Encapsulated instancer not recognizing inherited (string) attributes on prototypes

[mortimerb]

Version: Gaffer 1.5.16.1- VFX platform 2023
Third-party tools: Arnold

Description

When using (string) attributes on prototypes for the instancer to drive texture paths,
inherited attributes seem to get ignored when encapsulated.

If the attributes are localized, the issue goes away.

Steps to reproduce

See attached Gaffer scene file as simple repo case (as zip file because of github upload restrictions).

encapsulatedInstancer_inheritedAttributeIssue.zip

[johnhaddon]

I think this one might be relevant to a problem we saw at Cinesite the other day. We should give it some priority @danieldresser-ie.

[danieldresser-ie]

The issue here is that Capsules are intended to allow for efficient instancing, and we don't know where in the hierarchy they will be instanced when we output them to Arnold, so we can't access attributes outside the capsule in order to perform Gaffer string substitutions.

One possible solution is to use Arnold's string substitution instead - this has other issues, like not being able to access attributes at when doing displacement - but it works in this case. You should be able use Arnold substitution instead of Gaffer substitution by escaping the substitution: instead of <attr:user:texture> use \\<attr:user:texture\\>. ( The double backslashes are needed because one backslash is consume by the Gaffer string substitution system before the text even becomes a shader parameter ).

This was not working currently, so we've fixed it in Cortex here: ImageEngine/cortex#1496
Once that is merged, escaping the substitution will be a reasonable work around and last I heard, the user who first reported this has found their own workaround. We can't currently think of a better solution while preserving the performance of the current approach, so we're going to call this closed for now ( we can reopen if more problems with this approach come up ).