MR calculations based on distance from SSS object

# 1 28-04-2013 , 07:51 PM

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MR calculations based on distance from SSS object

I've got another annoying render problem. Doing some test renders, I realized that it takes some time for mental ray to do calculations before the render actually starts. After some research, it appears that the calculation time is proportional to the distance the camera is from my geometry. I'm trying to get my camera in nice and close for a shot, but this results in a verrrry long pre-render time. It was well over five minutes on a 640x360 before I killed it and it hadn't even rendered a pixel yet.

With a bit more research I've come to the conclusion that this is because I'm using a SSS material. Does anyone know how to circumvent or alleviate this problem? Thanks.

-stwert

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# 2 29-04-2013 , 12:07 AM

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You might be able to bake the FG, which may speed it up a bit. Try running the MR diagnostics and see if anything pops up.

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# 3 29-04-2013 , 01:32 AM

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I've got the FG map at freeze, is that what you meant? Or bake it to textures. I'll give the mr diagnostics a try.

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# 4 29-04-2013 , 01:46 PM

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If you have it set to freeze, you also have to supply a filename if I remember correctly.

You can also use an MR lightmap, which will write the SSS calculations to disk.

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# 5 29-04-2013 , 02:00 PM

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Yeah, the freeze is working fine, that's not a problem. I'll look into the MR lightmap, is that something that is used across all frames? Or does it cache the lightmap for each frame before doing the rendering?

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# 6 29-04-2013 , 06:31 PM

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I think this might explain it: https://employeepages.scad.edu/~kwitt...Scattering.pdf

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# 7 29-04-2013 , 11:24 PM

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scale is always an issue as well, check your scene unit size, meters or cm?

always take it into consideration. This is something that isnt very well documented

Jay

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# 8 29-04-2013 , 11:33 PM

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It's in cm, properly scaled. I'm just curious that there seems to be something related how close the object is to the camera. Or perhaps because the material fills the whole render, it has to do more calculations??

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# 9 30-04-2013 , 12:12 AM

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I don't think it's the distance that's the problem, but how much screen space the object takes up. (Which is inherently the distance)

The more pixels an object takes up, the more pixels that need to be computed with that material. If the material is expensive to calculate, it will take longer to render. With SSS, each traced ray that hits the material scatters. This creates many more rays that it needs to calculate, so you kind of get an exponential complexity increase.

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# 10 30-04-2013 , 01:52 AM

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That makes sense ND, I guess I was assuming there was something inherent to the distance, but it has to render a full frame of SSS goodness. The problem is that it's the most necessary to have a good looking shader when it's right close to the camera.

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# 11 30-04-2013 , 03:10 AM

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Well, if you save the lightmap of the SSS, you only need to render it once for a given light configuration.

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# 12 30-04-2013 , 02:10 PM

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I'm trying to save the lightmap, but I'm really confused as to the process. If I have "writable" checked, and don't specify a file, it does the full calculation. If I put a new filename there, it jumps right to the rendering, and when that's done, it says "no such file or directory, can't write". If I don't have "writable" checked, then it doesn't even render, it just errors out.

The pdf you linked me to says "You don’t need to specify an image name. But if you do, you can render the scene once, and uncheck the “writable” button."

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# 13 01-05-2013 , 10:20 PM

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Any ideas on how to write a lightmap to a texturefile to be read once? I feel like I'm missing something obvious.

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# 14 02-05-2013 , 01:30 PM

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Okay, I found this thread: https://groups.google.com/forum/?fro...3d/vfzbQqUeOwQ
So I created a blank white image in PS at the specified resolution, saved it as a 32-bit tiff, since the bit-depth in the Attr Editor was 32. I tried a render and the image now looks like it has some grayscale information for my render. So that's cool. I turned off writeable and am now rendering again. I think it might be going okay, but the output window says this:
PHEN 0.10 193 MB error: misss_lightmap_write: Lightmap 1073741828 is not writable, aborting lightmapping.

Is that expected? I guess I'll just see how the render looks.