Apologies for double posting this - I posted as a reply to a thread that has SOLVED in the title so I guess some people might not read it.
I've been doing lookdev on a scene that needs very heavy motion blur for something spinning very fast and it was looking great in the XPU viewport, also flipbooking. However if I render with a USD Render LOP I get MUCH less blur. I've tried all sorts of things, MBlur LOP, cache LOP, different settings on those but I can't figure out how to get a match to my viewport.
This pair of images are using the same settings, the one with the heavy blur is XPU viewport, the other is with USD Render LOP. Instantaneous shutter is off. You can see in the top right corner there is a very small amount of blur...
The thread I originally replied to - https://www.sidefx.com/forum/topic/93945/ [www.sidefx.com] - said that there had been some problems caused by using frame rates that were very different from 24FPS but mine is 25 and the thread says also that issue has been resolved. Essentially I just want to set my scene up with MBlur in the viewport and those results to be matched by my ROP.
Thanks for reading
Motion blur viewport vs USD Render LOP mismatch .
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Thanks for the direct message. The short story is that the USD Render ROP's defaults aren't great for your workflow, which is something for us to think about (i.e., you really shouldn't need to know all the details that follow).
To optimise on-disk storage, when we export USD files we cull time samples that are "out-of-range". For example, if you're only rendering frame 10, then you only need the samples from frame 10 ... ish. Motion blur complicates this, because actually you need to expand the "range-of-interest". In the ROP, we expand the range by one sample, under the assumption that there will generally only be useful time samples at shutter open, maybe on the frame, and at shutter close. The assumption fails in your case because you have many sub-frame samples between shutter open and shutter close, and we're only keeping the ones closest to the frame (e.g., frame 9.95, frame 10, frame 10.05). This is why you get *some* motion blur, but only a tiny bit.
While unfortunate that you need to know this and need to do something about it, this is easily addressed in the USD Render ROP. You can either change the "Filter Time Samples" mode to "Never", or set "Extra Frames" to 0.5 (or something similar), to further pad the "range-of-interest".
I won't post screenshots of your scene but, when I set the ROP as per the attached image, the viewport and mplay match.
To optimise on-disk storage, when we export USD files we cull time samples that are "out-of-range". For example, if you're only rendering frame 10, then you only need the samples from frame 10 ... ish. Motion blur complicates this, because actually you need to expand the "range-of-interest". In the ROP, we expand the range by one sample, under the assumption that there will generally only be useful time samples at shutter open, maybe on the frame, and at shutter close. The assumption fails in your case because you have many sub-frame samples between shutter open and shutter close, and we're only keeping the ones closest to the frame (e.g., frame 9.95, frame 10, frame 10.05). This is why you get *some* motion blur, but only a tiny bit.
While unfortunate that you need to know this and need to do something about it, this is easily addressed in the USD Render ROP. You can either change the "Filter Time Samples" mode to "Never", or set "Extra Frames" to 0.5 (or something similar), to further pad the "range-of-interest".
I won't post screenshots of your scene but, when I set the ROP as per the attached image, the viewport and mplay match.
Edited by robp_sidefx - June 24, 2024 17:14:35
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