NLA: Keyframe Remap Through Upper Strips

Add a new operator, "Start Tweaking Strip Actions (Full Stack)", which
allows you to insert keyframes and preserve the pose that you visually
keyed while upper strips are evaluating,

The old operator has been renamed from "Start Tweaking Strip Actions" to
"Start Tweaking Strip Actions (Lower Stack)" and remains the default for
the hotkey {key TAB}.

**Limitations, Keyframe Remapping Failure Cases**:
1. For *transitions* above the tweaked strip, keyframe remapping will
   fail for channel values that are affected by the transition. A work
   around is to tweak the active strip without evaluating the upper NLA
   stack.

   It's not supported because it's non-trivial and I couldn't figure it
   out for all transition combinations of blend modes. In the future, it
   would be nice if transitions (and metas) supported nested tracks
   instead of using the left/right strips for the transitions. That
   would allow the transitioned strips to overlap in time. It would also
   allow  N strips to be part of the (previously) left and right strips,
   or perhaps even N strips being transitioned in sequence (similar to a
   blend tree). Proper keyframe remapping through all that is currently
   beyond my mathematical ability. And even if I could figure it out,
   would it make sense to keyframe remap through a transition?

   //This case is reported to the user for failed keyframe insertions.//

2. Full replace upper strip that contains the keyed channels.

   //This case is reported to the user for failed keyframe insertions.//

3. When the same action clip occurs multiple times (colored Red to
   denote it's a linked strip) and vertically overlaps the tweaked
   strip, then the remapping will generally fail and is expected to
   fail.

   I don't plan on adding support for this case as it's also non-trivial
   and (hopefully) not a common or expected use case so it shouldn't be
   much of an issue to lack support here.

   For anyone curious on the cases that would work, it works when the
   linked strips aren't time-aligned and when we can insert a keyframe
   into the tweaked strip without modifying the current frame output of
   the other linked strips. Having all frames sampled and the strip
   non-time aligned leads to a working case. But if all key handles are
   AUTO, then it's likely to fail.

   //This case is not reported to the user for failed keyframe
   insertions.//

4. When using Quaternions and a small strip influence on the tweaked
   Combine strip. This was an existing failure case before this patch
   too but worth a mention in case it causes confusion. D10504 has an
   example file with instructions.

   //This case is not reported to the user for failed keyframe insertions. //

5. When an upper Replace strip with high influence and animator keys to
   Quaternion Combine (Replace is fine). This case is similar to (4)
   where Quaternion 180 degree rotation limitations prevent a solution.

   //This case is not reported to the user for failed keyframe insertions.//

Reviewed By: sybren, RiggingDojo

Differential Revision: https://developer.blender.org/D10504

1 files changed, 4 insertions, 2 deletions

diff --git a/release/scripts/startup/bl_ui/space_nla.py b/release/scripts/startup/bl_ui/space_nla.py
index 961d6525a69..f0e991c768d 100644
--- a/release/scripts/startup/bl_ui/space_nla.py
+++ b/release/scripts/startup/bl_ui/space_nla.py
@@ -214,7 +214,8 @@ class NLA_MT_edit(Menu):
             layout.operator("nla.tweakmode_exit", text="Stop Tweaking Strip Actions")
         else:
             layout.operator("nla.tweakmode_enter", text="Start Editing Stashed Action").isolate_action = True
-            layout.operator("nla.tweakmode_enter", text="Start Tweaking Strip Actions")
+            layout.operator("nla.tweakmode_enter", text="Start Tweaking Strip Actions (Full Stack)").use_upper_stack_evaluation = True
+            layout.operator("nla.tweakmode_enter", text="Start Tweaking Strip Actions (Lower Stack)").use_upper_stack_evaluation = False
 
 
 class NLA_MT_add(Menu):
@@ -288,7 +289,8 @@ class NLA_MT_context_menu(Menu):
             layout.operator("nla.tweakmode_exit", text="Stop Tweaking Strip Actions")
         else:
             layout.operator("nla.tweakmode_enter", text="Start Editing Stashed Action").isolate_action = True
-            layout.operator("nla.tweakmode_enter", text="Start Tweaking Strip Actions")
+            layout.operator("nla.tweakmode_enter", text="Start Tweaking Strip Actions (Full Stack)").use_upper_stack_evaluation = True
+            layout.operator("nla.tweakmode_enter", text="Start Tweaking Strip Actions (Lower Stack)").use_upper_stack_evaluation = False
 
         layout.separator()