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ruddiger52
09-28-2007, 12:20 AM
I need some advice on getting my rendertimes down with motion blur in mental ray. I have two cars racing eachother on a live plate. I want to render 3d motion blur but because of the speed at which the tires rotate the render time is unreal with the blur on. Does anyone know a good workaround for this?

royterr
09-28-2007, 03:40 AM
use full motion blur and try to lower it (motion blur by) as much as you can.

ruddiger52
09-28-2007, 04:50 PM
That seems to lower the amount of blur

sixbysixx
09-28-2007, 06:05 PM
Have You tried to use the Rasterizer?

ruddiger52
09-28-2007, 06:56 PM
wow the rasterizer really sped things up. Thanks. Am I sacrificing any quality on the rest of the car?

sixbysixx
09-28-2007, 07:04 PM
I don't have enough experience with it, but I think the Rasterizer has quite a few pitfalls.
Reflections don't blur properly and I think there might be problems with displacement as well - not sure.

jupiterjazz
09-29-2007, 12:08 AM
I don't have enough experience with it, but I think the Rasterizer has quite a few pitfalls.
Reflections don't blur properly and I think there might be problems with displacement as well - not sure.


My suggestions, in general:


- avoid raytracer+motionblur, slows down times of ~500% (5 times slower)
- use ratserizer (but rasterizer is not suitable with heavy raytracing techniques: AO, GI, FG, blurred reflections, area light)
- use 2D motion blur via output shader (lamaison motionblur shader or mip_motionblur)
- use renderman renderers :)


o

MasterZap
09-29-2007, 08:56 AM
My suggestions, in general:


- avoid raytracer+motionblur, slows down times of ~500% (5 times slower)


Actually, raytraced motion blur slows down pretty much exactly as many times as you have temporal samples. Which is farily obvious, if you think about it; it's pretty much rendering the fragment such many times. Duh. ;)



- use ratserizer (but rasterizer is not suitable with heavy raytracing techniques: AO, GI, FG, blurred reflections, area light)


True tip to a point. If the rasterizer has any issues together w. raytracing heavy effects is strongly scene-dependent. The rasterizer works on a "per object" basis and shades them as necessary. It tries to avoid double-shading something, but in some situation the rasterizer may shade a couple of pixels which ends up covered up by some other object. IF the shading on those pixels are very heavy, it is possible that the net result is that the rasterizer takes longer in that case.

A particular case to look out for is reflective things; set them to one-sided. Otherwize the rasterizer may accidentally shade the inside, and since you are INSIDE a reflective object... well... those rays will bounce a while. So set them to render "front" faces only and that problem goes away. (The "do not reflect inside" switch in mia_material does the same job)



- use 2D motion blur via output shader (lamaison motionblur shader or mip_motionblur)

True indeed, also.

However, the 2d blur is linear, and for rotating things you tend to need a 3d blur. So there is a cool thing you can do to get a "best of both worlds" blur very quickly: Turn on full 3d blur but with *1* temporal sample. It'll render as fast as a non-motion blur render, but grainy as heck. But then you arun mip_motionblur on top of that with a low shutter (0.1-ish). This will act as a "smoother" which smears out the 3d blur just a hair, to cover up the grain... very neat.


- use renderman renderers :)


No, keep using real renderers. ;)

/Z

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