motionblur looks like slowmo on heli rotor

[Morne]

Hi all

I have true 3D Vray motionblur on my tail rotor which is rotating extremely fast. The problem is that with the motion blur, it looks like it is actually turning very slow and a bit blurry. Imaging 4 blurry blades (blurry from motion blur) appearing asif they're turning as slow as the seconds indicator on a clock.

What I'm thinking is that the motionblur needs to be spread over more frames at a time, to "stretch" the blur on the blades so you almost don't see them so it just appears like a blurry wheel.

Any other thoughts or tips you have, please let me know...

[Morbid Angel]

if I understand correctly, this can happen when the rotations per minute sync-up with frames per second. Is your rotor rotating at an actual accurate to realworld rpm value? Also are you using geometric subsamples for correct motion blur interpolation, for rotors you would usually need 6-12 subdivisions in motion blur geometry.

[vlado]

If you have fast rotating objects, you might have to increase the geometry samples for motion blur for them.

Best regards,
Vlado

[joconnell]

You're quite right Morne, What happens is that if you work out the correct amount of degrees a helicopter rotates at per second, in 3d terms it can often end up lying on top of itself multiple times within the same frame and building up into a softer, wider shape. I've put in accurate rpm values for two different helicopter rotors and gotten something that looks like a soft "x" patter rotating around slowly, and another version of the same "x" pattern rotating slowly but strobing. It's a coincidence that happens with the rotors appearing in roughly the same place multiple times within the one frame. A friend who did a short film with a lot of propeller planes done in mental ray skipped the motion blur and used a flat card with a rotor that was radial blurred in photoshop and rotated that slowly over the shot. It looked the same and shortened render times.

[Morne]

a recent project involving a superspray fountain made me rethink this problem. Originally the heli was done with standard cam and image blur and all was fine. When I revisited the project and updated it to LWF, I used standard default settings for the VRayCAM and this resulted in the undesirable result of the "slowmo" motion blur rotor. All I had to do was change my F-number from F8 (default) to something like F11 or F16 and expose accordingly. It's one of those "duh" moments

[samuel_bubat]

Well that´s actually not an unrealistic effect and you often see somthing similar in footage of real helicopter rotors.
Here an extreme case
http://www.youtube.com/watch?v=jQDjJRYmeWg
But the solutions allready posted above should fix it. Rotos Speed, MoBlur Settings and so on...