= Volumes =

== Overview ==

Houdini supports a _volume_ primitive at the geometry level, alongside other primitives such as polygons and NURBS surfaces. A volume is a box with a position, size, and orientation, subdivided into a 3D grid of points, with a value stored at each point.

Different types of volumes store different types of information at the points:

- Scalar field - stores a single numeric value at each point. This is useful to represent smoke, where the value at each point represents the density of the smoke at that location.

- Sign distance field (SDF) - used to represent fluids. Stores a number at each point representing the distance to the fluid's surface, with the sign of the number (positive or negative) indicating whether the point is inside or outside the fluid.

- Vector field - stores a vector at each point. A common use for this type of a field is a _velocity field_ for a gas simulation, which each point's vector value represents the direction/velocity of the gas at that position.

== How to use volumes ==

- You can create a scalar volume with the [Iso Surface node|Node:sop/iso]. Set the __Implicit function__ to an expression representing the density, set the number of __Divisions__, and turn on __Build volume__.

- You can import data from a fluid or gas simulation as a volume primitive. See [how to import and export simulation data|/dynamics/io].

- You can manipulate or combine volumes with the [Volume Mix surface node|Node:sop/volumemix]. When you connect two volume inputs, you can use the __Mix method__ parameter to choose how to mix the values of the two volumes. When you connect one volume, you can set __Mix method__ to __User__ and set the __Value__ to an expression where `$V` represents the current value at each point and the result is the new value. 

- To learn how to shade and render volumes, see [volumetric rendering|/rendering/volumetricrendering].

- The [Iso Surface node|Node:sop/iso] can also convert a volume to a surface by creating a surface wherever the value of the field is 0, as in a sign distance field.