Houdini Cheat Sheet (from Andy Nicholas blog)

Common Houdini Surface operators (SOPs)

Box, Sphere, Grid, Tube, Circle, Line, Torus, Curve	Some basic operators that give you geometry primitives
File	Loads a mesh (obj, bgeo, etc)
Trace	Uses a threshold to generate geometry from an image
Add	Creates individual points and/or polygons
Merge	Combines multiple pieces of geometry
Point	Manipulates point properties and attributes. The second input allows operations between two sets of points from different sources providing they have the same number.
Primitive	Manipulates primitive (usually polygons) properties and attributes
Transform	Moves, rotates, and scales geometry
Facet	Most commonly used to calculate normals, but does a few other things too.
Group	Allows you to assign a group to a set of vertices, points, or primitives. (A bit like XSI’s Clusters)
Copy	Copies input geometry. The second input allows you to copy the geometry from the first input, onto each point of the second input. This operator allows loop-style processing by using the Copy/Stamp feature.
Delete	Deletes geometry. Can be used with a Copy operator to perform loop-style processing.
AttribCreate	Allows you to store custom information at each vertex, point, primitive, or object
AttribTransfer	Copies attributes from one piece of geometry to another. The geometry does not have to match (just like XSI’s GATOR).
AttribPromote	Allows you to do various statistics on an attribute (e.g. average, minimum, maximum, mode, sum of squares, etc.) and transfer it to the corresponding vertex, point, or primitive.
Measure	Allows you to calculate polygon area, perimeter or mesh curvature.
Cache	Stores geometry in memory to allow for faster previews.
Trail	Can calculate per a point velocity, or generate trails of points from animated geometry.
Skin	Performs a “loft” style operation
Resample	Redistributes points along a curve
Ray	Does ray tracing based on point nomal. Can be used to shrink wrap one mesh onto another.
Cookie, Boolean	Different types of Boolean style operations
Sort	Can sort point order based on various things.
Peak	Transforms points based on their normals
Connectivity	Creates an attribute with a value based on connectivity information. Works well with Partition operator
Partition	Assigns geometry to a group based on an attribute value. Often used with the Connectivity operator

 

Common Houdini Expressions

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

ch(<parameter path>)	Gets the current value of the parameter
chf(<parameter path>,<frame> )	Gets the value of the parameter at a particular frame
stamp(<Copy SOP path>, <stamp variable>, <default value>)	Retrieves the current value of the specified stamp variable (See Houdini docs for more info on the copy-stamping process)
point(<SOP path>, <point number>, <attribute name>, <index>)	Retrieves a point attribute value. If attribute is a vector (e.g. position), then index specifies vector component (e.g., 0=”x”, 1=”y”, 2=”z”)
prim(<SOP path>, <point number>, <attribute name>, <index>)	Same as point, but for primitives
clamp(<value>,<min>,<max>)	Keeps the value within the minimum and maximum values
fit(<value>,<min>,<max>,<newmin>,<newmax>)	Remaps the value from the old range to the new range and clamps it.
fit01(<value>,<newmin>,<newmax>)	Just like fit() but assumes value is already in the range 0 to 1.
smooth(<value>,<min>,<max>)	Outputs a smoothed value between 0 and 1 as value goes from minimum to maximum.
noise(<x>, <y>, <z>)	Returns a noise value for a given position in space.
rand(<seed>)	Gives a random number based on the seed value. The returned value will always be the same for a particular seed.
length(<x>, <y>, <z>)	Returns the length of the vector
distance(<x1>, <y1>, <z1>,<x2>, <y2>, <z2>)	Returns the distance between two points
if(<expr>, <value if true>, <value if false>)	Gives a choice of two values depending on the expression evaluating to true or false
opinput(<operator path>, <input index>)	Returns the path of the operator connected to the input specified by the input index.
bbox(<SOP path>, <dimension type>)	Returns the required measurement of the bounding box of some geometry. “dimension type” can be one of these: D_XMIN, D_YMIN, D_ZMIN, D_XMAX, D_YMAX, D_ZMAX, D_XSIZE, D_YSIZE, D_ZSIZE
centroid(<SOP path>, <dimension type>)	Returns the X, Y, or Z position of the center of the bounding box. “dimension type” can be one of these: D_X, D_Y, D_Z
pic(<COP path>, <u>, <v>, <color type>)	Returns the interpolated pixel color from a compositing operator at the specified UV. “color type” can be one of these: D_CR, D_CG, D_CB, D_CA, D_CHUE, D_CSAT, D_CVAL, D_CLUM

 

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.

$PT	Point number. Used in most SOPs that manipulate points
$PR	Primitive number. Used in most SOPs that manipulate primitives
$CY	Used by the Copy SOP to identify the current copy being made. Unless the second Copy input is being used, in which case $PT will be used instead.
$TX, $TY, $TZ	Position of a point
$NX, $NY, $NZ	Normal of a point/primitive
$CR, $CG, $CB, $CA	Color of a point/primitive
$VX, $VY, $VZ	Velocity of a point
$BBX, $BBY, $BBZ	The normalised (0 to 1) position of the point in the bounding box
$CEX, $CEY, $CEZ	Centroid of the geometry
$AGE	Number of seconds a particle has been alive
$LIFE	Normalised age (0 to 1) of a particle
$XMIN, $XMAX, etc.	Extents of the bounding box
$SIZEX, $SIZEY, $SIZEZ	Size of the bounding box
$TX2, $TY2, $TZ2	Used to identify the position of a point coming from the second input of a Point SOP. Similarly for other attributes, e.g. Normals, $NX2, etc.

Here’s a quick cheat sheet for Houdini that I wrote ages ago. It’s probably got about 95% of what you need to use when you first start out using Houdini.

Hope you find it useful.