# realistic lighting in mental ray

#1

Hello everybody!

I started this thread because I couldn’t find any other that gave good answers about accurate lighting in mental ray/3Dmax.
My goal is to be able to create a scene where I can simulate different light situations, as close to the real world as possible.

EXAMPLE:
In the posted scene I’ve tried to simulate daylight coming through the windows. I’m after a realistic looking room, where the indirect lightning doesn’t change the original colors to much.

I also want to be able to turn the electrical light on in the same
scene - adding that to the total lighting of the room. So that I can just switch on/off different lights and combine them as I like.
This combination of lights I find very tricky.

For months I’ve been puzzling with all the photon, energy, decay values i can find but I just can’t get it right!

RENDER1

[I]mr Area Omni

Intensity/Color/Attenuation
multiplier: 150
decay: inverse square starting at 0.29 meters

mental ray indirect illumination
energy: 4000
decay: 1.6
GI photons: 20000[/I]

RENDER2

[I]mr Area Omni

Intensity/Color/Attenuation
multiplier: 1
decay: none

mental ray indirect illumination
energy: 20000
decay: 1.6
GI photons: 20000[/I]

As you can see the top render is too warm. Something gives the white ceiling a orange tone. The indirect illumination effect is too obvious to be realistic.

The bottom one has a cooler, more even and natural tone. In color it looks more like the daylight here in Norway. But here the problem is how some of the faces, the ones who face the lightsource, are much too bright… As the light doesn’t have a decay, it doesn’t weaken as it travels through the room.

Another technique i’d like to understand is how to use real world light values in 3d scenes. So that a 60W bulb has the same effect in 3d as in the real world.

I think this technique is especially important for architect students (as am I) Only when we can solve these problems we can use 3d scenes as a realistic reference for the designing of spaces.

I hope you can help me…

the scene file:

#2

wow…dude…try to up the detail of the rendering to min-4 and max 16 at least to get those jagged edges out…

or use 16/64…but that’d take more time to render

#3

I will do that if this pleases you buggy166

But I am not looking for a smoother picture right now. I want to get the tones and indirect coloring right first…

I hope this thread can provide some numbers for realistic lighting in mental ray.

#4

There’s a wonderful example scene on the training/tutorial CD that comes with max. You have to dig for it, but it’s named “MR_LightGallery_Max6.MAX”. Give it a look and either disect the lighting or merge your scene into it.

#5

hi… i’m a maya user, so i have no 3dsmax… but this lightgallery example sounds very interesting… could someone show some renderings of this lightgallery? thx

#6

Here’s the rendered scene I mentioned above. (interior lights are off)

#7

Thanks folical9! I’ll have a look at it.

Do you know anything about taking real light values and putting them into 3d scenes?
I.e. If I want to test how five specific spots will affect the room??

also… Do you have any tips on how to make a lightbulb visible in a room, sp that you actually see the lightsource??

Feel free to experiment with my scene…

#8

First of all, always use decay with your lights when using mr’s global illumination. mr GI model is very physically accurate, and requires a physically correct behavior of light, wich means you need decay. Standard inverse square should do it.

I think the first image you posted is going in the right direction. But you’ll need A LOT more photons. In production scenes, it can reach up 2 millions! But with FG you can decrease it quite a bit. Absolutely consider using GI and FG together.

Oh also, don’t use light multipliers above 1.0. Again, for light propagation purposes.

Cheers
Bernard

#9

How many GIphotons do you suggest I use in combination with final gathering???

#10

I forgot to say that both of the above renderings are with Gi and final gathering. I will add final gathering numbers.

#11

Try something like 50,000 to 200,000, with GI accuracy up at 500. Also use Fast Lookup, this is quicken the calculation.

Cheers
Bernard

#12

Do I use decay for daylight too? If I use inverse square decay here the render becomes black, and if I cannot up the multiplier then what??

I’ve tried to set the energy level higher, but this only makes the colors of the scene affect eachother more. I.e. the white roof becomes brown because of the brown floor…

My problem right now is that the walls that face the light source directly becomes too bright. How can i simulate the more even light of a blue sky with no sun?

#13

Folical9 - breathtaking my friend. Good lord would I love to have your mastery of MR and materials.

You have to be in the top 10 of the cg artists I have run across over the years.

I first read your posts on the MR.mat files you posted with the sphere shapes. What a thread.

Thanks for the inspriation.

#14

if you are having a problem with certain lights over saturating then you need to use exposure control on the render.

turn off the global settings on each light and adjust the gi and caustic photons for each light. this is the most accurate way to get real world light simulation. obviously the sun would give off more and the other lights would give off less.

turn down the final gather samples to around 100 to speed things up but turn it back up when you want to make production renders. you might want to turn on the “all objects generate and receive caustics & GI”. turn on fast look-up also.

decay lights work wierd in Mental Ray. inverse square usually doesn’t give the results most people would expect. either it’s a bug or it’s what happens when you mix gi with light decay. regular inverse softens the light usually but inverse square never works for me.

#15

The lighting in the scene consists of two mr Area Omnis, one outside each window.

SETTINGS

multiplier: 0,67
decay: Inverse square starting at 5,85 meters

mental ray indirect illumination
energy: 450 000
decay: 1.6
GI photons: 200 000

render setting
final gather samples: 500
final gather max radius: 0,2 meters

This render is better, but I am not satisfied with the result yet.

The areas facing the two windows are too bright. It looks like the sun shines right on them. How do i simulate the more even light of a JUST blue sky outside??

The roof is, in my eyes, too affected by the indirect illumination.
Can one control how far the indirect illumination travels?

There are some errors under the nearest picture. Are these because of too few photons or too lfew fg samples?

Finally, the whole room is too dark. If I increase the multiplier to 1 (not above), the already bright areas become white.

#16

If anyone has a good idea whats wrong with my scene you could download my scene file, and try to make it look good by changing light settings?

#17

sdp777, thanks for the compliments but I didn’t create the interior scene in my previous post. It’s one of the MR samples that ships with 3DS. Just wanted to make sure I wasn’t getting credit for something I didn’t create.

Ligting is one of my weakpoints right now, I’m struggling with it as well.

Asomma, I like that last render, getting much better!

Jeff

#18

Good to hear that you struggle with the lighting too folical9

Do you use msn? I would really like to discuss some lighting issues with you sometime…

#19

I just added my info to the MSN thread (been meaning to do that for a while now).

#20

asomma,
have a look here at my suggestions man :

this is how I do it, it might not be THE ONE AND ONLY way to do it, but it works for me…
that might help :shrug:

first make sure you have your units correct :
a ‘cm’ in your units should represant a cm in real life
this is very important to get your sample size under control

your energy should be something between 20.000-100.000
not more… if that doesn’t do it… lower the decay (1.5 1.4 1.3 … until the scene is well lit)

keep your photons around 50 cm (this is why your units are so important) and don’t use more that 20.000 - 100.000 to start with

keep the FGather low (50-100) for test renders (when doing final renders increase, this will only improve the quality, not the brightness or anything, so you’ll get a good idea of the result and still have fast testrenders)
keep away from the FG radius, this will only slow down the renders, and can be fine tuned when all looks ok