Variables

Houdini's built-in variables reference

Overview

Houdini provides built-in variables for accessing file paths, geometry data, time, and other system information. Use these in parameter fields with HScript expressions.

Important Notes

• These variables use HScript syntax (e.g., $F*2)
• In VEX, use attribute bindings instead (e.g., @Frame, @Time, @P)
• Check Help → Expression Functions → Variables for complete list

File and Path Variables

Variable	Meaning
`$HIP`	Directory of the currently opened Houdini scene file (.hip)
`$HIPNAME`	File name of the current .hip file without path or extension
`$HIPFILE`	Full path to the current .hip file
`$JOB`	Project root directory (set manually in File → Set Project)
`$TEMP`	System temp directory used by Houdini
`$HOUDINI_TEMP_DIR`	Houdini's temporary storage directory
`$HOME`	User's home directory
`$HOUDINI_USER_PREF_DIR`	Houdini user preferences directory

Node and Geometry Variables

Variable	Meaning
`$OS`	Name of the current node (Operator String)
`$OBJID`	Object identifier
`$SOPPATH`	Full path to the current SOP node
`$PR`	Current primitive number being processed
`$PT`	Current point number being processed
`$VTX`	Current vertex number being processed
`$NPT`	Total number of points in the geometry
`$NPR`	Total number of primitives in the geometry
`$NVTX`	Total number of vertices in the geometry
`$CR, $CG, $CB`	Color components (red, green, blue) of a point/primitive
`$CA`	Alpha component of color
`$TX, $TY, $TZ`	Translate position (X, Y, Z)
`$CEX, $CEY, $CEZ`	Centroid position of geometry or group (X, Y, Z)
`$BBX, $BBY, $BBZ`	Bounding box center (X, Y, Z)
`$SIZEX, $SIZEY, $SIZEZ`	Bounding box size (X, Y, Z)

Time and Animation Variables

Variable	Meaning
`$T`	Current time in seconds
`$F`	Current frame number
`$FF`	Current floating-point frame number (includes subframes)
`$FPS`	Frames per second
`$RFSTART / $RFEND`	Start / End frame of the simulation range
`$FSTART / $FEND`	Start / End frame of the playbar range
`$TLEN`	Length of the playbar range in frames
`$NFRAMES`	Total number of frames in the animation range
`$TSTART / $TEND`	Start / End time in seconds of the playbar range
`$RSTART / $REND`	Start / End frames of render range

Transform Variables

Variable	Meaning
`$RX, $RY, $RZ`	Rotation values (X, Y, Z) in degrees
`$SX, $SY, $SZ`	Scale values (X, Y, Z)
`$PX, $PY, $PZ`	Pivot position (X, Y, Z)
`$LX, $LY, $LZ`	Local position (X, Y, Z)
`$WX, $WY, $WZ`	World position (X, Y, Z)

Render and Simulation Variables

Variable	Meaning
`$WEDGE`	Wedge index when running wedge renders
`$SLICE`	Slice number for distributed simulations/renders
`$NSLICES`	Total number of slices in distributed simulation
`$ITERATION`	Current iteration number in simulations
`$SUBSTEP`	Current substep in simulation
`$SIMFRAME`	Current simulation frame
`$SIMTIME`	Current simulation time

System and Miscellaneous Variables

Variable	Meaning
`$CH`	Channel value when used with functions like ch("parm")
`$USER`	Current username
`$HOSTNAME`	Computer hostname
`$OS`	Operating system (Windows, Linux, MacOS)
`$ARCH`	System architecture (x64, etc.)
`$HOUDINI_VERSION`	Current Houdini version number
`$PI`	Mathematical constant Pi (3.14159...)
`$E`	Mathematical constant E (2.71828...)
`$DEGS`	Degrees to radians conversion factor (180/π)
`$RADS`	Radians to degrees conversion factor (π/180)