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Extremely slow Mental Ray Renders with many overlapping unique textures: Optimizing settings?
show user profile  Stenrik

I am trying to render some unusual scenes. They consist of hundreds of low poly planes stacked closely up on top of each other, each with unique diffuse and alpha textures mapped to them for an interesting ethereal 3D effect.

Rendering these in scanline is pretty quick, and rendering them in Mental Ray with no extra lights is also pretty quick, but rendering with the daylight system is VERY slow.

More info about my scene setup:
- I have about 100 planes and plane-shaped "polys" combined, all low res.
- Each plane is about 500 x 500 px big.
- Daylight System is on, and set to raytracing.
A small 300 x 400 px preview spends a few minutes on "Computing Final Gather Points."
It then proceeds to spend about an hour rendering that preview, occasionally hanging for a few minutes at certain points. This is very long compared to my other renders and is making it very frustrating to work.

In order to try to speed things up:
- I set my "Samples per Pixel" Quality from 0.25 to 0.1.

Inside the "daylight assembly head" I have tried both "automatically calculate energy and photons" and putting it on manual settings with GI photons halved to 5,000 instead of 10,000. That didn't really seem to speed things up.
I've turned off "Use Advanced Lighting" in the "Common" tab of the Render Setup window. Again, hasn't sped things up.

I then tried the shadows, turning the "shadows" checkbox off within the mr Sun basic parameters. It surprisingly didn't seem to speed things up at all.
HOWEVER, when I turned off shadows from the "Shadows & Displacement" part of the Render tab in the Mental Ray render setup window, then it was noticeably faster. (About a minute or 2 to render, total.)

The problem is, I WANT shadows.

So I then played with the Global Illumination settings. I set Skylight Mode from "Final Gather" to "IBL" and it does appear to be faster... a little bit. I dropped shadow quality to 0.1 and it was about 7 minutes. I then dropped shaodw quality to 0.1 and found I could get the whole thing in about 5 minutes.
I then tried turning off Final Gather. It didn't seem to help speed up the render, still about 5 minutes.

I checkboxed "Draft Mode" in the Global Illumination tab, and it didn't really seem to do anything for the overall render time. I took the max trace depth down to 2. Still the same.

So is there more that I can do? I'd love to be able to look at "pretty good" render previews and get them at less than a minute per 300x400px render. One thing I suspect is holding me back is that the textures are large. Is there a way to adapt the texture mapping size to the relative size in the render?

As you can tell from my wall of text, I'm poking around in the dark and suspect I'm missing part of the equation for getting faster renders, especially faster PREVIEW renders. Any suggestions would be very welcome.

read 326 times
7/5/2016 10:30:14 PM (last edit: 7/5/2016 10:30:14 PM)
show user profile  HANZZ
Curious to see what you're seeing in the render. Post a shot.
Daylight is notoriously demanding. You don't have many options. Are you recreating a date/time setup that demands accuracy for a lawsuit previz or something? You don't have to use daylight for personal projects unless you're just wanting slow renders. And if you're trying to get shadows 'through' the alpha of the planes, you're out of options: it's going to take lots of time no matter what, because the alpha has to be calculated for every single pixel, then shadows for every single pixel, relative from one plane to the next. And that has to be done 100x. It's a massive calculation task. It's impressive that it's only taking around 5 minutes. Keep in mind that renders of a single frame in a modern effects movie can take anywhere from 15-60 hours. Ever see Starship Troopers? The mass bug attack scene (all done and rendered by Tippett studio) was originally taking upwards of 60 hours per frame.

Get a render farm, get multiple core xeon cpus and a terabyte of ram, or just accept that it's going to take tons of time.

 photo MorgothArisenSig_zpsup4yjp7o.jpg

read 322 times
7/5/2016 10:43:24 PM (last edit: 7/5/2016 10:43:24 PM)
show user profile  Stenrik
Hi Hanzz,

Thanks for the fast reply!

I do not demand accuracy for this work. It's actually for a psychedelic mural/illustration that will be printed at 30' x 10' for a festival.
You say that a daylight lighting scheme is not necessary. What is the fastest similar-looking alternative? I'd assumed that the daylight setting would have optimized settings, and it's pretty much the only thing I've been using thus far for this kind of lighting.

Here are some examples of what I'm doing:

The top one is an example of doing this in daylight mode. The others are scanline/default lights.

RE: render farms. I'm a single freelancer without super deep pockets, but I have seriously considered building a "render slave" machine with an amazing CPU and not much else. I may try a cloud render service like Rebus first, though.

read 311 times
7/5/2016 11:16:46 PM (last edit: 7/5/2016 11:20:10 PM)
show user profile  HANZZ
It looks like a kind of branching fractal in the bottom two pics. Very interesting effect, there. What were the render times for those two versus the top one? Honestly, the 2nd and 3rd look more interesting lighting wise than the top. Trying to put 'out in the daylight' styles of lighting and shadow on something as gossamer as a fractal pattern...seems like it might not serve so well. I'd say nix the daylight solver, put a point light and use standard mental ray. It won't really speed things up much, unless you turn off GI. Even then you'll get solid results. It's just all those layers cost sooooo much render time and passes, even more when you have shadows going with raytracing.

You might try knocking down the number of 'levels' in the shader. That always speeds things up too.

 photo MorgothArisenSig_zpsup4yjp7o.jpg

read 300 times
7/6/2016 1:52:41 AM (last edit: 7/6/2016 1:52:41 AM)
show user profile  HANZZ
And on a side-note, I employed a similar method to create volumetric fire effects, energy effects, nebulae.

 photo MorgothArisenSig_zpsup4yjp7o.jpg

read 298 times
7/6/2016 1:55:45 AM (last edit: 7/6/2016 1:55:45 AM)
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