Cached nParticles and motion blur

[SonyBoy]

So, seems like it's a known issue and cached nParticles rendered in Vray will not render with motion blur? Is that correct? I have found a few threads about this (though not many) dating back a decade. Hard to believe that this is still not fixed? Is there a workaround? In production it's simply not practical to render a simulation locally and without caching.

[hermit.crab]

Maybe this thread will answer your question.

[SonyBoy]

That's the thread I found and to which I was referring. Unfortunately it did not help, and Vlado's explanation (apart from being a decade old) is not very clear at all. However, I found that simply adding subframes (a single one at 0.5 will suffice) is enough to get cached particles to render motion blur. But why is this not simply made clear in the documentation? Or is this what this convoluted workflow described by Vlado is referring to -- simply adding subframe samples to the cache?? It's very unclear, and shouldn't be for something this simple and obviously necessary.

[hermit.crab]

Turns out it is already documented here.

[SonyBoy]

I assume you are referring to this?

"Use interpolated cache – This attribute is available only with nParticle and is intended as a workaround when using nCache. In this case, Maya does not interpolate the particle positions between cache frames. The recommended workflow is to always have cache for frame times that V-Ray renders. If this is not executed properly, V-Ray tries to find information about the cache and uses the velocity to estimate the next particle position. If this option is on, V-Ray samples the cache where it's available and interpolates the data. This way the end position of one frame better matches the start position of the next frame. This process can be much slower, and requires all dynamics on the scene to be cached!"

First of all, whether or not this attribute is added and the box then checked seems to have absolutely nothing to do with whether motion blur is calculated. If you cache with substeps you will get blur with it OFF. It seems redundant. However, perhaps this comes into play with more than one frame subsample (for my current needs I'm using only one, just get some A to B blurring).

Secondly, the wording of this entire paragraph is bad. It needs to be completely re-written using the correct established language to refer to the process of caching with FRAME or TEMPORAL SUBSTEPS or SUBSAMPLES so that's it's completely clear what on earth you are talking about. "The recommended workflow is to always have cache for frame times that V-Ray renders" is totally meaningless. In about 99 percent of cases users are rendering whole frames. Substeps are used to create finer interpolation (such as for curvature in motion). Please rewrite your docs to just clearly state that substep caching is necessary to get nParticles to blur. This shouldn't be this difficult.

[hermit.crab]

Thank you for the feedback. Just so you know, I've made a task for the documentation team to start working on a tutorial and better explanation of the methodology of working with motion blur and cached nParticles (bug-tracker id: DOC-10434).

[SonyBoy]

Thanks! I look forward to this being improved.