Motionblur for nparticles

[clabbster]

Hi.

I use nparticles but cant get motionblur to work, i use a physical camera with motionblur enabled.
We did some tests and got MB when disabled the caching for particles, but now its broken again.
Is there something i´m missing?

Best regards
// Daniel

[clabbster]

Found out the biggest issue was when we cached particles, it works without caching.
But then we have a problem rendering on the farm.

- Daniel

[vlado]

When rendering nParticles, you must make sure that your cached values in the particle cache are exactly at the time moments that V-Ray will use to sample the geometry for motion blur. You can see which those moments are in the Maya output window when rendering. The problem is that with the nParticles cache, if V-Ray asks for the particle position/velocity at a time moment that is not in the cache, Maya will just return the position/velocity at the closest cache keyframe, rather than interpolate smoothly in time. This is not a problem for the cache of the regular Maya particles where this works correctly. If we were to fix this, we would have to parse the nParticle cache file ourselves to find the time moments of the keyframes, but we decided this was too complicated to implement right now (and should not be the renderer's job really).

Best regards,
Vlado

[ivaylo.ivanov]

In fact this should work in the more recent nightlies (finding the keyframes). However the method that Vlado described is the better practice and will produce the most correct result.

[SonyBoy]

Can someone at Chaos please explain the best and most recent workflow for dealing with cached nParticles and motion blur in Vray? Vlado's post is 10 years old so I'm going to assume it's out of date. Plus, I am not really able to follow exactly what it is he is saying. I'm surprised that this is even still an issue in Vray, but I am currently experiencing the inability to motion blur cached nParticles.

[SonyBoy]

Actually, it seems that all that's necessary is to add at least one sub frame sample to the cache in order to get motion blur. However, can someone explain why even this is necessary? I'm only blurring over one frame, so surely whole frame values should be all that's needed to get a simple linear blur between e.g. frame 20 and frame 21.