CGTalk - rendering in passes and motion blur.

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smaug

05-26-2006, 03:08 PM

hi..

i am trying to create a mountain scene with jungle like forests..
i plan to render it in passes
i will divide it in both channels like color, occlusion etc. and layers like mountain, forest rocks. dust etc.

so in the end it will be mountain color, mountain occlusion, mountain diffuse shading, forest occlusion, forest color, haze, shadows etc.. lots of passes ;)

my problem is here..
i need motion blur on them so i think the bestway is to do it as the last step in the compositing to put in on top of all layers( please correct me if iam wrong)
i am rendering with mental ray so i can use lm2dv shader on everything to get the motion vectors in screen space and apply a motion blur using this.

but i want to use the motion bluroptio in mental ray because it looks really good and i can use lowerantialiasing samples with it because it blurs the image anyway.

if i render with motion blur how cani composite those layers. willi get problems at the compositing stage where my alphas or matte objects can mess with the compositing

and if i do that do i need to render the depth pass also with motion blur.

i am usings depth mapped shadow maps so will i get problems with them at post in both cases( render withmotion blur or apply blur at post)

so you see i am in need of suggestions while i am planning the shot..

can you help?

thanks in advance :)

oh by the way i am using xsi as main app. and i can use either its compositing or combustion

pgraham

05-30-2006, 06:08 PM

In most cases you can just do 2d blur at the end and it's fine, so I would recommend compositing everything before blurring and see how it looks. Often you'll have artifacts that you don't notice because they're hidden by all the motion in your scene. If you have a problem that you really notice, then you can break up your layers and blur them separately before comping.

I've only done 2d blur in post using maya software render and the blur2d utility, so it might not apply to your method, but there are a couple of inherent issues I've encountered that you might need to consider. If you have a background object move quickly behind a non-moving foreground object, the blur from the background object will bleed over the foreground object's edges. So you might want to plan your layers accordingly if you have objects moving quickly in the background. Also, there is only one motion vector per pixel, so when you have transparent objects or relfected objects, they will be blurred as if they are all moving the same way as the frontmost surface.

If you need to blur separate layers, make sure each layer is premultiplied before applying blur. Once again this is only from experience with maya's blur2d utility.

Shadows do not have any blur information. They will get velocity information from the surface they are cast on.

smaug

05-31-2006, 11:07 AM

thanks a lot for your reply..

i am renderign a couple of frames to see how it would work..ufortunatly i dont have acces to maya but i will try to add blur in post first and see id that works well..

thanks

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