[gimp] app: fix transform-grid center point for non-affine transforms; use as pivot

[Ell <ell src gnome org>]

commit cdc5782b3c0ad94beff4682e67503e86b15a6b78
Author: Ell <ell_se yahoo com>
Date:   Thu Feb 28 15:21:40 2019 -0500

    app: fix transform-grid center point for non-affine transforms; use as pivot
    
    In GimpToolTransformGrid, use the transformed center-point of the
    original polygon as the position of the center-point handle, and as
    a snapping point for the pivot when "cornersnap" is TRUE, instead
    of using the center-point of the transformed polygon.  These two
    points are different for non-affine transformations.
    
    Furthermore, when "use-pivot-handle" is FALSE, use the center-
    point as the reference point when transforming around the pivot,
    instead of the pivot itself, which might not be set.  In
    particular, this fixes transformation around the pivot in the
    perspective tool.
    
    Don't explicitly set the center-point as the pivot in the scale
    tool, since this commit makes it unnecessary.

 app/display/gimptooltransformgrid.c | 58 +++++++++++++++++++++++++++++--------
 app/tools/gimpscaletool.c           | 12 +++-----
 2 files changed, 50 insertions(+), 20 deletions(-)
---
diff --git a/app/display/gimptooltransformgrid.c b/app/display/gimptooltransformgrid.c
index 6c150b6130..df162b4a29 100644
--- a/app/display/gimptooltransformgrid.c
+++ b/app/display/gimptooltransformgrid.c
@@ -801,6 +801,17 @@ transform_is_convex (GimpVector2 *pos)
                                            pos[3].x, pos[3].y);
 }
 
+static gboolean
+transform_grid_is_convex (GimpToolTransformGrid *grid)
+{
+  GimpToolTransformGridPrivate *private = grid->private;
+
+  return gimp_transform_polygon_is_convex (private->tx1, private->ty1,
+                                           private->tx2, private->ty2,
+                                           private->tx3, private->ty3,
+                                           private->tx4, private->ty4);
+}
+
 static inline gboolean
 vectorisnull (GimpVector2 v)
 {
@@ -1309,16 +1320,29 @@ gimp_tool_transform_grid_motion (GimpToolWidget   *widget,
   newpos[3].y = oldpos[3].y = private->prev_ty4;
 
   /* put center point in this array too */
-  oldpos[4].x = (oldpos[0].x + oldpos[1].x + oldpos[2].x + oldpos[3].x) / 4.;
-  oldpos[4].y = (oldpos[0].y + oldpos[1].y + oldpos[2].y + oldpos[3].y) / 4.;
+  oldpos[4].x = private->prev_tcx;
+  oldpos[4].y = private->prev_tcy;
 
   d = vectorsubtract (cur, mouse);
 
   newpivot_x = &private->tpx;
   newpivot_y = &private->tpy;
 
-  pivot.x = private->prev_tpx;
-  pivot.y = private->prev_tpy;
+  if (private->use_pivot_handle)
+    {
+      pivot.x = private->prev_tpx;
+      pivot.y = private->prev_tpy;
+    }
+  else
+    {
+      /* when the transform grid doesn't use a pivot handle, use the center
+       * point as the pivot instead.
+       */
+      pivot.x = private->prev_tcx;
+      pivot.y = private->prev_tcy;
+
+      fixedpivot = TRUE;
+    }
 
   /* move */
   if (handle == GIMP_TRANSFORM_HANDLE_CENTER)
@@ -2307,14 +2331,24 @@ gimp_tool_transform_grid_update_box (GimpToolTransformGrid  *grid)
   private->tpx = private->pivot_x;
   private->tpy = private->pivot_y;
 
-  private->tcx = (private->tx1 +
-                  private->tx2 +
-                  private->tx3 +
-                  private->tx4) / 4.0;
-  private->tcy = (private->ty1 +
-                  private->ty2 +
-                  private->ty3 +
-                  private->ty4) / 4.0;
+  if (transform_grid_is_convex (grid))
+    {
+      gimp_matrix3_transform_point (&private->transform,
+                                    (private->x1 + private->x2) / 2.0,
+                                    (private->y1 + private->y2) / 2.0,
+                                    &private->tcx, &private->tcy);
+    }
+  else
+    {
+      private->tcx = (private->tx1 +
+                      private->tx2 +
+                      private->tx3 +
+                      private->tx4) / 4.0;
+      private->tcy = (private->ty1 +
+                      private->ty2 +
+                      private->ty3 +
+                      private->ty4) / 4.0;
+    }
 }
 
 static void
diff --git a/app/tools/gimpscaletool.c b/app/tools/gimpscaletool.c
index 10bbe18a0e..1924541a3e 100644
--- a/app/tools/gimpscaletool.c
+++ b/app/tools/gimpscaletool.c
@@ -270,8 +270,6 @@ gimp_scale_tool_get_widget (GimpTransformGridTool *tg_tool)
                                          tr_tool->y2);
 
   g_object_set (widget,
-                "pivot-x",            (tr_tool->x1 + tr_tool->x2) / 2.0,
-                "pivot-y",            (tr_tool->y1 + tr_tool->y2) / 2.0,
                 "inside-function",    GIMP_TRANSFORM_FUNCTION_SCALE,
                 "outside-function",   GIMP_TRANSFORM_FUNCTION_SCALE,
                 "use-corner-handles", TRUE,
@@ -291,12 +289,10 @@ gimp_scale_tool_update_widget (GimpTransformGridTool *tg_tool)
 
   g_object_set (
     tg_tool->widget,
-    "x1",      (gdouble) tr_tool->x1,
-    "y1",      (gdouble) tr_tool->y1,
-    "x2",      (gdouble) tr_tool->x2,
-    "y2",      (gdouble) tr_tool->y2,
-    "pivot-x", (tg_tool->trans_info[X0] + tg_tool->trans_info[X1]) / 2.0,
-    "pivot-y", (tg_tool->trans_info[Y0] + tg_tool->trans_info[Y1]) / 2.0,
+    "x1", (gdouble) tr_tool->x1,
+    "y1", (gdouble) tr_tool->y1,
+    "x2", (gdouble) tr_tool->x2,
+    "y2", (gdouble) tr_tool->y2,
     NULL);
 }