Fix Cloth Brush grab deformation mode

The grab mode was not correctly implemented, so the way it was working
was confusing for users.
- Grab delta was calculated in increments from the last stroke position, so it did not match the behavior of a grab brush. I refactored the grab delta calculation to make this change more explicit.
- Grab displacement was not calculated from the original coordinates
- Grab was using an incorrect strength

Grab is now setting the position of the affected vertices directly and
the constraints solve the rest of the cloth. I also tried to implement
an alternative version based on applying forces to move the vertices to
the grab position, but I think this is more controllable and the grab
falloff can be adjusted by tweaking the simulation falloff.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D7756

1 files changed, 7 insertions, 0 deletions

diff --git a/source/blender/editors/sculpt_paint/sculpt_intern.h b/source/blender/editors/sculpt_paint/sculpt_intern.h
index 191a6fdd62e..ec7ea1d85f4 100644
--- a/source/blender/editors/sculpt_paint/sculpt_intern.h
+++ b/source/blender/editors/sculpt_paint/sculpt_intern.h
@@ -333,6 +333,13 @@ void SCULPT_cloth_plane_falloff_preview_draw(const uint gpuattr,
                                              struct SculptSession *ss,
                                              const float outline_col[3],
                                              float outline_alpha);
+
+BLI_INLINE bool SCULPT_is_cloth_deform_brush(const Brush *brush)
+{
+  return brush->sculpt_tool == SCULPT_TOOL_CLOTH &&
+         brush->cloth_deform_type == BRUSH_CLOTH_DEFORM_GRAB;
+}
+
 /* Pose Brush. */
 void SCULPT_do_pose_brush(struct Sculpt *sd,
                           struct Object *ob,