nparticle Glow goes off when rendered in MR with Motion Blur

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  01 January 2013
nparticle Glow goes off when rendered in MR with Motion Blur

I am facing the glow shading issue when i am trying to render nparticles in Mental Ray with Motion Blur ON ( its set to full Deformation ). In the Glow shader, Auto exposure is unchecked , & in nparticles caching is set to ALL.

Without Motion blur the particle renders fine with glow but with motion blur set to ON ..even the particles which are not moving the effect of glow goes On & Off for some in between frames. I can understand the fast moving particles doesnt gives much color info but the for the static ones the effect is going On & Off which causes a flicker or a jump in the brightness values. is this a bug or am I missing something here ?

I ended up rendering them without glow & did it in post.
- Vik

Last edited by viki164 : 01 January 2013 at 05:33 PM.
  02 February 2013
hey vik

ive just done a shot where we used contribution passes and some nifty final gathering settings, in co-operation with the shading of the particles kinda done in the incandescence internal ramp.( well honestly i just rgb'd them and also adressed the color in nuke, )
however, doing this instead of the "glow" actually rendered faster and with the cont passes we had complete control over the glow's reflections separately from the particles glow.

i am still a green horn here, but i just wanted to share that little work flow for making fast rendering mental ray glowing particles

thanks for your time man,
  02 February 2013
Viki, I think you ended up doing it the best way.
My thought on why this was happening is that if the nParticles were caches only on each whole frame then there is actually little to no sub frame movement. So the moition blur is going to make the already finnicky glow even more finnicky.
nParticles annoyingly don't give you any subframe interpolation which is super annoying! A way to test is to just break the connection of the nCache to time and then connect some random controller that will let you scrub the value. You will see that there is really no movement between frames.
Totally lame.
anyway, just wanted to post a little fyi
  02 February 2013
Thanks Ritchi for the workflow tip , I will look into it more about contribution pass.
Thanks Benjamin , yes the sub frame interpolation issue sucks. I tried to notify Duncan few months before ..I hope they will fix it soon.
for now I find it easy to add it in post with some threshold value so only brightest particles glow.

- Vik
  02 February 2013
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