Houdini 20.5 Python scripting HOM Cookbook

HOM cookbook Wrapper library

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Overview

The code in this example defines a simple class that wraps functionality in hou to make it easier to build compositing networks in Houdini by calling methods on the class. For example:

>>> import comp

# Load in $HFS/houdini/pic/default.pic and convert it to a jpeg, writing the
# output to the current directory.
>>> comp.readFile("default.pic").writeFile("default.jpg")

# Load in a sequence of images (from $HFS/houdini/pic), brighten them,
# and convert them to jpegs.
>>> comp.readFile("butterfly$F.pic").bright(1.8).writeFile("butterfly$F.jpg")

# Load in default.pic, brighten it, composite it over a gray background,
# and write it to out.pic.
>>> comp.readFile("default.pic").bright(1.2).over(
...     comp.constant(0.3, 0.3, 0.3)).writeFile("out.pic")
...

As you call the methods on the object returned by comp.readFile, they COP nodes, set parameters on them, and wire the nodes together.

Tip

With some simple extensions to this example, you could create a Python equivalent of Houdini’s icomposite program.

Implementation

You can save the source code below to under $HOUDINIPATH/pythonX.Xlibs. See where to put Python code for more information.

import hou

"""
This module lets you create and evaluate a compositing network simply
by writing an expression describing the compositing operations to perform.
With some simple extensions to this example, you can create a Pythonic
equivalent of Houdini's icomposite program.

For example, you can write:

    import comp
    comp.readFile("default.pic").bright(1.2).over(comp.constant(0.3, 0.3, 0.3)
        ).writeFile("out.pic")

and Houdini will build a composite network that loads the default.pic image,
brightens it, composites it over a constant, and writes out the result to
out.pic.

Note that this module supports compositing over a sequence of images: simply
use a time-dependent expression (like $F) in the input and output image names.

If you use this module from a graphical Houdini session, you can inspect
the compositing networks it creates.
"""

def test():
    """This function creates a simple test case that evaluates the following:
       comp.readFile("default.pic").bright(1.2).over(
           comp.constant(0.3, 0.3, 0.3)).writeFile("out.pic")
    """
    readFile("default.pic").bright(1.2).over(constant(0.3, 0.3, 0.3)
        ).writeFile("out.pic")

class _Image:
    """This image class wraps a COP node and exposes image operations via
       methods that simply create COP nodes and return a new image wrapping
       that node.
    """
    def __init__(self, node):
        # The node parameter is a COP node.  The user of this module will
        # create images with the readFile and constant methods, and construct
        # _Image objects directly.
        self.node = node

    def __createNode(self, type):
        # Create and return a COP node of the specified type in the current
        # network.
        return self.node.parent().createNode(type)

    def bright(self, amount):
        """Brighten the image, returning a new image."""
        n = self.__createNode("bright")
        n.setFirstInput(self.node)
        n.parm("bright").set(amount)
        return _Image(n)

    def over(self, image):
        """Composite this image over the specified one, returning a new
           image."""
        n = self.__createNode("over")
        n.setFirstInput(self.node)
        n.setInput(1, image.node)
        return _Image(n)

    def writeFile(self, file_name):
        """Write this image to a file or file sequence."""
        n = self.__createNode("rop_comp")
        n.setFirstInput(self.node)
        n.parm("copoutput").set(file_name)
        self.node.parent().layoutChildren()

        # If we're called from a standard Python shell or hython, actually
        # write out the file.
        if hou.applicationName() == 'hbatch':
            n.render()

def __network():
    # This internal function just returns the COP network.  For this example,
    # it simply hard-codes a particular network.
    return hou.node("/img/comp1") or hou.node("/img").createNode("img", "comp1")

_lastResolution = None

def readFile(file_name):
    """Return an image object corresponding to a file or file sequence."""
    n = __network().createNode("file")
    n.parm("filename1").set(file_name)

    # Remember the image resolution.  If we later create a constant color,
    # we'll use this resolution.
    global _lastResolution
    _lastResolution = (n.xRes(), n.yRes())

    return _Image(n)

def constant(r, g, b, a=1.0):
    """Return an image that's a constant color.  The size of the image will
       be the same as the size of the last file read in."""
    n = __network().createNode("color")
    n.parmTuple("color").set((r, g, b, a))

    if _lastResolution is not None:
        n.parm("overridesize").set(True)
        n.parmTuple("size").set(_lastResolution)
    return _Image(n)

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