GP Interpolate Sequence: Tool settings for controlling the shape of interpolation

This commit introduces the ability to use the Robert Penner easing equations
or a Custom Curve to control the way that the "Interpolate Sequence" operator
interpolates between keyframes. Previously, it was only possible to get linear
interpolation between the gp frames.

Workflow:
1) Place current frame between a pair of GP keyframes
2) Open the "Interpolate" panel in the Toolshelf
3) Choose the interpolation type (under "Sequence Options")
4) Adjust settings (e.g. if you're using "Custom Curve", use the curvemap widget
   to define the way that the interpolation proceeds)
5) Click "Sequence" to interpolate
6) Play back/scrub the animation to see if you've got the result you want
7) If you need to make some tweaks, undo, or delete the generated keyframes,
   then repeat the process again from step 4 until you've got the desired result.

1 files changed, 18 insertions, 1 deletions

diff --git a/release/scripts/startup/bl_ui/properties_grease_pencil_common.py b/release/scripts/startup/bl_ui/properties_grease_pencil_common.py
index c43f56acb37..1f06b202adc 100644
--- a/release/scripts/startup/bl_ui/properties_grease_pencil_common.py
+++ b/release/scripts/startup/bl_ui/properties_grease_pencil_common.py
@@ -244,8 +244,8 @@ class GreasePencilStrokeEditPanel:
             layout.separator()
             layout.operator("gpencil.reproject")
 
+
 class GreasePencilInterpolatePanel:
-    # subclass must set
     bl_space_type = 'VIEW_3D'
     bl_label = "Interpolate..."
     bl_category = "Grease Pencil"
@@ -273,6 +273,23 @@ class GreasePencilInterpolatePanel:
         col.prop(settings, "interpolate_all_layers")
         col.prop(settings, "interpolate_selected_only")
 
+        col = layout.column(align=True)
+        col.label(text="Sequence Options:")
+        col.prop(settings, "type")
+        if settings.type == 'CUSTOM':
+            box = layout.box()
+            # TODO: Options for loading/saving curve presets?
+            box.template_curve_mapping(settings, "interpolation_curve", brush=True)
+        elif settings.type != 'LINEAR':
+            col.prop(settings, "easing")
+
+            if settings.type == 'BACK':
+                layout.prop(settings, "back")
+            elif setting.type == 'ELASTIC':
+                sub = layout.column(align=True)
+                sub.prop(settings, "amplitude")
+                sub.prop(settings, "period")
+
 
 class GreasePencilBrushPanel:
     # subclass must set