Motion blur problem I cant resolve

[kosso_olli]

Hey guys,
I have a serious problem with a rim I need to render for a car. We had a shooting back in Juli at night, involving a rig shot with spinning wheels. In reality, the camery will be attached to the car. The car is pulled slowly for about 3 or 4 meters, so that the wheel is rotating about 720°. Together with the long exposure time, in this case 30 seconds, you get a nice and fast looking motion blur on the rims. I am now trying to replicate that motion blur in Vray, without any success. Problem for me is the shutter speed. To get the 30 seconds exposure time from the real world set, I would need to type in 1/0.0333 in the dialog in the VrayPhysCam. However, I get a weird artifact when doing so. If I set the shutter speed to 1/0.1, equalling 10 seconds of exposure time, everything is okay, as in the first picture. With 1/0.0333 I get an error, as in the second picture. Geometry subdivs for the wheel are 32, which should be more than enough.

vray_motion_blur_prob.zip



Regards

Oliver

[joconnell]

What's happening is likely correct, with the geometry of the wheel overlapping itself multiple times within the same frame, and giving you that strobed effect. You get it quite often with helicopter blades where they look like a very blurred, but slow moving fan rotation slowly in the shot. Here's an extreme version where a camera shutter is synced up with the rotation speed of helicopter blades so they overlap each other as much as possible: https://www.youtube.com/watch?v=cxddi8m_mzk

[tashko.zashev]

Hi Olli,

If you make your animation larger, 2000 frames for an example, you will get the expected result i think.
900 frames are not enough for such an exposure.

[vlado]

This was a bug in the physical camera; it is fixed for newer builds, but for the time being, it might be better to use a regular camera and type in the appropriate frame duration.

Best regards,
Vlado

[kosso_olli]

Vlado,
thanks for the info. I tried to use a standard camera with motion blur. When I type in 900 (30seconds of exposure from the real cam, multiplied by 30 frames per second) for the duration, the render comes to a halt. Sent it to the farm, all buckets are stuck. Expected rendertime kept increasing to 999 hours. Geometry samples for the wheels were set to 12. You can see the other settings in the screenshot.



Regards

Oliver

[tashko.zashev]

Is there any warnings or errors in the Log window?

[kosso_olli]

Sadly not, no warnings at all, except for the subpixel mapping. I think it just takes that long to render.

[vlado]

Well, this worked perfectly fine for me and rendered in a couple of seconds. However there may be something else going on in your scene - can you re-post it with your new settings.

Best regards,
Vlado

[joconnell]

Just curious Vlado, what was the motion blur issue with the physicam? The folks I work with swear that it got cleaner results with motion blur with the same settings which I found hard to explain!

[vlado]

The motion blur interval was not entirely correct in certain cases, but that's all.

Best regards,
Vlado

[joconnell]

Super, cheers.

[kosso_olli]

Vlado,
I didn't render the dummy wheel from my first post, but the actual rim on the car. There is an HDRI and some reflection cards in the scene, but nothing too complicated. If I only save the wheel and the light, I think I can send it to you. But it will be some hundred megabytes. Can I upload these somewhere? It is confidential...

[tashko.zashev]

Please send us an email to vray@chaosgroup.com and we will provide you with account for our FTP.

[kosso_olli]

Thanks to Svetlozar I managed to get the scene working with the desired motion blur result. However, it still renders incredibly slow at full resolution with production settings. Will send it to the farm tonight and see if it gets rendered in a single night.

Regards and thanks again

Oliver

[vlado]

Thanks for the scene; I will profile it to see where is the bottleneck.

Best regards,
Vlado