Malcolm, I like your suggestions.

My thoughts:

* The name 'globals' is good.

* I like pre-declaring the list of allowed variables beforehand for the config.

* I like the use of env-vars as defaults. This seems very user-friendly.

* I like supporting only strings for now. In the future, I can also
see numbers being supported (such as allowing for CDL blocks that rely
on envvars), but this can be added as needed.

* The FileTransform will probably have to get a bit smarter. Say
you're using a pershot lut, and for some shots you dont want any such
table. This could be solved by either not installing a lut in the
expected location, or by installing an identity lut. Which is
preferable? In the current implementation, if a lut is not found
this is always an error condition. Maybe we should introduce on the
FilmTransform a 'silent error mode', where this would just skip the
application of that specific LUT?

* The "globals" pre-declaration could also define an optional default
(used if the envvar is not set). If the envvar is not set, and no
default is specified, any references to the global variable would
probably be an error.

* I have some apprehensions about the config->getGlobal,
config->setGlobal implementation. My primary concern is that the
GlobalConfig() is const (by design), so clients couldnt actually call
setGlobals on the config in use. Instead, they would have to do
something like:

c = GetGlobalConfig().createEditableCopy()
c.setGlobals(...)
SetGlobalConfig( c )

This is a lot of copying, particularly as the globals often are tied
to the clip. If you're A/B-ing two clips in an image viewer,
conceptually there really is only one config, and two sets of globals.
Why force the client to copy and have duplicate configs?

I will mockup some ideas which solve this and see what the possible
solutions look like.

-- Jeremy