From: Chris Zwar <chris@chriszwar.com>
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Subject: Using optical flow to speed up 3D renders
Message-Id: <9205B60B-FDBD-41BB-B242-750BA0228ECD@chriszwar.com>
Date: Wed, 5 Mar 2014 22:48:11 +1100
To: After Effects email list Mail List <AE-List@media-motion.tv>
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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. =20

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=85

-Chris=