Reference Expression cookbook

This cookbook refers to the old expression language. For information on Python expressions, see the Houdini Object Model section.

The rand(seed) function always returns the same number when given the same seed value. This makes “random” values repeatable between playbacks. However, you must be careful to supply a seed value that varies.

The current frame number ($F) is usually a good seed value, however:

• If you need random values for a sub-frame process, use the fractional frame number $FF instead of $F.

• If you need more than one random value, such as R, G, and B values for a random color, give each call to rand a different seed value by modifying the $F, otherwise each call will return the same number and the color will be gray.

  R	G	B
  rand($F)	rand($F * 10)	rand($F * 100)

• If you need random values that vary across geometry, use a variable like $PT (the current point number) as the seed. If the value needs to be random across the geometry and across time, combine $PT and $F.

    rand($PT / $F)
    

The rand(seed) function returns a random number from 0 to 1. Use the fit01(value, min, max) function to map the random value to a different range. For example, use fit01(rand($F), -10, -10) for a random number from -10 to 10.

(This is the same as rand($F)*20 - 10 but it saves you from having to do the calculation necessary to work out what to multiply and subtract to get the desired range.)

The modulus operator, a % b , returns the remainder after dividing a by b.

This is very useful in expressions, because no matter how big a gets, the result will keep cycling from 0 to b-1. For example, $F % 120 is a ramp from 0 to 119 which repeats at frame 120, 240, 360, and so on.

The modulus operator works on any real number, not just integers. Therefore, the value returned might not be what you expect. For example: 3.1415 % 1 returns 0.1415, and 34.999 % 5 returns 4.999.

Use an expression like sin($F * 10) in an object’s translate-X parameter to animate it moving smoothly back and forth.

Take the absolute (always positive) value of the expression, abs(sin($F * 10)), in an object’s translate-Y parameter to make it bounce up and down, reversing smoothly at the top and sharply at the bottom.

To make the bounce decay over time, create a spare parameter to hold the decay rate. Change the bounce expression to multiply by the decay parameter: (abs(sin($F * 10)) * ch("./decay"). Then, edit the curve of the decay parameter to go from 1 to 0 over time.

Use the ch("pathname") function to grab the value of another parameter.

For example, to have a Tube SOP change its height based on the translation of an object named logo, set the tube’s Height parameter to the expression ch("/obj/logo/ty").

To create ch() references easily, press on the source parameter and choose Copy Parameter, then press on the target parameter and choose Paste Copied References.

You can use the channel value within an arithmetic expression, for example ch("/obj/$OS/spare1") + ch("/obj/$OS/tx") + 2 takes the value of the channel $OS/spare1 and adds the X value of that point plus two.

The chf() function is the same as ch() function, but you can specify the frame at which to sample the value of the named parameter. For example, chf("/obj/logo/ty", 14) grabs the value of the logo’s translate-Y at frame 14. Use $F to offset from the current frame, for example chf("/obj/logo/ty", $F + 14) grabs the value 14 frames in the future.

The chs() function returns the value as a string. This is also useful for checkbox parameters because it returns “on” or “off” instead of float values.

You can get the path of the node connected to a node’s input with the opinput(operator_path, input_index) function. You can use this to automate path references in nodes. For example, if you want to get the number of incoming points from the node connected to the current node’s first input, use:

npoints( "../" + opinput(".", 0) )

The following expression examples were created by using a Grid op set to 1 row by 180 columns, and Connectivity set to Rows. To this, a Point op was appended. To use these as a segment expression, use the same expression in a channel, and replace the position variable ($PT, $TX or $BBZ) with $T.

You can use a Point SOP referencing a spare parameter to shear, taper, squash, bend, and apply other deformations to geometry.

You can also achieve these effects with the Twist.

Use the repeat function to copy or repeat repetitive motion. The basic form for the repeat function is:

repeat (start_keyframe, end_keyframe)

If you have keyframes at 0, 22, and 36, and you want to repeat the curve from 0 to 36, you select the segment from frames 36- , and enter:

cycle(0,36)

This works within a channel, without referencing another channel.

Your repeat range must be outside the range of the currently selected segment. For example, if you have keyframes at frames 0, 22, and 36, you cannot select the segment from 0-22 and assign a cycle() function with values within the range of 0-22, because it would be trying to repeat values of itself that have not yet been determined.

The Iso Surface SOP creates surfaces from equations.

You can set up the IsoSurface SOP with an expression having three dimensions, each taking a real value. The IsoSurface SOP can then create a surface from every point inside a bounding box where the expression equals 0.

For example, the default expression

$X*$X + $Y*$Y + $Z*$Z - 1

...plots a sphere (the expression is less than 0 inside a unit sphere, greater than 0 outside a unit sphere, and equal to 0 at the surface of a unit sphere).

This may seem like a difficult way to define a sphere, but combined with the rich array of functions available to expressions, it has the potential to create complex and strange surfaces. For example, noise($X, $Y, $Z) plots a random jumble of surfaces.