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View Full Version : Help concerning Indirect Illumination appreciated.

noviolence
09-30-2009, 04:05 AM
Hello everybody,

this post is possibly going to be a little longer and as I don't want to waste anybodies time I recommend to not read it if you're not really eager to help or to learn something (as i hope i am mostly right and others will correct me if necessary).

But: Any Help is Highly Appreciated, how small it may be.

I'm just preparing a 20 minute lecture I'll be giving in university soon (very soon unfortunately) and I want to make sure to get everything before I'll talk about it, .. at least most of it.

So,.. I picked the subject "Indirect Illumination". The negative point on that decission is that i don't know too much about the subject, the positive effect however is, that it brings me to learn a lot which will be of great value for me in the future (as I hope).

I'm trying to prepare the subject in a way that it is not only of value for people who use one particular renderer. However I'll mainly use modo to create sample pictures as i am used to it and mental ray(which i also wanted to use) is terrible to set up in order to get usable imagery.


Here are some statements which I'm not absolutely sure about if they are true. I'd be very happy if someone could tell me which are fully or partially false. If you can correct the wrong ones: Great! :) .. some also have questions in it.

1.
If Indirect Lighting is turned off, only direct lights will distribute to the color and value of every pixel. No light is being reflected from one surface to another. The render method used is ray tracing which shoots the rays from the camera into the scene.

2.
For direct lighting, no rays are sent from the lightsource into the scene or from the surfaces to lights. The lightinformation is calculated with the known information of the position, the value and color of the lightsource. (I didnt read anything about direct lighting but this is the only way that seems plausible for me.)

3:
Assuming a room with only a indirect light source (luminous polygon) and no direct light source. With indirect illumination (monte carlo or irradiance caching) the ray is sent from the camera and the first bounce is when it hits a surface, is redirected and hits another surface. The Value it can get from that first bounce is always black (except it hits the indirect lightsource directly), because the surface has no light cast on it.

If there is a direct light in the scene, then the lighting with that happens before the indirect illumination therefore the first bounce returns a color.

How does the second bounce work exactly, if there only is a indirect light in the scene? Are the bounces calculated one after the ather or is a ray(and all of its bounces) calculated one after the other? - Does the ray that travels from surface a to surface b bring back color from surface b only if it hits the light afterwards(on the 2. bounce) or does it return a color because surface a has been lit with the first bounce?

4:
In modo it is possible to render different surfaces with different methods(IC and MC) with the help of the shaders. Is this also possible in other renders, lets say mental ray? Or is it maybe possible with different passes?




I guess that's it for now..
I want to thank you if you took some time to read it, or part of it. I am not expecting anything from anybody and can't be mad if no one answers. Still, I am very thankfull for every answer.

General tips for the subject, or how I could present it are also very welcome.

thanks a lot
novi
playmesumch00ns
10-01-2009, 02:23 PM
I've started writing a reply, it's just turning out to be very long and quite difficult to make it clear and concise. I'll try and post something later on today...
ndeboar
10-06-2009, 12:48 PM
ill have a bash:
If Indirect Lighting is turned off, only direct lights will distribute to the color and value of every pixel. No light is being reflected from one surface to another. The render method used is ray tracing which shoots the rays from the camera into the scene.


Ray tracing is not requried for direct illumination.

For direct lighting, no rays are sent from the lightsource into the scene or from the surfaces to lights. The lightinformation is calculated with the known information of the position, the value and color of the lightsource. (I didnt read anything about direct lighting but this is the only way that seems plausible for me.)

It uses the surface normals and light position (that maybe over simplifing it :) ).

3:
Assuming a room with only a indirect light source (luminous polygon) and no direct light source. With indirect illumination (monte carlo or irradiance caching) the ray is sent from the camera and the first bounce is when it hits a surface, is redirected and hits another surface. The Value it can get from that first bounce is always black (except it hits the indirect lightsource directly), because the surface has no light cast on it.

I know irradiance caching shoots rays from the camera (importons) and i think it does this to work out the most efficent way to distbute rays, but this (the method you've discribed) isnt how gi works. There isnt really even a standard for GI, each method (monte carlo, final gather, irradiance caching, color bleeding) all use differnt math and functions to calculate indirect light.

Also, dont forget:

Ambient occlusion: Sends out a hemisphere of rays from a surface point, and returns a value based on the distance traveled before hitting another surface. Still the fastest and simpliest way of aproximating gi.

There are loads of resources online discribing how direct and indirect illumination works, a simple google should give you all the answers you need.

And, to be honest, you really should present a topic you know about, there is so much miss-information tought at uni, be good if the students are tought proper theory.
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