Expression functions Create your own expression functions

Tip

Instead of creating a custom expression function, use Python instead.

How to

Tip

Before you write your own function, double check the built-in library of expression functions. The function you need may already exist.

Custom function definitions have the following syntax:

[return_type] functionName([[arg_type1] arg_name1 [, [arg_type2] arg_name2] ...])
{
...
}

The return type and argument types can be float, string, vector, or matrix.

Note

If you don’t explicitly list a return type or an argument’s type, Houdini will assume it’s a float, and will silently cast any other data type to a float. If you forget to specify that an argument is a string, Houdini will cast it to float, potentially causing hard to find bugs.

The body of the function definition can use extra syntax such as assignment (=, +=, -=), if, for, while, etc. Use return to return a value.

Lines beginning with # are comments and are ignored by Houdini.

Open the custom function editor.

  1. Choose Edit > Aliases and Variables to open the Aliases and Variables editor.

  2. Click the Expressions tab.

Create a new custom function.

  1. Open the custom function editor (see above).

  2. Enter your new function in the large text box.

  3. Click Apply Changes. Your new function should appear in the list of functions on the left.

Load definitions from an external file.

  • Click the plus icon in the bottom right corner of the custom function editor and choose the file to load.

  • Also see the exread command.

Edit the function source code in an external editor.

  1. Click the Edit button in the bottom right corner of the custom function editor.

    Houdini uses the external editor defined by the $EDITOR environment variable. If $EDITOR is not set, Houdini uses a default editor for the platform (e.g. vi or notepad.exe).

  2. Edit the source code in the external editor, then save and quit to return to Houdini.

  3. Also see the exedit command.

Test your new function.

  • Click the Textport button in the bottom left corner of the custom function editor.

  • In the textport, type

    echo `yourfunction()`

    HScript evaluates expressions enclosed in backquotes (`).

Examples

# Function to find the minimum value of two
# floating point numbers

min(v1, v2) {
    if (v1 < v2) {
        return v1;
    } else {
        return v2;
    }
}

# Function to reverse the order of a string

string strreverse(string in) {
    float len = strlen(in);

    string result = "";

    for (src = len-1; src >= 0; src--) {
        result += in[src]; return result;
    }
}

# Example to find the minimum element in a vector

float vecmin(vector vec) {
    min = vec[0];

    for (i = 1; i < vsize(vec); i++) {
        if (vec[i] < min)  min = vec[i];
    }

    return min;
}

# Example to transform a vector into the space
# of an object passed in.

vector opxform(string oname, vector v) {
    matrix xform = 1;

    if (index(oname, "/obj/")) {
        xform = optransform(oname);
    } else {
        xform = optransform("/obj/"+oname);
    }

    return v * xform;
}

# Example to find all objects which have their
# display flag set

string opdisplay() {
    string objects = run("opls /obj");
    string result = "";
    nargs = argc(objects);

    for (i = 0; i < nargs; i++) {
        string obj = arg(objects, i);
        if ( index(run("opset " + obj), " -d on") >= 0 ) result += " " + obj;
    }

    return result;
}