Extending Houdini

Houdini’s Python API is implemented with the hou module. Each node is implemented as a class that inherits from a hierarchy of classes.

For example, a null SOP is a hou.SopNode inheriting from multiple other classes

flowchart TD
   A[hou.NetworkItem]-->B(hou.NetworkMovableItem)-->C(hou.Node)-->D(hou.OpNode)-->E(hou.SopNode)

This means that you can access any methods from the superclasses from the current subclass. However, there are some exceptions to this with regards to PDG and USD contexts.

Accessing a PDG class with Python

In order to access a pdg.Node object, you have to use the getPDGNode() method on a hou.TopNode

Credits to the following forum post that helped me discover this:

pythonscript to press button on node per work item | Forums | SideFX

https://www.sidefx.com/forum/topic/73862/?page=1#post-312290

top_node = hou.node('/obj/topnet1/genericgenerator1') #inherits hou.TopNode class
pdg_node = top_node.getPDGNode()                      #inherits pdg.Node class
for work_item in pdg_node.workItems:
    print(work_item)
 
#    use top_node.getSelectedWorkItem() 
#    and top_node.setSelectedWorkItem(id)
#    to get and set the current work item
#  THIS ONLY WORKS IF THE NODE IS ALREADY COOKED OR ELSE workItems will return NONE
#  WHICH YOU CAN COOK USING top_node.cookWorkitems() method

Sending Geometry across Houdini Processes with SharedMemory

Passing over geometry from one Houdini to another can be accomplished with file caches, but if you want to achieve a high-speed temporary transfer of geometry, you can push the geometry data as bytes into shared memory (RAM) and pick it up on the other client. Python SOP on the server Houdini: MAKE SURE MAINTAIN STATE IS TICKED OR THE PYTHON INTEPRETER IS KILLED

node = hou.pwd()
geo = node.geometry()
 
geo_data = geo.data()
 
print(len(geo_data))
from multiprocessing import shared_memory
 
shm = shared_memory.SharedMemory(create=True,size = len(geo_data))
 
for i,byte in enumerate(geo_data):
    shm.buf[i] = byte
 
#print(geo_data)
#print("-----------------GAPGAP---------------------")
#print(shm.buf.tobytes())
 
print(shm.name)

Python SOP on the client Houdini:

node = hou.pwd()
geo = node.geometry()
 
mem_name = 'wnsm_301104a4'
 
from multiprocessing import shared_memory
eshm = shared_memory.SharedMemory(name=mem_name)
data = eshm.buf.tobytes().rstrip(b"\x00")
 
geo.load(data)

Opening a socket and sending out data from Houdini\

On a shelf tool

import socket
import threading
 
 
def start_server():
    # Create a socket object
    server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    host = "127.0.0.1"  # Localhost
    port = 5000  # Port number
 
    # Bind the socket to the host and port
    server_socket.bind((host, port))
    print(f"Socket server started on {host}:{port}")
 
    # Start listening for connections
    server_socket.listen(5)
    print("Waiting for a connection...")
 
    while True:
        # Accept a new connection
        client_socket, addr = server_socket.accept()
        print(f"Connection received from {addr}")
 
        # Receive data from the client
        data = client_socket.recv(1024).decode("utf-8")
        print(f"Message received: {data}")
 
        # Close the connection
        client_socket.close()
        break
 
 
# Start the server in a separate thread to keep Houdini responsive
server_thread = threading.Thread(target=start_server, daemon=True)
server_thread.start()
 
print("Server thread started. Houdini is still responsive.")

Receiving the data with a Python SOP

node = hou.pwd()
geo = node.geometry()
import time
 
print(time.time())
 
import socket
 
client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
host = "127.0.0.1"
port = 5000
 
# Connect to the server
 
client_socket.connect((host, port))
    
    # Send a message
message = "Hello from client!"
client_socket.send(message.encode("utf-8"))
    
    # Close the connection
client_socket.close()
# Add code to modify contents of geo.
# Use drop down menu to select examples.

Adding a function into Houdini’s Event Loop using Callbacks

def LoopingEvent():
	print("LoopingEvent has executed")
	pass
 
hou.ui.addEventLoopCallback(LoopingEvent)
 
# To stop the eventLoop
hou.ui.removeEventLoopCallback(LoopingEvent)

Accessing a USD class with Python

To access a pxr.Usd.Stage object and use the USD functions in Houdini, you have to call the stage() method on a hou.LopNode.

Nvidia Python USD API Documentation

https://docs.omniverse.nvidia.com/kit/docs/pxr-usd-api/latest/pxr/Usd.html#pxr.Usd.Stage

Important

Take note, not all nodes that you create within Solaris is hou.LopNode, for example, the USD Render ROP which is unique to Solaris, does not inherit from hou.LopNode.

Example Code to get a Usd.Prim and attribute

node = hou.pwd()
stage = node.stage()
#Now that we have a pxr.Usd.Stage object
#We can start calling methods from the Usd-API
 
usdPrim = stage.GetPrimAtPath("/geo")
usdAttribute = usdPrim.GetAttribute("example")
value= usdAttribute.Get()

Incremental Save

So Houdini has an incremental save feature but with the default customization you can only choose to either go with incremental saves or to overwrite your saves as the only behavior. If you want to have a selectable option for incremental and overwriting your saves, you have to create a script. Luckily its only two lines.

import hou #not needed but I like to include it
hou.hipFile.saveAndIncrementFileName()

Save this script and add it to your File menu using the following method and you are set.

Adding a Script to Houdini’s built-in main menus

You can control any menu element in Houdini from the xml configuration files located in your Houdini installation’s folder, official documentation here.

But the summary is, after you create your script add this to your MainMenuCommon.xml

<mainMenu>
  <addScriptItem id="h.saveinc">
	 <parent>file_menu</parent>
	 <label>Save Incrmental</label>
	 <scriptPath>$Pipelinetools/scripts/saveIncremental.py</scriptPath>
	 <scriptArgs></scriptArgs>
	<insertAfter>h.save</insertAfter>
  </addScriptItem>
</mainMenu>

<parent> tag indicates which menu the new item will live under

<insertAfter> indicates the placement of of the additional item using the id of the element to insert after

The ids and parent menu names can be found under $HH / M ain M e n u C o mm o n . x m l or$ HFS/houdini/MainMenuCommon.xml

Collect Files (How to package a project)

So, I was making my own script for packaging a project using Python as an excuse to try out and learn Python within Houdini, but I also found out that there’s a script made by Aeoll that already accomplishes what I wanted to make, and also included features that I did not think to include, so here it is.

Regardless, here’s the code that I had typed that will serve as reference for me on how to find specific node types:

import hou
#returns all instances of nodes of type 'file'
#use hou.node('path-to-node').type() to return
#the type of a specific node you need
fileN = hou.sopNodeTypeCategory().nodeType('file').instances()
for node in fileN:
		#evalParm evaluates the current value of the selected parm
		#this means that $F or any expressions will return as the
		#current value, eg. on frame 86 $F will return 86
    path = node.evalParm('file')
    print(path.replace('$HIP',''))

This script will print out all directories from the ‘file’ parameter on all file nodes in the Houdini project.

CG Notes

Explorer

Python 🐍

Extending Houdini

Accessing a PDG class with Python

Sending Geometry across Houdini Processes with SharedMemory

Opening a socket and sending out data from Houdini\

Adding a function into Houdini’s Event Loop using Callbacks

Accessing a USD class with Python

Example Code to get a Usd.Prim and attribute

Incremental Save

Adding a Script to Houdini’s built-in main menus

Collect Files (How to package a project)

Graph View

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