Motion Vectors on Spinning car wheel

[Keithtron]

Have you tried just rendering out motion vectors in Maya Software and compositing them in afterwards? That's usually what I do, and Eyeon Fusion (v5.2) has a really nice built in plugin for tweaking the amount of motion blur.

Definitely a render time saver, and it allows a lot more flexibility in post.

[ruddiger52]

No I havent tried software only mental ray. Do you mind giving me a hint as to the process?

[Bitter]

lm_2DMV generally works fine, be sure you have motion blur still on, but zero or or negate the offset in the motion blur (begin and end shutter at .5 for example)

Also, rendertime for motion blur (regular) in mental ray is linear in time performance. Time Samples of 2 is about a 100% increase from 1, etc.

Try the Rasterizer instead. You can control the smoothness of blur with visibility samples and bring down Time Samples to 1 unless you need blurred raytraced shadows. (Try detail shadows instead.)

[ruddiger52]

yeah i tried that with the lm_2dmv. It just seems to smear the wheel instead of making it look like its spinning. The rasterizer is also a pretty long render. too long for the amount of frames i need to render

[yenvalmar]

just render it from dead on one frame with motion blur and map that render on a disc which you place on your wheel for the animated render from the final cam. done, zero additional render time past a little initial setup.

of course depending on what you are doing it may be too cheesy..

[ruddiger52]

yeah its kinda high profile and I need lighing changes etc

thanks though

[strange_quark]

two ideas.

rebuild a scene with just the wheels visible that has no GI/FG and no glossy effects (blurry reflections...) render this with rasterizer. you should be able to get 1/10 the render time i bet. this is the best idea. you would just comp the blurred wheels onto the car render.

2nd idea is to try the mip_motionVectorBlur lens shader. that might work for you. i dunno. never tried it on something with extreme motion.

unfortunately, these are the limitations of using mental ray. scanline/raytrace are really slow with motion blur. rasterizer is ultra fast, but you have to use it properly to get the bonus. if your rasterizer renders slow, you are just bending its arm to do somethning its not been designed to do.

[Keithtron]

No I havent tried software only mental ray. Do you mind giving me a hint as to the process?

It's easy. Just set it to render with Maya Software, and then find the motion blur settings in the render globals. Use a 2D motion blur, and check the 'Keep motion vectors' box. I think you'll want to set the blurring to 0 as well, for maximum control in post. Render in IFF, or I think TIFF can handle it too, and the image(s) will have the motion vectors embedded. Then, you can either use these vectors in Fusion (or maybe other compositing apps too) to dynamically tweak the amount of blur. Also, you can use a command line command to generate a blurred image sequence with your motion vectors, if you don't have a compositor that supports using motion vectors for tweakable blur.

[ruddiger52]

two ideas.

rebuild a scene with just the wheels visible that has no GI/FG and no glossy effects (blurry reflections...) render this with rasterizer. you should be able to get 1/10 the render time i bet. this is the best idea. you would just comp the blurred wheels onto the car render.



Im thinking this is the way to go. I'm also going to see what I can get out of the software render with this method. Thanks for the advice

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