Bugfix [#31834] Cycles materials cannot be manipulated using drivers

Until now, there was never any code for making drivers on materials get
recalculated when their dependencies were changed. However, since changing
material colors with drivers is something that is quite common, a workaround was
introduced to ensure that materials could still be driven (albeit with the
relevant drivers rooted at object level). This worked well enough so far with
traditional materials - though it was sometimes clunky and confusing for some
users - and would have been ok to tide us over until the depsgraph refactor.

The introduction of Cycles changed this, as it has in many other ways. Now that
people use Cycles to render, they'll need to drive the material colors through
the nested nodetree (and other things nested deeply within that). However, this
is much more difficult to generate hacks to create the relevant paths needed to
work around the problem.

== This Commit... ==
* Adds a recursive driver calculation step to the BKE_object_handle_update()
(which gets called whenever the depsgraph has finished tagging object datablocks
for updates), which goes through calculating the drivers attached to the object
(and the materials/nodetrees attached to that). This case gets handled everytime
the object is tagged as needing updates to its "data" (OB_RECALC_DATA)

* When building the depsgraph, every dependency that the drivers there have are
treated as if they were attached to object.data instead. This should trick the
depsgraph into tagging OB_RECALC_DATA to force recalculation of drivers, at the
expense perhaps of modifiers getting recalculated again.

== Todo ==
* The old workarounds noted are still in place (will be commented out in the
next commit). This fix renders at least the material case redundant, although
the textures case still needs a bit more work.

* Check on whether similar hacks can be done for other datablock combinations

* So far, only simple test cases have been tested. There is probably some
performance penalty for heavy setups still (due to need to traverse down all
parts of material/node hierarchy to find things that need updates). If there
really is a problem here, we could try introducing some tags to limit this
traversal (which get added at depsgraph build time).  <--- USER TESTING
NEEDED!!!

1 files changed, 4 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_material.h b/source/blender/blenkernel/BKE_material.h
index 8aa25a235a8..2407330a237 100644
--- a/source/blender/blenkernel/BKE_material.h
+++ b/source/blender/blenkernel/BKE_material.h
@@ -42,6 +42,7 @@ struct ID;
 struct Object;
 struct Mesh;
 struct MTFace;
+struct Scene;
 
 /* materials */
 
@@ -92,6 +93,9 @@ int material_in_material(struct Material *parmat, struct Material *mat);
 
 void ramp_blend(int type, float r_col[3], const float fac, const float col[3]);
 
+/* driver update hacks */
+void material_drivers_update(struct Scene *scene, struct Material *mat, float ctime);
+
 /* copy/paste */
 void clear_matcopybuf(void);
 void free_matcopybuf(void);