IM+LC brighter then BF+BF

[DANIELE_CASAROLA]

Hi!

I'd like to understand why if I render an interior using IM+LC (of BF+LC) it is brighter and less contrast then rendering with BF+BF. There's a lot of diffrentce

IM+LC:
http://www.ossosso.com/scambio/im_lc.jpg

BF+BF:
http://www.ossosso.com/scambio/bf_bf.jpg

No changes on color mapping or anything else, just GI method.
Maybe I've set something wrong.

Thank you very much!

[joconnell]

With brute force as secondary, you have a control over how many times you let the GI lighting bounce. More bounces mean more light energy is bouncing around and the scene gets brighter. By default this value is set to 3. Light cache on the other hand is hard coded to 100 bounces in max or it will keep bouncing until it isn't adding much more to the scene. Light cache isn't quite as accurate as BF but for every light bounce after the first (primary one) the light gets very ambient and soft, so you don't need as much detail in it.

LC means way more ambient brightness and less detail but it's far quicker. BF is more accurate but slower, and you don't really want accuracy in the secondary bounces anyway, you want smoothness.

[DANIELE_CASAROLA]

Thank you joconnell, very clear, I understand.

I've noticed the RT (cpu) render is similar to BF+BF (3 bounces), less bright and more contrast then IM+LC, I don't find a "bounce" value under RT options but I suppose here too is has you said.

Anyway the final task in my case is photorealism, maybe you can correct me, but if I'm not wrong the render of LC+LC and Progressive Path Tracing ON should gives the most realistic result (high lc subdivision and 0.0 sample size). And the result (as I can see with the scene I'm working on) is that PPT is nearer to BF+BF (3 bounces) then IM+LC... I mean.. I suppose the number of bounces for LC secondary rays are too high under a lighting quantity point of view (not under detail point of view). And I think this is the reason why raw final render appears a bit "flat", not in agreement with what is my common life experience of the world I see around me; well.. no problem, with Post Production and a bit of settings tweaking I have exactly what I want, but do you think I'm wrong?

[joconnell]

Hmm, with the flatness thing it might be your white level in that case? If you've got your white set at 255, 255, 255 then this could be bouncing way more light than actually occurs in real life. Under measured circumstances the "whitest" substance known to man is fresh snow which is about 95% white or 242, 242, 242 so if you've got something over this, nearly all of the energy of your light is being kept at every bounce and washing out your scene. There's a discussion here - forums.chaosgroup.com/showthread.php?74362-Typical-White-Shader-for-Ceilings-and-Walls-4-LWF/ about levels of white for walls and most people seem to be going a good bit lower - each bounce of your GI is going to kill off a small percentage of the light and so it won't wash everything out as much.

Maybe Vlado can let us know how many bounces the LC calculations have in rt?