原文地址：http://blog.sina.com.cn/s/blog_6856498b0100m55n.html
= Global expression variables =
== Playbar variables ==
$FPS:
Playback speed in frames per second (as set with the Playbar controls).
$FSTART:
Frame number of the first frame of animation (as set with the Playbar controls).
$FEND:
Frame number of the last frame of animation (as set with the Playbar controls).
See also $RFEND below.
$F:
The current frame, (as set with the Playbar controls). This is a very useful variable, especially for rendered picture filename numbering.
$FF:
Floating point frame number.
$NFRAMES:
Number of frames in the animation.
$RFSTART:
Frame number of the first frame shown in the playbar. The playbar can show a subset of the total number of frames, allowing you to focus on a particular section of a long animation.
$RFEND:
Frame number of the last frame shown in the playbar.
$T:
Current time in seconds. Equals (
$TLENGTH:
Total length of animation in seconds.
$TSTART:
Start time of animation in seconds.
$TEND:
End time of animation in seconds.
== Global Variables ==
$ACTIVETAKE:
Contains the name of the current take.
$E:
The mathematical constant e (2.71828…).
$HFS:
The directory where Houdini is installed.
$HH:
$HFS/houdini.
$HIP:
Job directory. This defaults to the directory where you started Houdini. You can set it through the Textport.
$HIPNAME:
The name of the current .hip file.
$HOME:
Your home directory.
$JOB:
A custom variable that determines where your jobs are located.
$PI:
The mathematical constant pi (3.1415926…). You can use it to calculate the lengths of arcs. 2*$PI*r (radius of circle) equals the circumference. Also, pi = 180藲 expressed in radians.
$WEDGE:
Contains the wedge name when using the Wedge ROP.
== Channel Variables ==
$OS:
Operator String. Contains the current OP’s name.
$CH:
Current channel name.
$IV:
In value (value at start of segment).
$OV:
Out value.
$IM:
In slope
$OM:
Out slope
$IA:
In acceleration
$OA:
Out acceleration
$LT:
Local time - not including stretch or offset
$IT:
Start time of segment
$OT:
End time of segment
$LIT:
Local start time of segment
$LOT:
Local end time of segment
$PREV_IT:
Previous segment start time
$NEXT_OT:
Next segment end time
== COP-specific variables ==
$CSTART:
Start frame of the current COP.
$CEND:
End frame of the current COP.
$CFRAMES:
Number of frames for the current COP.
$CFRAMES_IN:
Number of frames available from the first input COP.
$CINC:
Gets the global frame increment value.
$W:
Current image width.
$H:
Current image height
== Output Driver Specific Variables ==
$N:
Current frame being rendered.
$NRENDER:
Number of frames being rendered.
= Standard Variables =
NOTE:
Variable names ending with 2 refer to the geometry from the second input, if applicable. For example, PT2 is the current point number from the second input.
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鈥檚 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|Node:sop/measure]).
PERIMETER:
The perimeter of the primitive (created by the [Measure SOP|Node:sop/measure]).
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|Node:out/ifd] for rendering curves & polywire for generating trees.
SEGS:
Number of segments to divide a curve up into in Polywire (created by [L-systems|Node:sop/lsystem]).
DIV:
Number of divisions to build the circle swept with Polywire (created by [L-systems|Node:sop/lsystem]).
LAGE:
The age of the point in the [L-system|Node:sop/lsystem] computation.
ARC:
Arc length distance from root in the [L-systems|Node:sop/lsystem].
GEN:
Generation in the [L-systems|Node:sop/lsystem].
CURLAYER:
Stores the currently processed layer for texture/materials (set by the [Layer SOP|Node:sop/layer]).
MAXLAYER:
The number of layers that are considered active for display.