Writing into velocity volumes (or any vector field) question?

[catchyid]

Hi,

If I have a density volume (i.e. a scalar volume), then I can use volume vop and connect something to output "density" to write to it.

However, if I have vel.x, vel.y, vel.z, (i.e. a field volume) then how can I then write to them inside a volume vop? I found out that "parameter" operator will do that, however, I am confused, vel.* is a voxel data and not the "typical" definition of a parameter (which is a value per an operator)? I know it's not that important to know why (as long as it works), however I would appreciate it if someone sheds a light on this logic or maybe different way to write into field volumes (either VDB or native Houdini volumes)

 

Thanks

[coltonmil]

Hey catchy,

So vel is a vector field, which Houdini simply treats as 3 scalar fields within the singular volume.

You can actually think of them as very similar to point attributes.

At any given voxel for a vel.x field in a bounding box, you'll have a value (even in "empty" space within the bounding box). The same goes for vel.y, and vel.z. These are combined to give you the full vel field.

VDB works similarly it doesn't use a bounding box, and also uses very little processing for "empty" space. VDB also has scalar and vector fields, so your vel would be just one field (as opposed to being treated as 3 scalar fields like the standard Houdini volume).

[catchyid]

Hey, thanks for your answer  I think my question was not clear I wanted to know how to read/write vel.* in volume vop? I found out one can do that using "parameter" operator in SOP context, OR "bind" operator in DOP context. I am not sure why there are two ways to read it (why not just use bind in all contexts). I will create a sample file to show you what I mean (but I am about to take off now, so most probably will do that on Monday)....Thanks Colton for your quick reply...

[woodenduck]

You can bring vel in to a volume vop using a 'bind' vop. Name = vel, type = vector. Do whatever operations you want on it then you can use 'bind export', again set to name = vel and type  = vector. 

[catchyid]

Thanks for Joe