FG & GI Questions in 3ds Max

[debendraroy]

Hi all,

 

Got a bunch of questions about Final Gather & Global Illumination in MR. Hoping you'll be kind enough. Thanks for your time.

 

1. What does the 'incrementally' in 'add fg points' for the fg map creation mean? I thought that it meant that when going from draft preset to low, only the additional points would be calculated, thus saving time. But when I actually tested it, I found no difference in render times. So I'm confused. (I'm only talking about a still, not animation, if it makes any difference).

 

2. Could someone explain what the bounces in FG are? I've understood the point density, rays per point & interpolation but not sure where the bounces fit in.

 

3. It is possible to render out a low-res FG map solution & yet use it for the final full res render. But how low-res can it be? Maybe half of the final res? Any other best practices in this regard?

 

4. With certain scenes viz. Interiors lit by mr sun & sky with sky portals, (in my limited experience) I've found that FG doesn't quite create enough illumination in certain areas. What does one tweak to increase illumination in such problem areas with FG? Maybe increase FG rays per point or interpolation?

 

Or is it a limitation with FG & one would be better served by adding a GI solution or fake lights in the scene?

 

Conversely, how does one decrease illumination from GI. In the same scenes where I felt the FG solution was underwhelming, the GI solution felt a bit overwhelming.

 

Or should I be adjusting the mr Photographic Exposure instead?

 

5. When using both FG & GI, does the order in which they are cached matter? Is the correct order always GI 1st, FG later?

 

6. Does increasing FG quality on a per material basis not increase the initial point density? Why does it not reflect in the Max diagnostics?

 

7. In a youtube tut by I3d, the tutor used 10% of the room diagoanls to arrive at a value for the GI interpolation min & max radius. Is this a good starting formula?

 

8. In a tutorial by Ramy Hanna, he calculated the GI photon map without all of the lights visible. In the same tut, he then calculated FG without the mr sky portals. Is this the norm or was it something he felt was appropriate for that particular scene? (He did mention that his method isn't entirely physically accurate.)

 

& on a slightly unrelated note,

9. Where do I to plug in the spec map in the Arch & Design material? From reading the Max help files, my understanding is that it is in the Reflection Glossiness & Reflection Color both. Is that correct?

 

Much obliged.

Edited September 3, 2014 by debendraroy

[Matt McDonald]

Wow! That's a lot of questions. Here goes:

 

1. The incrementally is really for animation. Because you can only have one FG map you might (should) pre-render your FG map every nth frame. The additional FG will be added to the map for final rendering.

 

2. The bounces just indicate how much a ray will propagate throughout the scene. With bounces set a zero, a ray fired from an FG point will go looking for illumination until it strikes an object if you add a bounce it will the go until it strikes the next bounce.

 

3. Yes, if render time is an issue. I think people usually say half-res.

 

4. You can add FG bounces. Adding GI is useful as well as it is relatively cheap in terms of render time. Increase initial FG point density if you feel like there are areas which aren't being sampled, increase rays per FG point if you don't feel that your indirect illumination is accurate. Increasing interpolation will help smooth out a smokey FG solution at the cost of accuracy.

 

5. GI first then FG as the GI helps to inform FG.

 

6. Controls the number of FG rays shot from the material which I believe will not be reflected in the diagnostic.

 

7. Its not a bad place to start. If your GI photons are too big you will have a wall or object which looks like it has a big GI splat on both sides.

 

8. He's probably trying to hot rod the sky portals which are great but can add a lot of render time. Another trick is to use the architectural: sky portal shader on the portal. With that you can have the portal effect either FG or GI but not both (as it does by default).

 

9. Spec map would go in the reflection map. There is an artist who goes by Rawalanche on the forums who insists that it should in fact go in the glossiness map. He did a mental ray tutorial over and Ronen Bekerman you can read if you want to hear his argument.