Hi all,

I have searched extensively to find a “car-rig”-like solution to make wheels rotate based on the distance they have traveled.
I came across: Lyonz thread: (http://forums.odforce.net/index.php?/topic/13258-direction-spin [forums.odforce.net]) but that didn't came up with a solution.
I found tyson ibele's elegant solution in 3Dmax over here: http://www.simplycg.net/viewtopic.php?f=32&t=4630 [simplycg.net]

So i started to try to get a simple version of that in Houdini.
For this i want to calculate the total distance traveled and here comes the first trouble. I calculated the distance traveled in the last frame.
I created a null that does follows a path, and in chops offset it to 1/$FPS. Then subtract the second from the first and you've got the distance traveled in the last frame.

So my idea was: get the rotation of the wheel and add (distance * (360/2*pi*wheelradius)
But this isn't working because in sops += doesn't work as explained here:
http://www.sidefx.com/index.php?option=com_forum&Itemid=172&page=viewtopic&t=20063 [sidefx.com]

Alright, so i thought i'dd get the totaldistance covered so i brought it into dops and use a sop-solver, similar to how Peter Quint explains here:
http://vimeo.com/6663811 [vimeo.com] but i'm getting into trouble again. I CAN get the distances calculated in a vopsop in the sopsolver in dopnetwork but i just can't get them properly added to the already existing distance.

Needless to say, I'm starting to feel a bit stupid and i could really use some help. Maybe someone could shed some light on this?

Thanks in advance.

Rudi

I have found a solution to the “total distance” problem using a Python script in chops that is very well explained by Steve Twist on 3DBuzz's Technical Rigging V.

This is quite easy, assuming you're writing out some steps to disk. Once your car animation is complete, delete all but 4 points, one from each wheel (you could use some point on the axle, or really any point if the wheels aren't turning). Write out this as a sequence to disk. This will be fast to read back in because it's only 4 points per file. Next, load back in with a file SOP followed by a trail SOP. Set the trail SOP length to $F and ‘connect as geometry’. Presto, you have your 4 tire marks. Now use a ‘measure’ SOP to measure the perimeter of these. Use a prim() function to get access to the primitive attribute, and use that in your rotation calculation.

Antoine's method works well.

Here's another that uses CHOPs. I have my own Distance CHOP HDA that I use to quickly calculate distance of anything travelled. Then you can do the trivial trig to rotate the wheel.

Have a look at the attached hip file on one way to do a rotating wheel along a path..

Hi everyone. I stumbled on this thread and was wondering if you've tried using the Area Chop. It's my go-to node for measure distance. Usually I use a trail sop ( like Antoinne did ) but I only trail it for one frame. I measure t he distance between current and previous frame. Then I store that in an attribute that I pull into chops using a geometry chop. Following that with an area chop gives me a cumulative distance traveled between whatever frame range you choose.

hope this helps.
jonah