LightEditor support for light filters

[ericmehl]

This adds support for editing light filters in the Light Editor. This includes blockers, which are distinct scene locations apart from lights and filters applied to lights using ShaderAssignment.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[ericmehl]

I added a couple of fixes :

• 2828a9b Creates the window needed to trigger the lazy column update with sets __muteColumn and __soloColumn. The existing test was trying to toggle values without the column update which caused errors finding those variables.
• 3ec46ff Modifies the logic preventing mute and solo toggling of light blockers. We also need to allow PathMatcher.DescendantMatch to allow toggling groups.
• b6d701c Adds a test for the light blocker mute and solo toggle prevention.

I kept the first two as fixups instead of squashing into the original so they can be discussed as desirable or not. Maybe there are better solutions?

[ericmehl]

Rebased to clean up the changes.md conflict. After talking live on the three fixup commits, I squashed them down to their origins and added the LazyMethod.flush() treatment to avoid the need for a window and idle checking.

I also improved the naming of the scene processors created to do the edits and added the summary registration for Edit Scopes.

I think that should be it for my commits for now, so ready for a review.

[murraystevenson]

Thanks Eric, this is going to be really useful! A couple of thoughts inline.

[murraystevenson]

What do you think about including the filteredLights attribute on a blocker as a column? Seems like it would be very useful to be able to quickly see the lights the blocker is affecting.

I could then see people wanting to be able to right click on this and "Select Affected Objects" similarly to what we've just added to the Render Pass Editor but maybe I can take that bit on as part of the great LightEditor/RenderPassEditor refactor...

[ericmehl]

Yes, that's a nice attribute to include for sure. I added that in 583ca50 and 5f0036f.

Adding "Select Affected Objects" seems nice too. I added that to keep parity with the Pass Editor (and you had already done the hard part). It was a pretty straightforward repurposing of the Pass Editor code so maybe it won't be too much trouble to unify the two. That's done in 844837e

[murraystevenson]

Thanks! I think we'll just need to add "filteredLights" to the attribute registry over in EditScopeAlgo, so this can still be edited in an EditScope when there's no upstream attribute.

There'd also be an argument for allowing this to be edited using the new SetExpressionPlugValueWidget, but I think we can handle that in a separate PR as it'll require a little additional metadata and AttributeTweaks plumbing along the lines of what was done for OptionTweaks over here: 4bc8ef6

[ericmehl]

Right, that explains why I needed to set the value in the unit test. I added that registration, and removed the manual value setting in the unit test to verify the registration worked. Those are in bdf665d

[ericmehl]

There'd also be an argument for allowing this to be edited using the new SetExpressionPlugValueWidget

I looked at adding that too, but I realized that it may not be as simple. If you're editing the plug on the blocker itself - not a tweak or Edit Scope - the SetExpressionPlugValueWidget will only be somewhat useful, and might even be misleading. Since there is no input to a light blocker, the widget doesn't add anything useful by way of autocomplete or browsing existing sets.

It would be useful for modifying filteredLights in a tweak plug / Edit Scope though. Maybe we could manage a conditional where we decide which widget to use based on the source plug?

[ericmehl]

Thanks @murraystevenson! Those are all nice additional points. I noted the changes in the comments, ready for a fresh look.

[ericmehl]

My latest push is a rebase onto 1.4, fixing a few conflicts that came up after merging other PRs. Otherwise this is the same as the fixes I pushed on Friday.

[ericmehl]

Thanks for the additional review @murraystevenson! I squashed down the fixups from Friday and added a couple new ones. Besides the inline comment, 2f2427b fixes the test failure.

Should be good to go assuming CI passes.

[murraystevenson]

Thanks Eric! This looks good to squash and merge.

[ericmehl]

Great! Squashed and merged