How do I properly 'set up' a 'global' preference variable?

[darksaga]

Hello,

I'd like to add a bar value to my preference screen by which the user will control the opacity of the user interface.
The 'linked' preference variable should be persited globally for the whole game, e.g. like the text speed you can adjust on the preference screen, and not persisted in every individual save file.

Somehow I only get this to work properly, when I persist the variable on a 'by save file' basis, using the following bar value methods to access it (global var, object var):

Code: Select all

FieldValue(object, field, range, max_is_zero=False, style=u'bar', offset=0, step=None, action=None)
VariableValue(variable, range, max_is_zero=False, style=u'bar', offset=0, step=None, action=None)

Nonetheless I'd like the preference variable the bar value can access to be save file independant and to be set globally for the game.
I assume it's related to this doc topic: https://www.renpy.org/doc/html/persistent.html or this post: viewtopic.php?t=3514

Can anybody tell me how to do this properly?