$TX, TY, TZ : Point position.

$TW : See WEIGHT.

$CEX, CEY, CEZ : The centroid of the geometry.

$XMIN, XMAX : The X extents of the bounding box of the geometry.

$YMIN, YMAX : The Y extents of the bounding box of the geometry.

$ZMIN, ZMAX : The Z extents of the bounding box of the geometry

$SIZEX, SIZEY, SIZEZ : The size of the bounding box.

$BBX, BBY, BBZ : The point’s relative position in the bounding box.

$WEIGHT : Point spline weight.

$PT : The point number of the currently processed point.

$PR, NPR : Primitive number & total number of primitives.

$VTX, NVTX : Vertex number & total number of vertices.

$NPT : The total number of points in the template geometry.

$LIFE : Percent of total life used (from 0 to 1).

$NX, NY, NZ : Normal vector.

$MAPU, MAPV, MAPW : Point or vertex texture coordinates.

$VX, VY, VZ : Velocity direction.

$MAT : Material name specification.

$CR, CG, CB : Diffuse point or vertex color.

$CA : Point or vertex alpha value.

$COMX, COMY, COMZ : Center of mass.

$LOD : Level of detail.

$RESTX, RESTY, RESTZ : The rest position.

$DIST : Distance from particle to last collision.

$MASS : Point mass.

$DRAG : Point drag.

$TENSION : Spring tension of an edge.

$SPRINGK : Elasticity of a point.

$PSCALE : Particle Scale.

$AGE : The seconds a particle in the template has been alive.

$CREASE : Point or vertex crease weight value.

$AREA : The surface area of the primitive (created by the Measure SOP).

$PERIMETER : The perimeter of the primitive (created by the Measure SOP).

$ID : The ID of the particle in the input.

$UPX, UPY, UPZ : The vector pointed in the up direction.

$WIDTH : The width of the curve. Used by mantra for rendering curves & polywire for generating trees.

$SEGS : Number of segments to divide a curve up into in Polywire (created by L-systems).

$DIV : Number of divisions to build the circle swept with Polywire (created by L-systems).

$LAGE : The age of the point in the L-system computation.

$ARC : Arc length distance from root in the L-systems.

$GEN : Generation in the L-systems.

$CURLAYER : Stores the currently processed layer for texture/materials (set by the Layer SOP).

$MAXLAYER : The number of layers that are considered active for display.

Common Houdini Surface operators (SOPs)

Common Houdini Expressions

You can find the full list of expressions here:
http://www.sidefx.com/docs/houdini11.0/expressions/

Houdini expressions also contain the common mathematical functions that are found in most scripting and expression languages. Here are just a few:

sin, cos, tan, asin, acos, atan, atan2, abs, pow, sqrt, ceil, floor, atof, dot, cross, deg, rad, exp, frac, hsv, rgb, min, max, log, log10, round, substr, strreplace

Note: Trigonometric functions accept angles in degrees.

The modulus operation is achieved using the % character, and boolean logic is achieved using the standard C operators, i.e. <. >, <=, >=, ==, !=, !

Common Houdini Expression Local Variables

Important: The following variables are NOT available in every operator. Each operator has its own local variables, and you can find which ones in Houdini’s help documentation. The ones listed here are the ones you’ll most commonly encounter.