Cycles motion blur: how to get predictable render times?

Hi all,

trying to render about 250 frames of animation in Cycles I run into a massive render time problem: most frames get along pretty well with render times about 10 minutes, but unfortunately some freak out completely, there’s a single one rendering right now for more than 7 hours. The crazyness is that there are only marginal, incremental changes compared to previous frames, which still render at about 10min. Background plus one character travelling from left to right, camera panning, scene split in two render layers, foreground char, and background. One more frame, and I have to interrupt Cycles because it will go nowhere, the foreground layer rendering forever.

Is this a common issue with motion blur renders?
Is there a way to debug it, single out conditions which would lead to that behaviour and work around them? I could of course add motion blur in post pro, but that’s a different game quality-wise …

You could use the new embree branch form @skw maybe.

In my experience, often "any blur," especially vector blur (done in post-processing) is usually quite satisfactory.

“Absence of” a blur is what draws the eye and looks like a mistake. Accuracy of that blur really doesn’t.

If you’ve suddenly gone from 10 minutes to 7 hours, then, “undoubtedly, someting is wrong,” and we’d need to see your blend-file to find out what it is.

True, and I tried already, but probably dropped it too fast, for quality AND - funny enough: speed, looking back and ahead one frame to compute motion vectors slowed things down for most frames.

Well, I’ll take another look then.

If you’ve suddenly gone from 10 minutes to 7 hours, then, “undoubtedly, someting is wrong,”[/quote]

Yes, I thought so. :slight_smile:
However it seems to me I’ve run in the very same problem Tangent Animation hit during their production, the one Stefan Werner tackled with Embree. Only I thought the patch was already part of 2.79 (which I am using).

Yep, I know.
Unfortunately it is not that simple to upload however, the file makes use of linked libs and alembic caches …

that embree branch, are there builds available somewhere I could download to give this a quick try?