Opacity modifier implementation based on GP2.
Functionality is largely unchanged.
_Color Mode_ is either `Stroke` or `Fill` for modifying color opacity or
`Hardness`.
_Uniform Opacity_ does two things at once (!):
- Sets the same opacity value for every point in a stroke.
- Sets opacity as an absolute value rather than a factor.
_Weight as Factor_ (button to the right of Opacity Factor): Use the
vertex group as opacity __factor__ rather than an overall __influence__.
This is confusing and hard to convey, but copies behavior from GP2.
The _Influence_ panel contains the same filter settings as the GP2
modifier, with some small changes:
- _Layer_ selects only strokes in the respective layer (with an _Invert_
option)
- _Material_ selects only points with the respective material (with an
_Invert_ option)
- _Layer Pass_ and _Material Pass_ select only strokes/points which are
rendered in the respective pass.
_Note 1: Layers don't have UI for setting a pass yet, this will be a
generic layer attribute. This can be set through the API for testing._
_Note 2: In GP2 a pass value of zero was used to disable pass filters.
Since zero is a valid pass ID an explicit flag has been added for the
purpose of turning pass filters on and off._
- _Vertex Group_: This can be used as an additional influence filter on
points. If _Weight as Factor_ is enable the vertex group instead
replaces the opacity factor. In _Fill_ mode the vertex group weight of
the stroke's first point is used as influence for the entire stroke.
- _Custom Curve_ is another possible influence factor per point. The
curve input value is the relative position of a point along its
stroke.
The Influence settings have been moved into a separate DNA struct, which
should help with reusability in various modifiers. Various utility
functions can be found int `MOD_grease_pencil_util.hh` for handling
influence settings and generating `IndexMasks` for modernized C++ code.
Pull Request: https://projects.blender.org/blender/blender/pulls/116946
c02f3c94d9