Houdini supports 2 volume types, its own volume format, and VDB. These are treated as primitives like polys or nurbs, if you middle click and hold on a node, you'll see it say '1 VDB' or '1 Volume' or similar.
The most simple way to describe it is as Alembic, for volumes. It's an open source, standardised format so you could write a VDB in Houdini and render in Vray or Arnold, has several companies helping drive it, and is generally a good thing.
It's a little more than Alembic though, in that Alembic is mainly a file format, and a handful of example tools to manipulate and view Alembic files. VDB within Houdini is a primitive, and comes with a large set of tools to manipulate volumes, a lot (all?) of which are exposed as sops in Houdini.
The name VDB as stated in the original paper comes from "...Volumetric, Dynamic grid that shares several characteristics with B+ trees.".
Further reading: http://www.openvdb.org/about/
VDB vs Houdini volumes
Houdini's own volume format had been around for a while before VDB arrived, so there's a large amount of nodes in there for working with them (you can see this in sops by typing 'volume' in the tab menu). VDB has its own collection of nodes (either prefixed or suffixed with VDB).
While I haven't played with all the volume and VDB nodes, an area that VDB definitely wins is size on disk. A VDB when cached out can be easily half or less the size of its Houdini volume cousin.
There's a .vdb file format (which you'd use to export to other apps), but you don't need to write out a .vdb file to get the space savings, just having a VDB primitive in Houdini is enough. A Houdini volume primitive can be converted to a VDB primitive at any time, just put down a convertvdb node, set its mode to VDB. If you middle click on the node, you'll see the Volume primitive has been replaced with VDB. You can now cache this out to .bgeo like any other piece of Houdini geo, but it'll be much smaller than if you didn't convert.
Also, most of the core Houdini volume tools work with VDB primitives anyway. Volume VOPS and Volume Wrangles both work with VDB, I've been told a good number of the other Volume tools do too. There's a lot of overlap between the Houdini tools and the VDB tools, having seen a few production scenes, artists currently favour the VDB tools.
Further reading: http://www.sidefx.com/docs/houdini15.0/model/volumes
A volume (or vdb) can represent an amorphous shape like fog or fire, or it can represent a solid surface. When doing the latter, each voxel stores the distance to the closest point on the surface, and if its on the inside or outside of that surface. Because these values can be fractions (eg 1.42 units from the surface rather than just 1), they can give a pretty good approximation of the input geometry. To tell if a voxel is on the inside or outside of the surface, it becomes positive or negative; positive is outside, negative is inside. If you visualise it, it would look like a gradient that is 0 on the silhouette of the shape, and a smooth gradient that extends inside and outside the shape. This volume is called a Signed Distance Field, or SDF.
Positive values are red to yellow, negative values are dark to light blue.
This can be handy for many things. If you have a robust way to generate a SDF (which of course we do in Houdini), it's a great way to generate collision geometry for simulations. Particles or RBD objects can very quickly sample the SDF, if their sign is negative, they're inside the shape. They can then measure the gradient of the SDF, and determine exactly how far and how much to be pushed to stop intersecting.
If you take the SDF and convert it back to polys, you get an evenly meshed, watertight surface. You can smoothly expand or dilate the surface, you can combine SDFs and get very clean booleans, they're handy in lots of situations.
Houdini gives you 2 ways to generate an SDF from poly geo; the native Houdini way ( IsoOffset SOP in SDF volume mode), and the VDB way ( VDB-from-polygons sop). My 30 second tests imply VDB is much faster for more detailed geometry.
Houdini visualises SDF volumes with a sprite per-voxel where the SDF field is 0. It looks like it uses the SDF gradient to derive a normal for the sprites, so they respond to lighting.
Viewing volume data
Download scene: File:vis_volume.hip
Because a volume or vdb is treated as its own primitive type, the voxel data can't be inspected from the geometry spreadsheet as you'd initially expect. If you try, all you'll see is a primitive number for each volume in the shape, no data is displayed.
Instead, you're supposed to use the visualise tools within the viewport. A Volume slice lets you see a 2d slice of voxel data mapped into a colour ramp, a volume trails node lets you follow lines of direction within a volume, say graident, through the volume.
The example hip is an example of the two different ways of working with volumes; it calculates the gradient in 2 ways, one with a vex 'volumegradient' call, the other with the 'VDB analysis' sop. The VDB nodes are presented a little differently than regular Houdini nodes, but are very powerful and very fast.
Volumes, SDFs, Trails
Download scene: File:volume_trails_V01.hip
Saw an interesting vimeo tutorial on generating swirly lines ( https://vimeo.com/134057856 ), and an odforce post asking how to generate tracer lines from noise ( http://forums.odforce.net/topic/23535-noise-how-to-convert-into-line/ ), this is my attempt to combine the two. The volume trail sop is used to visualise fields in volumes, but you can use it for your own purposes if you get volumes setup the right way.
Here, the idea is to take a shape and create an SDF, or Signed Distance Field out of it. This is a volume where each voxel stores the distance to the closest point on the surface. Voxels on the surface have value 0, voxels outside the surface have positive values, voxels inside the surface have negative values. This makes them very handy for checking collisions, among other things (here's a nice shadertoy demo to visualise SDFs in 2d: https://www.shadertoy.com/view/ltBGzK )
I then take another volume, where each voxel can store a vector value, called 'vel' (the houdini standard for storing velocity in volumes). I then clip the vel field using the SDF, so only voxels within 0.1 of the surface remain, the rest are clipped.
I then run that volume through curl noise, so the vel field now has groovy lumpy swirls through it. Feeding that to a volume trails sop generates the sweet lines.
While this sort-of follows the pig surface, the lines tend to wobble above and below the surface quite a bit. To make them stick directly on, I just ray the curves onto a slightly fatter copy of the pig (to avoid clippy intersection issues).
There's probably a way to generate the curl noise directly from the SDF, so that the resulting lines stick dead on the surface. A ray is quick n lazy tho. :)
Volume magnetic lines
Download scene: File:magnetic_lines_vdb.hipnc
Found this post ( http://pepefx.blogspot.com.au/2015/08/how-to-create-magnetic-vector-field-in.html ) explaining how to setup magnetic lines. Tried to follow it once when very tired, then tried it again a month later without looking at his notes to see if I could get a similar effect, and without using his clever vex.
Two things that might be interesting in this setup:
- Creating vdbs from polys, specifically creating density and vel vdb's simultaneously from the one poly. The vdb-from-polygons sop has a multilister similar to creating aovs/image planes on a rop. Click to add a new one, tell it what attribute is the source (eg, v for velocity), and what kind of vdb it will map to (vector).
- Combining vdb's. I setup a positive and negative vdb, then combine them with the 'add' mode. It just occurred to me I could have merged the poly shapes first, then made the vdb in one step, which would be much more efficient. Still, handy learning exercise eh?
The end result I'm sure is a scientific abomination, but looks pretty. To be honest I was sure the setup wouldn't work; just creating a negative and positive velocity field seemed too simple for the volume trails to do its thing, but here it is, working. The lines and colours are my usual abuse of volume trails and point wrangles.