Questions about lighting

[graphicsgriffin]

Hi everyone,

Last month I had to do an interior of a very large guest hall within a palace (about 400 square meters), the client wanted me to make visualization for the lighting system, containing neons and spots, there were about 175 spot lights, (other than neons).

Well, rendering a scene with so many lights is very slow, so I did reduce the number of the spot lights a little, to 48 lights, and since the neons are placed beside each other, I did place one planar Vray light for each row of neons, but still very slow(using direct for secondary).

Can you please tell me how can I make real interior light study;
---How many bounces?
---first and secondary bounce values? To simulate real-life lighting.

Another question, about methods to speed up rendering? I mean having so many lights, is there any real reason to use 10 bounces? We can get a good light contribution with much less bounces, right?

Last question, If I am going to use photon mapping, is there any need for high diffuse subdivs? Can I reduce the subdivs when adding so many lights?

Thanks in advance.

Yours.

[tom schuelke]

Hi

many questions at one time.

Usualy, when you use the 1.09 version of Vray, and also with 1.5 and also with max radiosity system, you will not have the possibillity to realy simulate real world light distribution because of several reasons.

first of all, you don´t have the real world parameters, of material light reflections. those are more complicated as it seems. Max radiosity overrite mat goes a step ahead and alows to give the overal light reflection value in % steps, which gives you a bit more of control. Vray only gives you a way to do this via rgb values or Gi multiplier, which is wrong at last, compared with realworld parameters and also dont give real control. Also, light reflection to a scene is partial diffuse and partial specular, which means, only highend lightplaningsystems like "spectar" or for example "radiance" alow such materials and also alow realworld lamps with an energy amount. At last if renderers like these are able to calculate the relistic energy distribution, they still have to deal with the problem of transforming them to a rgb valu on your screen, and at last here, they have to deal with all the problems, a photograper has, because a screen can´t display wether the wavelength of light of realworld, not the differences between brighter and darker areas, as realworld does. So all of them have some sort of exposure, to translate energy values to a predictable screenbehaviour, as one would expect it. (This is why it is only called photorealistic and not realistic)

OK i think this doesn´t help you so far, but shows, that realworld lightsimulation isn´t the total aim of renderers like brazil, vray or finalrender. They have the aim to make nice and fantastic images very fast and intuitiv, and nothing more.

So with 1.09, you might use photonmapping to solve a bit your problem. I-map calculation as you might know, chooses (very intelligent) single points in your visible image areas, and only at these points calculates the incoming light. This hapens by trycing a bounce of dozensz of secoundary rays away from this point, to see spots of light around this first point.
When you render an interior, usualy, you get less and smaler areas with light,compared to exteriors, wich means for the i-map calculation, that you have to trace many many more rays, to hit statisticaly correct those smal areas of light, and this also is the reason, why i-map calculation is usualy much slower in interiors as in exterior renderings, where you have much more bright areas, to be seen from your samplepoints.

photonmapping tries to solve this problem. It shots virtual photons away from your lights, that hit the surfaces, lose energy, bounce to the next walls, and by hitting, bouncing, and loosing energy very fast can make an averaged lightsimulation of your overal lightning.
this is calculated very fast, but can be quite difficult to be tweeked with so many lights.
But this way, your secoundary bounces are calculated very fast, and your imap takes this allready good overal lighting and now has to trace much less rays to get an god aproximation to the real light distribution in your scene. with this method, you don´t anymore need quasimontecarlo for secoundary bounces. photonmapping can speed up scenes between two to three times in interiors.

(you can find a good tutorial for Photonmapping when you look in the announcement section. Vlado has posted a link to the new internal textes for 1.5, and there is also the tutorial section)

you also should see, if you use softshadows, and how low you can get with your softshadow samples settings to still get good rendertimes. (many softshadows can be a timeeater).

at last, if you use many softshadows, don´t try to use analytical antialiasing (adaptive subdev), but simple two level. the analytical cant deal that good with al those pixel based effects like soft shadows, and very smal details the analysing process here can get slower than the rather brutal antialiasing effort of simple two level. Simple two level, in such a case can make renderings two times faster.

OOps, i think i´ve just written a book.
Hope there´s something that helps you


Tom

[graphicsgriffin]

Hi Tom and thanks very much for your informative and detailed post,

How can I get to know more about these aspects of the program -Vray and max-? ( I mean scientifically and not just broad descriptions )
Maybe a site or a link?

It's nice to know how to make good images, but I think what's really nicer is to know how the program do it, and how everything works.

I would appreciate that very much.

One last request, could you please make it clearer about abandoning quanti monte carlo?

Yours faithfully.

[tom schuelke]

Hi Graphicsgriffin

nice some of my explanations seemd to help a bit.

i am of your oppinion. Most of the functions in Vray are much easier to be set up, when you understand the structure of the underlaying processes.

thats allways , what i try to do. Well some of the backroundknowledge, you get here slowly in the forum, some things, i knew, when i was reading the manual of finalrender stage 0 in the earlier days.

A lot of the concepts of rendertechnology stay the same in vray, brazil, mentalray, and finalrender.

so if you understand one of the concepts, the other are nearly the same.

of course in detail, the manual of 1.5, vlado posted, can help better.

Well and Quasi montecarlo... (don´t know the meaning of the word abandoning).

Tom

[graphicsgriffin]

Hi Tom,

Thanks alot for help.

Actually there are some nice sites about theory of lighting in the real world and how to simulate, just go CGArchitect, and then to the links section.

Your's faithfully.