How to build an automatic Write node

Generally, most of the information needed for the output of a Nuke script is contained in it's name. We can reduce manual name-changing on behalf of comp artists and simultaneously increase consistency by creating a Write node that creates (and updates) it's own output path by parsing the script's name. I call a Write node that's customized in this way an AutoWrite node. We're only adding a little functionality to a Write node, so instead of wrapping it in a gizmo, I find it simpler to create a Python script. This is how I set it up.

First, we drop the Write node that will become our AutoWrite.

w = nuke.createNode('Write', inpanel=True)

Then, we look for AutoWrites that we've previously made (if any) so that our new one can have a unique name.

count = 1
while nuke.exists('AutoWrite' + str(count)):
    count += 1
w.knob('name').setValue('AutoWrite' + str(count))

Most projects at most facilities will have a standard set of path fragments. My list includes: project root, sequence name (or abbreviation), shot name. Feel free to add your own. We will create a new tab on the Write node to help us isolate each fragment so we can easily re-assemble them for the output path. This tab will contain knobs for each path fragment.

t = nuke.Tab_Knob("Path Fragments")
w.addKnob(t)
w.addKnob(nuke.EvalString_Knob('proj_root', 'Project Root', ''))
w.addKnob(nuke.EvalString_Knob('seq', 'Sequence', ''))
w.addKnob(nuke.EvalString_Knob('shot', 'Shot Name', ''))
w.addKnob(nuke.EvalString_Knob('script', 'Script Name', ''))

If you run this now, you'll get a tab with a bunch of empty knobs. This is usually how I start when I'm setting up a new show. I suck at TCL expressions, so I use the trial-and-error technique for chopping up my script path into the appropriate fragments. And for that technique you need feedback about what your expression is doing. For that reason, I use the AutoWrite's label to display the output of the individual fragment knobs as well as the full output path.

feedback = """
Output Path: [value file]

Project Root: [value proj_root]
Sequence: [value seq]
Shot Name: [value shot]
Script Name: [value script]
"""
w.knob('label').setValue(feedback)

The next step is to figure out what expressions you need to use in the path fragments tab to hack your useful bits out of your path. Here's what a common path loks like for me:

/Volumes/PICO/show/testproj/abc/abc123/nuke/abc123_comp_v01.nk

And these are the expressions I use to create each chunk:

Project Root: In this case, the show root directory consists of the first 5 path components, so I can just split them, limit the range and re-join.

[join [lrange [split [value root.name] / ] 0 4 ] / ]

If your facility uses environment variables, you may be able to use something nicely simple like [getenv SHOW_DIR].

Path component #6 is my sequence abbreviation, so Sequence goes like this:

[lrange [split [value root.name] / ] 5 5 ]

Repeat that idea for Shot Name:

[lrange [split [value root.name] / ] 6 6 ]

Script Name is nicely simple -- we can just take the last path component and split the extensinon off of it.

[file rootname [file tail [value root.name] ] ]

Once you've figured these out for your facility, you can put them in the knobs during creation. So now the section that created the tab should look like this:

t = nuke.Tab_Knob("Path Fragments")
w.addKnob(t)
w.addKnob(nuke.EvalString_Knob('proj_root', 'Project Root', '[join [lrange [split [value root.name] / ] 0 4 ] / ]'))
w.addKnob(nuke.EvalString_Knob('seq', 'Sequence', '[lrange [split [value root.name] / ] 5 5 ]'))
w.addKnob(nuke.EvalString_Knob('shot', 'Shot Name', '[lrange [split [value root.name] / ] 6 6 ]'))
w.addKnob(nuke.EvalString_Knob('script', 'Script Name', '[file rootname [file tail [value root.name] ] ]'))

All that's left is to re-assemble the path components in the file knob:

output_path = "[value proj_root]/[value seq]/[value shot]/comps/[value script]/[value input.width]x[value input.height]/[value script].%04d.dpx"
w.knob('file').fromScript(output_path)

And that's it. Running this script will create and customize a Write node and set it to follow along as the script's name is updated.

Here's the catch:

This script creates output paths that often don't exist on the filesystem. This will cause an error from Nuke when you start a render. The solution to this problem is to implement the beforeRender callback (createWriteDir()) described in the Nuke documentation (pg 575 in the Nuke6.1v2 manual.)

My finished version of this script is here on Nukepedia if you just want to skip to the end.

I hope this has been helpful. Drop a comment if I need to clarify something further.