MBlur missing buckets

[tricky]

I have a rotating...rotor. A fan essentially.

I am enabling motion blur but some buckets are rendering black, towards the outer edge of the blur. This is local rendering only. VRay 3.40.01

[tricky]

It could be related to the default minimum image samples. At '1' I get some missing buckets. Increasing that to '2' and it appears to solve the problem. I will keep testing.

[vlado]

Yes, increasing the min. AA subdivs is the solution.

Best regards,
Vlado

[tricky]

Yes, increasing the min. AA subdivs is the solution.

Best regards,
Vlado

OK. I guess this is where my understanding of VRay starts to fall apart, big time. To me, AA (antialias, right?) is the slight blurring around edges of objects/colours/mattes etc. In the Photoshop world, it affects the fringing of one layer over another. Why would something like this yield black buckets in motion blur? Clearly my way of thinking is wrong.

[vlado]

OK. I guess this is where my understanding of VRay starts to fall apart, big time. To me, AA (antialias, right?) is the slight blurring around edges of objects/colours/mattes etc. In the Photoshop world, it affects the fringing of one layer over another. Why would something like this yield black buckets in motion blur? Clearly my way of thinking is wrong.

It is related to how the feature is implemented in a raytracer - by shooting more camera rays into the scene. If you only have a few rays, it may happen that none of them hit the fast moving object, and V-Ray (wrongly) assumes that the bucket is empty. If you add more AA samples, then more camera rays are shot and there is a greater chance that some of them will hit the object. Then the V-Ray adaptive sampler can take that and explore the area around to shoot more rays as needed.

Best regards,
Vlado

[tricky]

Thanks for the explanation.

I guess another thing that surprised me with this mblur missing bucket 'issue' is that even with min set to 1 and max set to 20, I am still getting some buckets on the edge of the blur that are black. I wouldn't have expected that. I'd expect an overall loss in quality/increase in granular noise rather than blackness.

[Morbid Angel]

Tricky any raytrace renderer is blind when it begins to render, it does not know what is seeing from camera until a ray intersects a polygon. If min aa is set to 1, as vlado explained few rays that are shot miss that tiny polygon and vray assumes there is nothing in that bucket.

Often times I have to set to min aa 3, and never set anything below 2 for that reason.

[tricky]

Often times I have to set to min aa 3, and never set anything below 2 for that reason.

Understood. I guess my general thinking was that the default settings are reasonably OK for all situations. Its all getting a bit clearer to me now.

[tricky]

Could you suggest the kind of settings you'd expect to have to push to for something such as this as I am still getting missing buckets even with min=4, max=40. As I haven't really changed the minimum from anything but the default of '1' for many years, and we have rarely, if ever used motion blur in our work, I'm not sure what I should be expecting.

[Morne]

try min AA of 2 or 3

EDIT: on top of this Vlado recommends making buckets larger

[tricky]

try min AA of 2 or 3 and make your bucket size smaller. Bucket size of maybe 10 or 8?

So is changing bucket size part of people's workflow to achieve 'correct/proper/good' results then? I thought bucket size was purely concerned with render efficiency (i.e. the way something is divided up between CPU cores)

[Morne]

...increasing min samples can help.

EDIT: Vlado recommends making buckets larger

[Zdravko.Keremidchiev]

Smaller bucket size might help in this situation. Since those camera rays will be shot in a smaller area, the probability for them to hit the moving object are greater.

[anthonyh]

hi,
i`m coming across this black square issue on helicopter rotors a fair bit now. what`s the solution, as raising min aa/smaller buckets isn`t solving the problem. only rendering with noise threshold of 0 or using progressive or fixed seems to work but that comes at the cost of slower render speed. it means i can`t benefit from adaptive rendering speed benefits now ?
is there a solution ? will there be one ?

thanks
anthonyh

[vlado]

I think Zdravko meant *larger* buckets, not smaller ones. Smaller buckets are likely to make the problem worse.

We have only two solutions to this problem:
*) Increase the min. AA rays and increase the bucket size, or
*) Use the progressive sampler.

This is just a tough case for the bucket sampler to resolve.

Best regards,
Vlado