motion blur rims/tires

[jbeau3d]

I'm new to vray and wanted to know what the best approach is for rendering motion blurred rims and tires. I'm trying to create something similar to this image.


3d motion blur vs velocity pass?

[Dariusz Makowski (Dadal)]

More like...100 layers in photoshop mate. Forget about 5 min job...

[MoonDoggie]

If you really want to do it in the render. Animate your wheels, and make a VRayPhysicalCamera and turn on motion blur, check the manual on spot3d.com.

I would do it in photoshop as suggested by DADAL

[jbeau3d]

I don't mind rendering out passes, but I'm thinking like 5 or 6...
45 secs with 12 passes in render.
I'm really curious if anyone has good results from RSMB-->nuke.

[Morbid Angel]

you need to increase motion blur geometry samples to make the arch effect correct. Something like 6-12 samples should do. Now you cant do it on the whole scene you will run out of memory, but in object properties (vray) there is override default motion blur samples, you can use that to explicitly specify which object gets how many samples. Unfortunately you wont get the RSMB to arch like that.

[simmsimaging]

I would try and hit it in Vray if you can, at least for the rim. The post radial blurs are never really the same for that sort of anisotropic kinda look.

/b

[LarsSonparsson]

you need to increase motion blur geometry samples to make the arch effect correct. Something like 6-12 samples should do. Now you cant do it on the whole scene you will run out of memory, but in object properties (vray) there is override default motion blur samples, you can use that to explicitly specify which object gets how many samples. Unfortunately you wont get the RSMB to arch like that.

But is more geometry samples really needed?
If the object is just rotating and doesn't have any deforming animation,
Vray will render it fine with default settings, right?

EDIT: I was wrong. More geometry samples are definitely needed.

[Dariusz Makowski (Dadal)]

But is more geometry samples really needed?
If the object is just rotating and doesn't have any deforming animation,
Vray will render it fine with default settings, right?

EDIT: I was wrong. More geometry samples are definitely needed.

Nop u need more geometry... try going at 10/15 for wheels... that what I use normally for tire...

[joconnell]

The problem with post motion blur using something like nuke or rsmb is that it only gets one bit of information about the way each pixel in your render is moving, so for example if you put out a velocity pass with your render, it will tell your motion blur filter that a pixel is moving left, right, up or down or whatever direction. The motion blur plugin then smears the pixels in the direction that the velocity pass specifies. The main problem is that only having a single direction value for a frame means that the blur is always a straight line blur along the direction. This is fine if everything in your scene is moving in a straight line such as the background of the below picture:



The problem is that in a lot of cases the objects don't move in a perfectly straight line - they turn in an arc, like your wheel. Let's take a classic example of where post motion blur falls down - a propeller.



So if you tried to motion blur an object turning using post motion blur which only gets one direction to blur the pixels on you get this:



Not so good. The motion blur is looking at where the pixels are in frame one, then where they are in frame two, drawing a straight line between the two points and then blurring along that line. The problem is that a single direction is not good enough to properly describe the shape of an object that rotates or moves in an arc. What vray and other renders do instead is start to take directions for the pixels more times between frames. This is called sub sampling and in the case of vray, the setting that does this is geometry samples in the motion blur part of the camera dialog. The very minimum amount of samples you can use is 2 for motion blur, and with this vray records where an object is at frame 1, then where it is at frame 2. This gives you a linear blur between these two points. If you set this to 3 samples, vray records where the object is at frame one, then half way between frame one and two, then frame two. With this if you had an object rotating, you'd end up with an arc with two segments in it - a linear blur between where the object is at frame 1 and 1.5, then another linear blur from frame 1.5 to frame 2. If you turn the geometry samples up to 4, you get an arc with frame 1 to 1.33, then 1.33 to 1.66, then 1.66 to 2. So this will look like an arc with three segments to it. Effectively what you need to is put in enough samples so that the arc of the blur gets curved enough to look good. Here's the same propeller image with six motion blur samples:



You can see that it's still not perfectly smooth along the shape of the blur, but it might be good enough when moving to fool our eye into thinking it's a nice smooth circular blur.



The same thing applies for the headlight streaks above - to get a nice continuous line, you just have to turn up your geometry samples to a high enough amount that it looks like a smooth streak to you. In vray you can use the right click > Vray properties and set higher or lower motion blur values for individual objects - for example a helicopter propeller will need more motion blur samples than its main body. The more motion blur samples you use for an object, the more memory it will take up so it can probably help in heavy scenes to use 2 or three motion blur samples, then turn up the samples for objects that either move very quickly, or rotate around a lot.

[Alan Iglesias]

Interesting thread.

The bottom line is that, for the most part anyway, motion blur in your 3D renderer is designed primarily to smooth motion for animation - the same way photographic blur on film smooths motion. You are trying to create a still effect, and that is probably best done in post, as advised. That being said, I did get a fairly good still/blur tire/rim effect with Mental Ray years ago (see attached). You can probably do as good or better in V-Ray if you follow the advice here.

-Alan

[Dariusz Makowski (Dadal)]

Interesting stuff joconnell I wish only that Vray would actually re-sample geometry over period of time like we would set long exposure in camera...

Anyway we can get great results with some post work, check my profile pictures I got aston&bmw with spinning wheels. Neither are fully pushed in quality since they aren't the subject of render but still good enuf for commercial level.

[jbeau3d]

tossed a few quick shaders on this setup, (needs tweaking) but it still lacks the long exposure quality as you mentioned. I was thinking of creating a couple of a splines to simulate the exposure or maybe an expession. With my current settings the motion blur looks faceted, even though a single tire and rim geo are over 600k polys. As I increase the rotation value (currently at 1500) the blur starts creating some weird arc shapes, not what I want but still interesting to see.
Any idea on how to smooth out the faceted look? perhaps motion blur values in the properties as Joconnell mentions or maybe this is a byproduct of the rim shape.

[Dariusz Makowski (Dadal)]

U need to go to either object proporties or f10 settings and set the geometry samler higher... I usually stick to 10-15 value...

[simmsimaging]

Looks pretty cool but I would try higher mo blur samples and see if you cant smooth it out. Something to note about some of the reference images is that many show the effect of combined strobe and ambient exposures, so reproducing some of the artifacts will be impossible without post work.

[jbeau3d]

Hmm, now the faceting is even more noticeable on the interior rim at 15 geo and 15 mo blur samples; but the tire and outside part of the rim smoothed out. I going to try and up the rotations and see if it smooths out the interior.