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View Full Version : What does the "Ray Cutoff" setting actually *do*?



stiff paper
03-14-2013, 04:07 PM
Just what the title says. What does it do? What does the number mean? What units is the number in? If you know then pleeeeaase tell me.

Is it documented anywhere? Even slightly? If it is, sorry, I did try but I can't find it.

Celshader
03-14-2013, 04:47 PM
Just what the title says. What does it do? What does the number mean? What units is the number in? If you know then pleeeeaase tell me.

Is it documented anywhere? Even slightly? If it is, sorry, I did try but I can't find it.

From what I remember, Ray Cutoff uses the same value as the Threshold in the antialiasing settings: the brightness difference between two given samples.

Threshold cuts off the Adaptive Sampling if the brightness difference between two adjacent pixels drops below the Threshold amount. Instead of controlling Adaptive Sampling Antialiasing, however, Ray Cutoff controls ray recursion. If the brightness difference between the last two bounces drops below the Ray Cutoff amount, that specific ray will bounce no further no matter how many ray recursions you allow.

So, if you set the Ray Recursion Limit to 64, but the Ray Cutoff is set to a sloppy brightness difference value of 0.1, the chance of a given ray actually taking 64 bounces in a hall of mirrors is slim to none -- and the render may look noisy/grainy. To force LightWave to take all 64 ray recursions in a hall of mirrors for every single bouncing ray, you would have to set the Ray Cutoff to 0.0. The default of 0.01 attempts to strike a balance between quality and efficiency, since it stops a given ray from recursing if its last two bounces have less than a 1/100th difference in brightness.

I don't know if it's formally documented. This is the kind of information that Dave Jerrard drops whenever a hapless passerby walks within conversational distance. ;)

stiff paper
03-14-2013, 05:20 PM
So, if you set the Ray Recursion Limit to 64, but the Ray Cutoff is set... (snip)
Aah... that makes perfect sense now. Thank you very much!


I don't know if it's formally documented.
Yeah. No.


This is the kind of information that Dave Jerrard drops whenever a hapless passerby walks within conversational distance. ;)
We need to arrange for three or four people to hem him in by walking a tight circle around him until he tells us everything.

shrox
03-14-2013, 05:23 PM
....This is the kind of information that Dave Jerrard drops whenever a hapless passerby walks within conversational distance. ;)

I wish I lived near the same pub Dave goes to...

khan973
03-15-2013, 06:24 AM
I wish that too!

erikals
03-15-2013, 09:35 AM
...I don't know if it's formally documented. This is the kind of information that Dave Jerrard drops whenever a hapless passerby walks within conversational distance. ;)

http://erikalstad.com/backup/misc.php_files/smile.gif