Light Cutoff - how to setup it?

[Micha_cg]

Hello,

doe's somebody know, how to use the cutoff option? I would like to optimize a scene for more speed and ask me, how I could change the cutoff. Or is trail-and-error the way? Maybe it's good to disable GI and make some quick tests, where the light is cuted. What's the best workflow?

Ciao,
Micha

[Micha_cg]

Re: Light Cutoff - how to setup it?

I tested it a little - a big plane a point light in the middle. It seems to be, that the cutoff is a light intensity threshhold in output grey intensity.

cutoff max pixel grey value before cutout

0.2 34
0.1 20
0.05 10
0.02 4
0.01 1
0.001 1

If I increase the color mapping multiplier, the values stay approx. the same. Also I tried the phys. cam and different light intensity/ f-stop values - the cutoff seems to stay in the range of grey value 10. Are my results ok? If yes, than a cutoff of 0.02 should be good for the most cases. At bright scenes 0.05 or more could be not visible or? It seems to be, the default value of 0.005 is quite low.

[dalomar_cg]

Re: Light Cutoff - how to setup it?

The cut off is used to prevent light from being traced when its too far away from a light to have any significant contribution to the scene. Let me explain the process as a whole, then you'll see why cut off helps things out. When V-Ray is trying to determine the appearance of a given point, one of the first things it does is figure out the direct light contribution. This entails going through each light in the scene and seeing if the light rays from that light have a "clear shot" to the point being sampled. This involves raytracing, and as you know, it can be an expensive process. The cutoff value kinda sticks its head in the middle of that process and figures out what the distance between the sample point and the light point are. Using that distance, the light intensity, and the decay settings, it determines what the illumination level would be IF that point were visible to the light. If the illumination level is above the cutoff, then V-Ray will continue on with the raytracing to see if its actually visible. If the illumination level is below the cutoff, then V-Ray won't perform any raytracing at all, thus speeding up the scene.

The question of "what's a good cutoff value" is not an easy one to answer and depends on your scene, the number of lights, and how much you're looking to speed things up. Ultimately, what you're trying to achieve is the highest cutoff value that does not wind up causing odd results in the rendering. From what I've found a scene with a lot going on and a larger number of lights (like a cafeteria) will work with higher cutoffs.

[Micha_cg]

Re: Light Cutoff - how to setup it?

Thank you for the detailed answer.

PS: At my current scene I found now, that 0.02 works fine - at 0.05 the cutoff was visible.