Unreliable Motion Blur

[danio1011]

Hello - I've recently started to use lights + motion blur to generate headlight streaks (archviz) in the occasional render.  Unfortunately I'm finding it a little unpredictable when it comes to results.  With keyframed lights that should blur past the camera I often get no result at all (black VFB).  I have to turn on and off motion blur, start and stop the render, move the keyframe around, and then 'bam' it renders as it should.  Then I move the keyframe slider or some other minor variable and the lights disappear altogether (i'm rendering them isolated with a direct viz override of black, so I can composite the beauty pass right on my base render.)  If I change the value back to where it was, they still are gone.  It then takes more fiddling to get it to reappear.  It really seems a little random.

Any ideas?  Would love to use this technique more in the future, it's just not proving reliable.  Probably user error.

Thanks!
Daniel

[romullus]

Sounds more like a bug. Can you provide more info? Is this happens with interactive or with regular rendering? Couls you recplicate that in simple scene and attach it for devs to look at?

[pokoy]

A general note, not sure if it applies here: if those lights and their streaks are small (a few pixels and less) it might be that it's just the sampling not picking them up and you need to wait longer. I had a scene where I needed this and ended up rendering elsewhere. Raytracers like Vray/mental ray will produce better results, as strange as it sounds.

[danio1011]

Thanks both.  Yes I have a stripped down scene I can send.  And I also get different results (sometimes) between IR and final.  It may be what you're saying, just need to wait longer.  But I have given it some time and not seen anything show up.  I tried UHD vs Path and also tried disabling the new light solver.  I probably should do the streaks in PS but these look so nice...

Anyway, I'll send the scene tonight.