Using the velocity channel of an object in that object's material?

[HowardDay]

Is there a way to do this? I'll give you an example of *why* I'd want to:
I've got a background sphere with stars mapped on it - as the camera moves, this motion blurs and looks really cool. Unfortunately the faster it moves, the less visible the stars are - while this is physically correct, I'd like the ability to control this a bit more. if I increase the brightness of the stars so that they're correctly visible while turning, they look great, but then when the camera *isn't* turning, they're so bright the stars are triggering my post-process lens flares.
I'd like to be able to link the brightness of the stars to the in-camera velocity speed. Is there any way to do this, currently?
Thanks!

[HowardDay]

Any thoughts? Or am I completely out of luck on this one.

[theedge]

Hi HowardDay,
Animate the stars brightness would work maybe ?
Or apply motion blur in post to tweak it as you want ?

[HowardDay]

I'm already hand-animating the brightness of the background stars - unfortunately quite a few of my camera moves are highly procedural - and even a slight change can dramatically alter timing. I'm really hoping for an automatic solution.

[hermit.crab]

Have you tried playing with the sphere's Motion blur samples and duration (in the V-Ray object properties)?

[HowardDay]

I have - those settings are disabled in the GPU and RTX renderers. Fiddling with them has no effect.

[HowardDay]

Any further suggestions? Or solutions?

[gandhics]

DCM to get velocity > Put in UV > Use VertexColor map to render UV channel color
Oh.. only cam is moving.. well,, that's more complicated...

[HowardDay]

Indeed! But you did put me onto a solution. It's hacky, and not completely automatic...but I can stick a linked ball around the camera, use the DCM to get the translation-less velocity in world space, shove that into a vertex color channel, then use the ABS value of each channel, additively to figure out a overall "velocity magnitude". I can then render a 256x256 image sequence from the same camera, then use that grayscale animation as an additive multiply for the stars. I've tested, it works. If there was some way to automatically get the vert colors from the speed-ball to the star model (which is just another bigger, non-rotating sphere) then I could cut out that velocity mag image pass. I cant think of a way to do it, tho.
Thanks!