why is it different?(images included) qmc and light cache

[d7man2000]

I did 2 renderings: one with QMC and one with Light Cache,

same setting, same light intensities and I get something different.

Light Cache scene is too bright

can someone tell me why is it like that?

[d7man2000]

difference is only the secondary bounce

[AJLynn]

Huh. Looks like the non-physical-shaders-causing-light-bounces-to-multiply-by-pi problem, but I've only seen that in mental ray. Anybody know if Vray does the same?

 

Try a test: set your lights to be using inverse square falloff and in the Vray settings turn on material override and use a VrayMaterial that's just got a diffuse gray at a value like 170.

[d7man2000]

I changed the multiplier of secondary bounce to .9 and I get something closer to what I wanted , do you think this is acceptable?

[Ismael]

Actually it is acceptable. C. Nichols mentioned somewhere that he favors LC mostly but renders are brighter and he reduces it like you did. Always, what works best is the right approach.

 

Ismael

[AJLynn]

Generally I'll use LC with multiplier 1.0, use the "linear space" trick that Chris posted something about a while back, and adjust my light multipliers as needed. Not sure if I started doing this because it seemed like an elegant solution or because I liked the midrange balance better, but it's been working well.

[Christopher Nichols]

Yeah it seems strange. The more I read this the more it seems that your lights are not decaying correctly or, if they are, the room is the wrong scale. LC is essentially an inifinite light bounce, so if your light does not decay with a the correct inverse square decay, you have problems. Also, your room color should never be pure white. Always a shade darker (max 245 or so). Doing the .9 on secondary basically means that it looses 10% of the light on each bounce. By reducing the color of your walls by 10%, you do the same thing (essentially), but it would be the more accurate way to do it.

[d7man2000]

what's "linear space" trick?

[AJLynn]

The linear space trick uses a levels correction in the Vray frame buffer to get your image into an sRGB profile, which is what your monitor is probably set up for. Similar effect to gamma correcting your image in Photoshop.

 

Download this file - I did it by eyeballing but I think it's correct.

 

Then use the Vray Frame Buffer, and click the button all the way on the bottom left. Right-click in the area that looks like a graph with a diagonal line, and click Load. Choose that file, and the line will change to a curve. Now, there's a button a few to the right of the one you clicked to get the graphs, that looks like a multicolored boomerang on a gray background. Click that, and the correction will be applied.

 

A lot of the time I expect a render to have a certain level of brightness, but it's too dark especially in midtones, and this corrects that, but in a more general sense it makes your images display in the correct way for your monitor.

 

Chris: did I get that all right?