Motion blur in moving wheels

When a wheel, or fan or something similair, rotates very fast, you obviously see a blur. When you capture it on film, you get the added effect of sometimes seeing the wheel rotate backwards.

In my experiments with motion blur, vector and “normal”, I have been unable to create either properly. Both suffer from the problem that they don’t capture the movement the wheel makes between frames.

Take a look at this example for example. The tips of the fans stick out of the blur and it doesn’t look very good in general.

Is there a way to create motion blur so that rotating wheels look like they do when filmed, with the backwards rotation effect and such?

I do know it’s possible in 3D software, because the intro movie of Need for Speed Hot Persuit 2 has it.

The reason That the wheel looks like it rotaing backwards in films is because that the camera only captures the points where the whell allready has turned ones… So maybe if U take down the frame rate… Ask if U don’t get it…

I know all (well, some) about nyquist frequencies and sampling and such. With fiddling, I can sort of make it appear it turns backwards, but the blur is still wrong.

A very important reason, I think, is that when a real object is blurred on film, you don’t see the real object + some blur. Rather, the object is blurred over the area it moves from beginning to end of the frame. In my tests, I can always see the base object, and blur added to it.

A second attempt. As you can see, the backwards motion is there somewhat, but the blur exists only like a spider web in the middle of the “wheel”.

hm… Maybe U should adjust the vector-blur in the node-system… But I don’t know then… hm…

try combining the node vector blur with the standard motion blur, that should help smooth those lines.

I thought I’d give it a go. So I made a propellor, put little holes in it, 4 blades, and a nice red stripe…,

It’s turning 180 degrees every 10 frames. Anti-clockwise. The holes appear to turn clockwise.

I did this with vector blur using 96 samples and turned right up to 2.00.

Check out my rotor…,
http://www.youtube.com/watch?v=MWXgf8YiTpc (you may have to wait a while, I just uploaded it) :smiley:

Do ya know what? It looks crap on YouTube. So, if you like, you can download it from here. Probably best to right click and Save as…

try combining the node vector blur with the standard motion blur, that should help smooth those lines.

That does improve things a bit. It unfortunately makes rendering very slow… The scene I’m planning will easily take about 10 minutes per frame, I guess. With mblur added, this five-folds…

thought I’d give it a go. So I made a propellor, put little holes in it, 4 blades, and a nice red stripe…,

It’s turning 180 degrees every 10 frames. Anti-clockwise. The holes appear to turn clockwise.

I did this with vector blur using 96 samples and turned right up to 2.00.

180 degree per 10 frames is not very fast, it’s 75 RPM. A car wheel will turn quite somewhat faster. A quick calculation gives about 2000 RPM for 200 kph.

I made a new test, with the wheel of my F50. It slowly increases speed. As you can see, it’s downright ugly… This is without the normal mblur option BTW, it’s just vector blur.

Normal blur also has the effect that you can fully see what is behind an object. I mean, try to put up a note behind a fan. You won’t be able to read it when the fan is turned off, but you will when it rotates. It would be cool if I can render my moving wheel so that I can see the brake calipers through the rim, but with vector blur at least, this is not possible, since it only smears existing pixels.

I will re-render this animation tomorrow with the normal mblur enabled as well. But that will take some time. This took about 1.5 hours to render already…

I do hope I can find a way, because proper wheel movement is quite important for the feel of the animation I’m planning.

BTW, how did you put in that text? I’ve been wondering what method I should use in my final image.

I think the secret you are looking for can be summed up with two words…,

fake it

Created a pre-blurred texture
Move it backwards
Take down the alpha
Obviously, change the texture back when the wheel slows down.

I make it sound easy, don’t I :wink:

by the way, I added the text with EditStudio 4.

Hmm. I’ve got no problem with faking, but your suggestion involves quite a lot of work. Also, I’m not sure I can get the rim-caliper effect I mentioned.

But you know, perhaps this is how they did it in that Need For Speed HP2 intro movie, by faking it. It’s available as google video, I discovered. So, now you all can see what I mean :slight_smile: That is, if the low quality of google video permits it.

How about actually rotating it backwards on the high speed portion? Or maybe rendering twice, one with normal forward rotation and the other with backwards rotation and superimposing them later?

Rotating backwards is not an option, because then there is no blur. Especially the logo on the tire will look weird.

Rendering the wheels twice might be an idea, I will have to look into that.

In the mean time, I started the rendering with mblur enabled. Going at a whopping rate of 2 minutes per frame.

The rendering with mblur+vector has finished. The result is hardly better.

I guess I will need to explore all the fake methods.

have you cranked up the mblur factor all the way to 5? It seems to distribute the motion blur better, at least on my simple test.
Also, try playing with the speed of the rotation. It doesn’t have to be really fast, just fast enough to fool the eye. At certain speeds, the backward rotation illusion kicks in…

I have been experimenting some more, and trying to figure out what all the parameters do. The normal mblur value determines the area outside normal frames in which intermediate steps are calculated. This means that at 1.0, the blur is exactly calculated between two frames. i.e. it renders frame 1.1, 1.2, 1.3, etc and blurs it into one.

Now, something that is new to me, is how to control the amount of intermediate steps that are rendered. This is done with the OSA level setting. When at 16, it calculates 16 in-between-frame steps, which are then blurred into one. This looks a lot better, but still a bit too discrete. I added a 0.1 vector blur to try to smooth this out.

It’s rendering now. I guess it will be done in 24 hours or so :slight_smile:

I’ve made a new test with my simple rotor. With the new settings, it looks quite good. It takes quite a long time, however. I’m continuing the render with my wheel.

Can you render in separate passes (i.e. wheels by themselves of course) and composite?

Or are you doing that already?

I am currently only rendering wheels, there is no scene yet. But for the final product, that might be a good idea. I have to figure out how first; I have no idea how to do that and if the passes interact correctly with eachother (lightning and shadows etc).

I have completed another test. It’s almost looking good enough… There is still this annoying contraction to the middle at some point. I wonder why.

The mid point you speak of (where it stuffs up) I think is where it is getting close to rotating 180 degrees per frame. I think it’s because the ‘blurs’ are trying to converge on the center, or really, the other side of the wheel. I think you should add in the ‘fake’ texture just after it gets to the point where it is rotating 1/4th of the way around per frame.

I also noticed in that need for speed video, they hardly ever show the blurred wheels you spoke of, and when they did, it was from a distance, or in slow motion. When I saw it from a distance, there was no blur.