Number of Prepasses

[Arkitec]

Hello ALL,

When rendering an image I normally set the irriadance map to "very low" when test rendering. The total prepasses is usually 2 when using this setting. The past several days I noticed it has gone up to 5 prepasses....

Question: Are the prepass only related to the irradiance maps and/or materials or just the map settings?

TIA,

Arkitec

[metroberlin]

did you use glossy reflections with interpolation ? normaly the prepass for that is 4-5 times. when you use "very low" presets, vray calculates 3 more passes for the glossy reflections. the best way is to set the number of the prepasses for the glossy reflections equal to the number of passes you have i the rendersettings. you can find that in the material settings

[Arkitec]

metroberlin,

That did the trick THANKS A MILLION!

Arkitec

[proces2]

i have the same issue, but i don't see where you set the no. of prepasses in the material settings.

[GoncaloP]

Under the reflect or refract interpolation options in Vray material you have Min and Max rate for the interpolation passes.

Gonçalo

[proces2]

so if you have settings of min=-4 and max=-2, for example, how does that translate into the number of interpolation prepasses?

[GoncaloP]

Here you go:

-4 prepass 1
-3 prepass 2
-2 prepass 3

Gonçalo

[flipside]

it does one at -4, one at -3 and one at -2, so that's three in total.

[proces2]

okay. this will probably sound stupid but ... is there a difference between using values of -4,-2 or -3,-1?

i am getting pretty good with the program, but i don't quite grasp the concepts behind all the settings.

thanks for the help, though!

[Da_elf]

there is a difference. the amount of pre-passes doesnt affect your irradiance map quality. its the max rate that does. 0 means that samples are taken for every pixel -1 for every 2 pixels, etc. if you go above 0 then your getting sub-pixel irradiance map calculations.

[proces2]

okay. now its starting to make some sense.

thanks for the explanation.

[flipside]

Mmm, I think you should read a bit more in the help files, and at vray.info also. There's already a lot to read about irradiance map settings.

To explain it very easy:
a prepass is a rendering of a lower resolution version of your image (-3 means 4 times smaller I think, 0 is original resolution). Because the irradiance calculation is based on comparing neighbouring samples, a small image is calculated faster than a big one. So after the first pass (=the lowest resolution image), vray has some info about where many samples are very different from neighbouring ones (curvy areas) and areas where all samples are equal (flat areas). This is very rough, but based on this info, in the next pass vray knows where to put more samples (more GI calculations) and where there are less needed. This goes on in the next pass and the next etc... The higher the pass, the more detailed the samples will be placed and thus the more detailed and artifact free your GI will look.

So -4,-2 means the most detailed pass is still calculated at 3 times smaller resolution than normal. For scenes without much detail this can be enough. But -3,-1 (same number of passes than -4,-2) is better because it initially starts at a higher resolution image and it also ends with a more detailed one.

General rules (that I use...)
testing -5,-3
final -3,0

This also means that if you render all the time at 800*600 with -3,0, but then you need a higher res image like 1600*1200, you can set irradiance at -4,-1 and this will give you the same irradiance map detail as the 800*600 at -3,0.

The treshold values are also very important, but I think you can do a search for that yourself. I found a lot info about it when I started with vray, and I wrote my experiences with these settings more than once in this forum.

hope this helps,

flipside

[flipside]

Ok I'm too late

[Bolkar]

Here is another question.
Why I would do -4 -2 calculations, instead of directly -2 -2 ?
Does Calculations get help from their previous lower res. passes?
(I guess so, otherwise this would be stupid)...

[flipside]

Yes, that's what I tried to explain