Hi,

i have a obj Sequence imported in to Houdini and i got the velocity already computed using a trail sop, but now i want to have the acceleration. on an old thread from 2004 i found an hint to use a slope chop to get this done, but since i have point based data i have no clue how to import the per point available velocity to chops to use the slope operator.

could somebody give me a hint how to get this workin or an other clue how to get the acceleration calculated ??

thx

Sebastian

Read the data in to CHOPs using a Geometry CHOP. Point it at your SOP to reference in your points, Attribute scope left at P.

At this point you have the choice of bringing in the geometry data as Static or Animated.
Static gives you three channels with samples representing each point in the data. The result is your channels will have the same number of samples in time as points in your geometry.
Animated gives you three curves per point over the duration of the scene ($FSTART, $FEND).
You want Animated so turn that on.

Append your Slope chop and set to Acceleration.

Note: You can also bring in the velocity attribute above and change the Attribute scope to v. Then you will need to set the slope chop to Slope…
Or, you can bring in the points P and use two slope chops: One to calcuate velocity and the next to calcuate the acceleration.

Exporting your new attribute to SOPs requires that attribute to already exist.
Use an Attribute Create to create an attribute called “accel” to be consistent with POPs for no other reason. It should be a vector type and just leave the values at default.

Append a Channel SOP to the attribcreate SOP and change the method to “Animated” to match the Geometry CHOP.
Leave the channel scope to t if you fetched in the Position P attribute or change to v if you decided to fetch the velocity attribute (no real need in this case).
Change the Attribute Scope to accel to match the attribute you just added to the geometry.

Done.

The geometry spreadsheet for the Channel SOP will indicate that yes there are acceleration values picked up from CHOPs.

@thx Jeff ;-) (even if its some time ago)

sorry to dig out this thread but i got an problem direct connected to this.

According to Jeffs example i now have the acceleration & velocity curves for every point of the mesh.

what i now want to do is to combine the x,y,z channels of the acceleration curve to get the length of the acceleration vector each frame.

so i found the math chop and the length operator but it doesn work on the “combine chanels” because it combines _all_ chanels and not the x,y,z for every vertex. and the combine chops does not work too … i dont know why.

if somebody has an idea it would be great to share it with me ;-)

length($ACCELX,$ACCELY,$ACCELZ)

thank you edward

but is there a way to to this with chops ?

Sebastian

It just occurred to me that you can probably just change the Scope parameter on the Math CHOP so that it only computes the length based specific channels.