GPencil: Changes in Fill and new 3D Cursor View Plane

This commit groups several options that were tested in grease pencil branch:

- Changes to fill algorithms and improves, specially in small areas and stroke corners.
  New options has been added in order to define how the fill is working and internally there are optimizations in detect the small areas in the extremes.

  Kudos to @charlie for coding this fill improvements.

- New 3D cursor view plane option.

  Now it's possible to lock the drawing plane to the 3D cursor and use the 3D cursor orientation. This allows more flexibility when you are drawing and reduce the need to create geometry to draw over surfaces.

- Canvas Grid now can be locked to 3D cursor.
- New option to reproject stroke using 3D cursor.
- Small tweaks and fixes.

Changes reviewed by @pepeland and @mendio

1 files changed, 46 insertions, 10 deletions

diff --git a/source/blender/editors/gpencil/gpencil_brush.c b/source/blender/editors/gpencil/gpencil_brush.c
index 0403a42a2c9..2df0edd3bf7 100644
--- a/source/blender/editors/gpencil/gpencil_brush.c
+++ b/source/blender/editors/gpencil/gpencil_brush.c
@@ -176,18 +176,54 @@ static void gpsculpt_compute_lock_axis(tGP_BrushEditData *gso, bGPDspoint *pt, c
 		return;
 	}
 
-	ToolSettings *ts = gso->scene->toolsettings;
-	int axis = ts->gp_sculpt.lock_axis;
+	const ToolSettings *ts = gso->scene->toolsettings;
+	const View3DCursor *cursor = &gso->scene->cursor;
+	const int axis = ts->gp_sculpt.lock_axis;
 
 	/* lock axis control */
-	if (axis == 1) {
-		pt->x = save_pt[0];
-	}
-	if (axis == 2) {
-		pt->y = save_pt[1];
-	}
-	if (axis == 3) {
-		pt->z = save_pt[2];
+	switch (axis) {
+		case GP_LOCKAXIS_X:
+		{
+			pt->x = save_pt[0];
+			break;
+		}
+		case GP_LOCKAXIS_Y:
+		{
+			pt->y = save_pt[1];
+			break;
+		}
+		case GP_LOCKAXIS_Z:
+		{
+			pt->z = save_pt[2];
+			break;
+		}
+		case GP_LOCKAXIS_CURSOR:
+		{
+			/* compute a plane with cursor normal and position of the point
+			   before do the sculpt */
+			const float scale[3] = { 1.0f, 1.0f, 1.0f };
+			float plane_normal[3] = { 0.0f, 0.0f, 1.0f };
+			float plane[4];
+			float mat[4][4];
+			float r_close[3];
+			
+			loc_eul_size_to_mat4(mat,
+				cursor->location,
+				cursor->rotation_euler,
+				scale);
+
+			mul_mat3_m4_v3(mat, plane_normal);
+			plane_from_point_normal_v3(plane, save_pt, plane_normal);
+
+			/* find closest point to the plane with the new position */
+			closest_to_plane_v3(r_close, plane, &pt->x);
+			copy_v3_v3(&pt->x, r_close);
+			break;
+		}
+		default:
+		{
+			break;
+		}
 	}
 }