help with slow car render

[Hos]

Hi,
I am working on a project and I have a lot of trouble getting acceptable render times.
I know the type of environment and light setup (in-door, lots of lights, highly reflective subject) can give really long render times, but I don't know if that is the only reason.
There are a lot of different render settings I tried and they gave me a lot different render times and this is the lowest rendertime I could get for this render, that produces somewhat acceptable results.
This frame renders in about 3 hours on a dual xeon with each 8 cores@2.7ghz.
But it still gives too much noise and jitters in the small highlights. When I raise the samples to a point where I have no jitters anymore, render times will be around 6 hours per frame.
Is that acceptable for this type of render or could there be something wrong with my settings or even with my scene?

[Morbid Angel]

you would have to provide scene for detailed support if you want proper analysis.

[Hos]

If anyone could have a look at it would be really great.
I uploaded a zip with the project to mega. I fixed all the paths to relative, so maya should find everything.
The scene is made in maya 2014 and where rendering in the vray 3 beta.
The render scene can be found in assets\scenes\cgcar_shot09_forum_scene_r001_v001.m b
https://mega.co.nz/#!dkgmUT7S!UDySmI...cWliZgp07uCHcM

[Morbid Angel]

hey, had a quick look at your scene. I don't have good news for you in terms of rendertime. You are trying to render full hd interior with multiple lights using brute force gi for both gi methods. This is the slowest method possible. For complex scenes like this, you would most likely want to separate the car and the bg, render the bg with irradiance map and lc using multiframe incremental, then render the car with bg hidden using brute force and comp them together.

you don't need to set min dmc to 4. Rather set it to 2 and max to 16 or 24 with threshold of 0.01 or lower.

I hope your exrs are mipmapped also, they can slow down the rendering if they aren't.

[Hos]

Hi Dmitry,
Thank you for checking out my scene. I thought that this kind of scene would be really hard to render, but I didn't expected these extreme long render times. I am going to try render with your recommended sampling settings. Maybe I'll get it to render a bit faster. I didn't tried to convert the textures to tiled exr yet. I thought on these long render times, the difference would be minimal. But I'll try it.
Thanks again for having a look at it. It is hard to get an expert opinion on these things.

[griml0ck]

If you want a better understanding how to optimize your render settings you can read this guide: http://www.cggallery.com/tutorials/vray_optimization/

[ivaylo.ivanov]

The scene seems pretty sane setuped, except maybe the min shading rate like Dmitry said. However it's full of reflections/brute force GI inside a closed box, and just can't render fast... I doubt that any change will improve something, except start reducing glossy and gi rays with irradiance maps for example.

[Hos]

Lowering the min shading rate helped a little bit. The thing that worked the best for me for reducing the render time, was to lower the intensity of all lights and brighten all the diffuse textures.
This way the garage had the same brightness because the brighter textures, bounced the light more and Vray would spend less time sampling the super bright highlights.
I also made sure the self illuminating shaders didn't cast reflection and illumination and used lights for the illumination instead. This way all the bright reflections would become speculars in the shaders with a low gloss.
Clamping the secondary rays prevented the fireflies to becoming to apparent, so I could lower the sampling a bit. Although it will give you a less correct result.
I didn't try to render the car and the background separate and render the background with irradiance and the car with brute force. I don't know if that would help that much, because I still need to render the garage to catch the bounce light, reflections and shadows. And the because the car is so reflective the garage would also be rendered in brute force for a part in the reflection.
But next time I need to render a similar scene , that would be the first thing I will try.
We ended up rendering only 1 shot on the highest setting, because that was the only shot, where there weren't that much things moving and the low sampling become the most apparent because of that.
That shot would render 5 hours for 1 frame on a dual 8core xeon and 10 hours on a dual 6core xeon. Thank god we have a few of them so the shot only rendered for 4 days or so.
thanks for the tips.

[Svetlozar Draganov]

Making the texture brighter and reducing the light-intensity is a very clever optimization technique in order to reduce render-time.
Thank you very much for the feedback.