Almost all ROPs will have a section for scripts:

• Pre-Render: Before the whole sequence.
• Pre-Frame: Before each frame.
• Post-Frame: After each frame.
• Post-Render: After the whole sequence.

These are pretty self explanatory. In the box you can type some Hscript (visit www.houhscriptwiki.com... jk) or some Python 😎. Make sure to change the right hand dropdown to Python for the fun stuff.

These boxes can run their own scripts written straight into them, or you can alt+E and get the big editor box and write some multiline stuff.

In the post render box, add:

import LVRender
from imp import reload
reload(LVRender)

LVRender.post_render()

Standard reloading stuff here, then we call a function in the file we loaded.

Now that we have conquered that, we we can get into the next bit below.

In almost every shot, you will be comping your work in some way, but sometimes it can save time and have a video that you can use for quick preview or dropping into Slack, etc.

Let's create a new script eg. LVRender.py and inside it, we can add a function.

import hou

def pos_render(node=hou.pwd()):
	print(node)

We pass in an optional argument of hou.pwd(). This allows up to run the script elsewhere, not just from the post render. If we run it, we get our ROP the script was called from.

Now we can get a few values that we will need.

import hou

def post_render(node=hou.pwd()):
    start = node.parm("f1").eval()  # type: ignore
    parm = node.parm("RS_outputFileNamePrefix")
    wide = parm.unexpandedString()  # type: ignore
    wide = wide.replace("$OS", node.name())

    if "$F4" in wide:
        wide = wide.replace("$F4", "%04d")
    elif "$F3" in wide:
        wide = wide.replace("$F3", "%03d")
    elif "$F2" in wide:
        wide = wide.replace("$F2", "%02d")

    full = hou.expandString(wide)
    print("full: ", full)

    video = "".join(full.split(".")[:-1]) + ".mp4"

    video = video.replace("%04d", "")
    video = video.replace("%03d", "")
    video = video.replace("%02d", "")

A bunch of string manipulations here to get our unexpanded output path, and replace the $F4 with the %04d that ffmpeg uses to glob all our output images.

At the end we replace that with nothing to get a clean no digit version of the path for our video

Now we need to run ffmpeg and create our video, for this I chose subprocess as its pretty simple to construct a string and pass it in.

To build our string:

    command = f'ffmpeg -framerate 25 -y -start_number {start} -i "{full}" -c:v libx264 -crf 23 -pix_fmt yuv420p "{video}"'

Then we can finally call the full command with subprocess.Popen():

def post_render(node=hou.pwd()):
    start = node.parm("f1").eval()  # type: ignore
    parm = node.parm("RS_outputFileNamePrefix")
    wide = parm.unexpandedString()  # type: ignore
    wide = wide.replace("$OS", node.name())

    if "$F4" in wide:
        wide = wide.replace("$F4", "%04d")
    elif "$F3" in wide:
        wide = wide.replace("$F3", "%03d")
    elif "$F2" in wide:
        wide = wide.replace("$F2", "%02d")

    full = hou.expandString(wide)
    print("full: ", full)

    video = "".join(full.split(".")[:-1]) + ".mp4"

    video = video.replace("%04d", "")
    video = video.replace("%03d", "")
    video = video.replace("%02d", "")

    command = f'ffmpeg -framerate 25 -y -start_number {start} -i "{full}" -c:v libx264 -crf 23 -pix_fmt yuv420p "{video}"'

    subprocess.Popen(command)

    hou.ui.showInFileBrowser(video)

Cherry on top is that it opens and explorer window with our file selected.

If you were gonna be really snazzy, you could break this into 2 functions, one that renders the video, and a preprocess that you can use all over Houdini to create videos.