Python SOP Example: Color Falloff

Overview

This example defines a Python SOP that copies its input, creates a Cd (diffuse color) point attribute, and assigns each point a color based on the distance to a position.

Location

Supporting files for this example are in $HFS/mozilla/documents/hom/cookbook/color_falloff, also found in the cookbook/color_falloff directory of cookbook_files.tar.gz.

Viewing the SOPs Output

Load color_falloff.hip.
Press enter in the viewer to enter the handle tool, enabling the SOP’s translate handle.
Move the handle around to change the position parameter and change the falloff parameter, and how it affects the SOP’s output.

Exploring the SOPs Implementation

The color falloff Python SOP is stored in PythonSops.otl. Right-click on the color_falloff1 SOP and bring up the Type Properties dialog.
Click on the Parameters tab to see the pos and falloff parameters. These parameters were created exactly the same way you would for a normal digital asset.
Click on the Handles tab and you’ll see that a translate handle is bound to the posx, posy, and posz parameters.
Click on the Code tab to see the Python code that Houdini runs when the SOP cooks.

# When an instance of this Python SOP cooks, Houdini will have set hou.pwd()
# to the Python SOP instance.  Access the hou.Geometry object for this SOP.
# Since we're calling from hou.SopNode.geometry from a Python SOP
# implementation, we'll have write access to the geometry.
geo = hou.pwd().geometry()

# Create the "Cd" point attribute value, giving it a default value of white
# (1, 1, 1), and store the returned hou.Attrib object.
cd = geo.addAttrib(hou.attribType.Point, "Cd", (1.0, 1.0, 1.0))

# Evaluate the pos parm tuple, and create a hou.Vector3 out of it so we can
# later do vector subtraction.  Also evaluate the falloff value.
pos = hou.Vector3(hou.parmTuple("pos").eval())
falloff = max(hou.ch("falloff"), 0.0001)

for point in geo.points():
    # Compute the distance from this point to the position parameter, divide
    # the distance by the falloff value, and clamp it to be between 0 and 1.
    # Note that hou.Point.position will return a hou.Vector4 containing the
    # weight value, so we convert it to a hou.Vector3 to throw away the weight
    # component.
    distance = (hou.Vector3(point.position()) - pos).length()
    value = min(distance / falloff, 1.0)

    # Create a color object, and set the hue and value of the color based on
    # the normalized distance.
    color = hou.Color()
    color.setHSV((value * 256, 1.0, value))

    # Extract the RGB values from the color object and store them in this
    # point's Cd attribute value.
    point.setAttribValue(cd, color.rgb())

Creating a new Python SOP

Note that if you want to create your own Python SOP, you can do so by selecting File → New Operator Type and picking “Python Type” as the operator style.