Geometry Nodes: Support modifier on curve objects

With this commit, curve objects support the geometry nodes modifier.

Curves objects now evaluate to `CurveEval` unless there was a previous
implicit conversion (tessellating modifiers, mesh modifiers, or the
settings in the curve "Geometry" panel). In the new code, curves are
only considered to be the wire edges-- any generated surface is a mesh
instead, stored in the evaluated geometry set.

The consolidation of concepts mentioned above allows remove a lot of
code that had to do with maintaining the `DispList` type temporarily
for modifiers and rendering. Instead, render engines see a separate
object for the mesh from the mesh geometry component, and when the
curve object evaluates to a curve, the `CurveEval` is always used for
drawing wire edges.

However, currently the `DispList` type is still maintained and used as
an intermediate step in implicit mesh conversion. In the future, more
uses of it could be changed to use `CurveEval` and `Mesh` instead.

This is mostly not changed behavior, it is just a formalization of
existing logic after recent fixes for 2.8 versions last year and two
years ago. Also, in the future more functionality can be converted
to nodes, removing cases of implicit conversions. For more discussion
on that topic, see T89676.

The `use_fill_deform` option is removed. It has not worked properly
since 2.62, and the choice for filling a curve before or after
deformation will work much better and be clearer with a node system.

Applying the geometry nodes modifier to generate a curve is not
implemented with this commit, so applying the modifier won't work
at all. This is a separate technical challenge, and should be solved
in a separate step.

Differential Revision: https://developer.blender.org/D11597

1 files changed, 6 insertions, 4 deletions

diff --git a/source/blender/makesdna/DNA_curve_types.h b/source/blender/makesdna/DNA_curve_types.h
index 520fc6c1b00..77ab2b67f3d 100644
--- a/source/blender/makesdna/DNA_curve_types.h
+++ b/source/blender/makesdna/DNA_curve_types.h
@@ -302,8 +302,11 @@ typedef struct Curve {
   float fsize_realtime;
 
   /**
-   * A pointer to curve data from geometry nodes, currently only set for evaluated
-   * objects by the dependency graph iterator, and owned by #geometry_set_eval.
+   * A pointer to curve data from evaluation. Owned by the object's #geometry_set_eval, either as a
+   * geometry instance or the data of the evalauted #CurveComponent. The curve may also contain
+   * data in the #nurb list, but for evaluated curves this is the proper place to retrieve data,
+   * since it also contains the result of geometry nodes evaluation, and isn't just a copy of the
+   * original object data.
    */
   struct CurveEval *curve_eval;
 
@@ -344,8 +347,7 @@ enum {
   CU_DS_EXPAND = 1 << 11,
   /** make use of the path radius if this is enabled (default for new curves) */
   CU_PATH_RADIUS = 1 << 12,
-  /** fill 2d curve after deformation */
-  CU_DEFORM_FILL = 1 << 13,
+  /* CU_DEFORM_FILL = 1 << 13, */ /* DEPRECATED */
   /** fill bevel caps */
   CU_FILL_CAPS = 1 << 14,
   /** map taper object to beveled area */