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, 1 insertions, 2 deletions

diff --git a/source/blender/editors/sculpt_paint/paint_stroke.c b/source/blender/editors/sculpt_paint/paint_stroke.c
index 7887aaaf658..463334720c5 100644
--- a/source/blender/editors/sculpt_paint/paint_stroke.c
+++ b/source/blender/editors/sculpt_paint/paint_stroke.c
@@ -230,8 +230,7 @@ static bool paint_tool_require_location(Brush *brush, ePaintMode mode)
                SCULPT_TOOL_THUMB)) {
         return false;
       }
-      else if (brush->sculpt_tool == SCULPT_TOOL_CLOTH &&
-               brush->cloth_deform_type == BRUSH_CLOTH_DEFORM_GRAB) {
+      else if (SCULPT_is_cloth_deform_brush(brush)) {
         return false;
       }
       else {