displacing volumes using a velocity field in SOPs

[bhaveshpandey]

Hi, I have a particle sim and a density field generated from them.

I wanted to know if we could displace volumes using the velocity of the particles? (without using DOPs for advection)

I did a little test by extracting the velocity from particle into a vel field. I merge this field with the density field and

then within a volume vop use this vel field to predict the next position. Using this, I displace the original position

and then do a point cloud look up on the Particle's age (which creates the density) using the new displaced position and add it to my density.

I'm sort of lost here.Not sure if the logic makes sense?

Its creating sort of weird Xray kind of effect on volumes

Displacing geometry is pretty straight forward but how does this work with volumes?

Edited July 30, 2011 by bhaveshpandey

[anim]

this may help you

http://www.sidefx.com/docs/houdini11.0/examples/nodes/sop/attribfromvolume/DeformVolume

[old school]

Displacing a volume by another field or point data is more than possible in SOPs using a Volume VOP SOP to do the work. The idea is to build the displacement vector then index in to the volume from P-vector then using this new position look up in to a Volume Index From File VOP.

When working with grids it's easier if everything is a grid, especially if you use the Volume VOP SOP. It's easy to convert a point attribute in to a field using the Volume From Attribute SOP. If you really want points you have to use point clouds in VOPs.

There is a cool thread that shows what is possible using this technique here inspired by Jerry Tessendorf's work:

I also attached a hip file that goes through the process step by step with some comments. I like experimenting wrapping up noise in For Each SOPs to pull the volume about.

[bhaveshpandey]

Erm..did you post the example on the above link? I cant see it. Some really cool workflows in there.

So we can do most of the volumetric displacement effects which were possible in i3D using volume vop?

The volume gradient from file can get us the gradient and the volume index from file can look up any volume

field?

What would be the difference between i3d and houdini's native volumes then? Or is it that there wont

be any difference in near future?

I was looking into this, to create a volumetric splash. I have a fluid sim and I was having issues in

shading the particles to match/look more like the ones in my reference. So I thought if I could get/mimic the

same using volumes without having to resim again(I try to keep things in SOPs as much as I can),

I could get nice volumetric details (instead of faking it by upping the particle count)

So if we could convert it to a density field and then somehow use the pyroclastic workflow using velocities etc

we could get good results??

I'll give it a try once i'm done with my projects.

Thanks.

[old school]

Here's the file.

i3d vs native volumes? They're both volumes. Since later versions of Houdini 11 save volumes in a tiled format, the line is now quite blurry. i3d can encode many different attributes per voxel while you have to save a discrete volume per attribute in the .bgeo file. i3d isn't nearly as seamless to work with as volumes in .bgeo format.

You can get at the same results with either format so use the one that is more convenient. For me that's .bgeo files for most things.

Yes you can use point clouds to form volumes. Best to use the VEX Volume Procedural VOP being fed by the generic CVEX VOP that accesses your point cloud and builds density from that. You can also use the point instancing technique wrapped around the VEX Volume Procedural and CVEX in one of the files that was posted in the thread mentioned above.

volume_intro.hip.zip

[bhaveshpandey]

Thanks a lot!