Motion Blur, add in render or post processing?

Just seeking what general opinions regarding motion blur.

Is it better to render animations with motion blur or add the effect in post processing?

At this stage I’m thinking post processing, as you then have a clean render to work with.

post motion blur unless you are getting artifact problems. rendered motion blur is performing the full render for each sample.

Rendered is slow, post is (relatively) fast. Beyond that you need to do tests to decide for yourself which works for your project.

Post is great, but bear in mind it doesn’t work in combination with DOF… If you want to do focus pulls etc you may want to think differently! (do DOF in post but motion blur rendered, or render different elements as differnt layers and animate blurs to fake DOF…

post is more control. I don’t know if sub-frame sampling is considered in post processed motion blur. I’ve always prop for post mo-blur. but I guess you’ll have to go with rendered with some renders.

just wanted to point out, that post-motion-blur also doesn’t work with things like raytraced mirrors or transparent objects.

so as long as you don’t run into such limitations, go with the much faster and flexible post-blur.

So how would you do “post” motion blur? You have to get the speed pass into your compositor, somehow, right?

What would I output from Blender so I could do my motion blur in After Effects?

Thinks like after effects and such to my knowledge use interpolated motion blur, where one frame is compared to the following frame and then the blur is calculated based on the difference in the frames. From my experimentation rendered blur will produce superior results on fast moving objects, but you really have to crank up the samples, which means long render times. However, I think post blur produces more realistic results.

@ronnke: it’s also possible to use speed/vector passes in AE to create motion blur.

our postpro guy used maya/mental ray in combination with AE and did post-motion blur on a project. as far as i can remember the speed/vector pass from maya was just a 2D vector that indicated the speed and direction in 2D space. i cannot remember the name of the AE plugin we used, but it must have been something like “reelsmart motion blur”.

however, blender’s speed pass works differently and i don’t know if there’s any AE plugin that works with blender’s native speed pass. blender delivers RGBA values. the position of the pixel in the previous frame (RG) and the position of the pixel in the next frame (BA). This is superior since it also allows curved motion. However, you need some math to convert blender’s speed pass to be usable with the AE plugin we used.

here’s a maya tut for general information:
http://aloedesign.com/2008/12/maya-2d-motion-vectors-tutorial

I often use the “timewarp” plugin that ships with AE. Timewarp can motion track the footage and then intelligently blur the stuff that’s changing from frame to frame. It’s quite good in most cases.

In AE, apply the timewarp filter to your blender animation. Change the speed setting from 50% to 100%. Then find the Motion Blur section, enable Motion blur, set Shutter Control to “Manual”, then tweak your shutter angle (180 to 220 usually – larger numbers create more blur) and increase the shutter samples until you have a satisfactory result.

These settings work for me most of the time; if it isn’t working, there are more resources floating around online that describe in more detail how the plugin works. It can get pretty complicated if you want it to…

http://blender.mostlydocumentary.com/images/basicTimewarpMBSetting.png

Edit: If this approach works for you, then exporting vectors from blender is unnecessary.