View Full Version : relationship between final gather and global illumination?

02 February 2005, 06:37 AM
I am having trouble understanding the need for global illumination if you have final gather. From my tests, final gather seems to be used to do the same thing, but it seems to do it much better and faster. Can someone please explain to me the reason why you would use one or the other or both together?

02 February 2005, 07:39 AM
GI is from lights (such as the sun for example), where FG is used to simulate a very soft sky glow. Think about GI as a particle emitting light, and then think about FG as a shadowed ambient.

11 November 2005, 08:05 PM
The real light calculation is done in Global Illumination phase infact. The correct way of working with radiosity with mental ray should be first turning on global, adjusting overall scene illumination, then turning on final gathering for the final quality image. Final gathering creates the finishes, correcting the artifacts caused by the limited number of GI photons.

It s also a nice experience to try achieving the beautiful color bleeding effects by ONLY "Global" turned on. Try that if you can. First decrease the photon radius, Increase photon amount from the light parameters -but gradually to stay away from crashes- You can also increase the overall photon amount multiplier from the render scene dialog. It's a global value that determines the amount of photons for all lights in your scene. The amount of photons is determined by this value x GI photons value of the light - when automatic.
Low radius values will create grainy GI.

Theoretically, in order to have a realistic lighting effect we should have infinitely many photons while each of them being invisible because of its size, but a software has to be limited otherwise rendering time goes to infinity.
So in order to improve rendering quality of Global illum., we don't increase the photon amount, while it should be the physically real thing. We keep the radius high -increase samples. And turn on Final gather to see details.

Youll be amazed how global might also create smoothly global illuminated scenes.

11 November 2005, 11:44 PM
Yeah what he said! :p

It takes some time to experiment with and hair pulling is inevitable! :banghead:

11 November 2005, 06:30 AM
Global illumination is pouring the cream in the coffee. Final Gathering is stirring it.

11 November 2005, 04:19 PM
Global illumination is pouring the cream in the coffee. Final Gathering is stirring it.

haha nicely put..

so guys would you say fg is an approxmation method to fill in the holes in GI photon emissions?

i tend to use just FG on its own a bit at the moment - doing all sortsa freezing and baking for my aniamtions. But am wondering if i was to implement GI and use FG on top of that if it would be more friendly to animation.. im almost certain it just complicates things. I will be doing some tests as i my next enviroment for my reel i will try bake down with both GI and FG...

11 November 2005, 10:28 AM
It takes some time to experiment with and hair pulling is inevitable! :banghead:

:eek: You won't try that with a scene with several zillion polygons of course. Put just a plane and a sphere on it -maybe another wall to see some more reflections. It doesn't take more than 10 minutes to see crashing point lol =)

- -
You might think of GI as a directional illumination source in your scene where fg is non directional. (Meaning coming from all directions- illuminating all directions)

By the way FG alone, might also give beautiful effects. It might apparently seem %90-95 correct. But that is because the effect we like comes from the gradients caused by the color bleeding on objects. And if there is a light object you ll see almost the same effect (only with some differences in the illumination values. With GI on, your scene lighting adjusted for fg will be overexposed)

Think of it this way: Delete all lights from scene, GI will warn you of it. GI needs light to work. But FG will still work. This might give you an idea why GI is the real thing. I don't mean you "MUST" use GI to reach the right effect. Whichever fits your scene and time is OK but;

1. GI calculates the effect of the light. You can cause that directed light to reach other rooms in your scene,
2. while FG calculates a diffuse lighting effect caused by any illumination (let's say any Value) in your scene. You may think that you can illuminate the entire scene by FG also and the same effect might be achievable by increasing the effect radii.
But the overall effect will not be correct if you're using one light and several rooms which are not directly illuminated by that single light.

You may as well place fill lights, where it would -in one sense- be meaningless, while trying to use global illumination.

so guys would you say fg is an approximation method to fill in the holes in GI photon emissions?

Approximation is the exact right word.

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11 November 2005, 10:28 AM
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