I use AE with depth passes to simulate DoF in 3D scenes, that is a major time saver as true 3D DoF can triple render times. In my experience using reelsmart motion blur, a vector pass is definitely required for anything that's going to be seen close up. Neat video is great but there is little substitute for properly-tuned GI. Might be worth investing more time to learn the renderer than trying to cheat it -  Vray can produce super clean results quickly if you invest the time in tweaking the settings and using cached light maps, etc.

Overall, I think there would be too much judder for anything with reasonable motion to it with an optical flow plugin replacing frames. However it could be great for slow-moving or stable 3D objects like a CG set extension or other BG replacements. Something that is a principle onscreen like a character or CU of a vehicle etc, you may not be able to get away with, as you could have artifacts and / or framerate issues.

In my experience RSMB, even with good vector maps, is still not that close to photorealistic motion blur, and it operates on the same principles.

The hours you spend looking for a simple AE cheat may end up better spent on just renting some distributed render farm hours to render everything at full res, prices are pretty reasonable these days. I've had good experiences with renderrocket.com Just my 2c

-TG

On Wed, Mar 5, 2014 at 6:48 AM, Chris Zwar <chris@chriszwar.com> wrote:

I have this vague recollection that optical flow technology (e.g. twixtor and reel smart motion blur) was originally developed to help speed up 3D rendering workflows.  I think the idea was that 3D animators could render out every 2nd or 3rd frame, and use an optical flow plugin like twixtor to create the in-betweens.  Even if that's not strictly correct, the potential is there.  Some of the photo-realistic projects I've worked on have had 3D render times of 3 - 5 hours a frame.  Rendering every 2nd frame is effectively halving the overall render time, which can be a massive saving.  Even a slow After Effects plugin is usually only seconds per frame, not hours.

So I was wondering if anyone has actually done this, or tried using other 2D techniques to help speed up 3D rendering.

I can think of 3 ways in which slow 3D renders can be compensated for by faster compositing techniques:

1) Up-resing.  For example rendering at 720p instead of 1080p and scaling up the finished renders.  If compositing multiple passes, only the slow renders need to be smaller and scaled up.
2) De-noising.   Forgive me for not knowing the correct terminology, but when rendering with global illumination it seems that there's an overall quality setting that directly determines both the speed of rendering and the noisiness of the image.  Rendering with a lower setting can make renders noisier, but a de-noising plugin such as Neat Video can fix this.
3) As stated above, rendering every 2nd or 3rd frame and using something like twixtor to create the missing frames.  A motion vector pass would make this more accurate.

So I'm familiar with 2 of those 3 approaches - I have worked in situations where 3D passes are rendered at smaller sizes and then scaled up.  It works very well and the time savings can be dramatic when dealing with very long renders.  3D renders can be so clean that they scale up very well.

I have also worked in situations where the neat video de-noiser was used to compensate for noisy GI renders, and again the savings can be dramatic - in some cases this can almost half 3D rendering times.  Neat video seems to be an incredible plugin, so much faster and so much better than the AE equivalent.

So that leaves the optical flow technique as the one I haven't tried yet.  Has anyone done this?  I'd love to hear from real-world examples where people were able to render every 2nd or 3rd frame.  Is a motion vector pass essential for it to work properly?

Any other thoughts or insight welcome…

-Chris
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