Python: Add to_curve method to the object API

This patch adds a to_curve method to the Object ID. This method is
analogous to the to_mesh method. The method can operate on curve and
text objects. For text objects, the text is converted into a 3D Curve ID
and that curve is returned. For curve objects, if apply_modifiers is
true, the spline deform modifiers will be applied and a Curve ID with
the result will be returned, otherwise a copy of the curve will be
returned.

The goal of this addition is to allow the developer to access the splines
of text objects and to get the result of modifier applications which was
otherwise not possible.

Reviewed By: Brecht

Differential Revision: https://developer.blender.org/D10354

doc/python_api/examples/bpy.types.Depsgraph.7.py

@@ -0,0 +1,64 @@
+"""
+Dependency graph: Object.to_curve()
++++++++++++++++++++++++++++++++++++
+
+Function to get a curve from text and curve objects. It is typically used by exporters, render
+engines, and tools that need to access the curve representing the object.
+
+The function takes the evaluated dependency graph as a required parameter and optionally a boolean
+apply_modifiers which defaults to false. If apply_modifiers is true and the object is a curve object,
+the spline deform modifiers are applied on the control points. Note that constructive modifiers and
+modifiers that are not spline-enabled will not be applied. So modifiers like Array will not be applied
+and deform modifiers that have Apply On Spline disabled will not be applied.
+
+If the object is a text object. The text will be converted into a 3D curve and returned. Modifiers are
+never applied on text objects and apply_modifiers will be ignored. If the object is neither a curve nor
+a text object, an error will be reported.
+
+.. note:: The resulting curve is owned by the object. It can be freed by calling `object.to_curve_clear()`.
+.. note::
+   The resulting curve must be treated as temporary, and can not be referenced from objects in the main
+   database.
+"""
+import bpy
+
+
+class OBJECT_OT_object_to_curve(bpy.types.Operator):
+    """Convert selected object to curve and show number of splines"""
+    bl_label = "DEG Object to Curve"
+    bl_idname = "object.object_to_curve"
+
+    def execute(self, context):
+        # Access input original object.
+        obj = context.object
+        if obj is None:
+            self.report({'INFO'}, "No active object to convert to curve")
+            return {'CANCELLED'}
+        if obj.type not in {'CURVE', 'FONT'}:
+            self.report({'INFO'}, "Object can not be converted to curve")
+            return {'CANCELLED'}
+        depsgraph = context.evaluated_depsgraph_get()
+        # Invoke to_curve() without applying modifiers.
+        curve_without_modifiers = obj.to_curve(depsgraph)
+        self.report({'INFO'}, f"{len(curve_without_modifiers.splines)} splines in a new curve without modifiers.")
+        # Remove temporary curve.
+        obj.to_curve_clear()
+        # Invoke to_curve() with applying modifiers.
+        curve_with_modifiers = obj.to_curve(depsgraph, apply_modifiers = True)
+        self.report({'INFO'}, f"{len(curve_with_modifiers.splines)} splines in new curve with modifiers.")
+        # Remove temporary curve.
+        obj.to_curve_clear()
+        return {'FINISHED'}
+
+
+def register():
+    bpy.utils.register_class(OBJECT_OT_object_to_curve)
+
+
+def unregister():
+    bpy.utils.unregister_class(OBJECT_OT_object_to_curve)
+
+
+if __name__ == "__main__":
+    register()
+