with or without it.