Particle Creep Instances Correct Rotation

[natalia]

Hello everyone.

I've been working with Maya for quite some time and I'm quite new to Houdini. I'm trying to achieve a bunch of particle instanced objects to creep along a surface. I've managed to start that with the particle creep operator putting an expression on the U V parameters. (What local var is the U V param as I want to be able to check if they are over 1 or under 0 so I can loop. A modulo %1 loops only if it is above 1)

But now I was wondering how to calculate it's velocity and getting the surface normal so that I can orient my instanced objects with expressions based on the values I've gathered. For the velocity It's simple I wish to get the world position of the particle at frame T-1 and at frame T and substract the results to get the velocity. How do you achieve this in Houdini as I'm so used to write code for particles in Maya. I would normaly update the old position after I've calculated the velocity.

For the normals I simply can't figure out wich node could help me.

Any Ideas anyone ? that would be great

Nat

[Wolfwood]

For velocity look at the Trail SOP.

If you are using the Creep SOP then the points will inherit whatever attributes are on the surface, if you are using a Creep POP then you can use either an AttribTransfer SOP or the primuv() expression to lookup attributes.

[natalia]

For velocity look at the Trail SOP.

If you are using the Creep SOP then the points will inherit whatever attributes are on the surface, if you are using a Creep POP then you can use either an AttribTransfer SOP or the primuv() expression to lookup attributes.

Hey Thanks alot !! got it working. I used primuv() in the upvector POP and computed the velocity with the trail SOP. The only thing I would like to add is that to compute velocity with the trail SOP is that I had to birth all my particles on frame 1 so that there is always the same number of points for the SOP operator to work properly.

Many Thanks

Nat