[gimp/gimp-2-10] app: in GimpCurveView, use relative motion when dragging point

[Ell <ell src gnome org>]

commit 1ecb868821da0f3d3499591df4d159e169a93d6a
Author: Ell <ell_se yahoo com>
Date:   Wed Apr 17 17:33:24 2019 -0400

    app: in GimpCurveView, use relative motion when dragging point
    
    In GimpCurveView, when dragging an existing curve point, don't
    immediately move the point to the cursor position uppon button
    press, but rather move it relative to its current position as the
    cursor moves.  This allows selecting a point without moving it, and
    adjusting its position more easily.
    
    Additionally, when the cursor hovers above a point, or when
    dargging a point, have the coordinate indicator show the point's
    position, rather than the cursor's.
    
    (cherry picked from commit 0b9737a3ed8cac8b49c18c9c6fa7bd8bd3343c8b)

 app/widgets/gimpcurveview.c | 40 ++++++++++++++++++++++++++++++++--------
 app/widgets/gimpcurveview.h |  2 ++
 2 files changed, 34 insertions(+), 8 deletions(-)
---
diff --git a/app/widgets/gimpcurveview.c b/app/widgets/gimpcurveview.c
index e723dd61ff..1a0fa09240 100644
--- a/app/widgets/gimpcurveview.c
+++ b/app/widgets/gimpcurveview.c
@@ -212,6 +212,8 @@ gimp_curve_view_init (GimpCurveView *view)
 {
   view->curve       = NULL;
   view->selected    = 0;
+  view->offset_x    = 0.0;
+  view->offset_y    = 0.0;
   view->last_x      = 0.0;
   view->last_y      = 0.0;
   view->cursor_type = -1;
@@ -781,6 +783,8 @@ gimp_curve_view_button_press (GtkWidget      *widget,
   gint           width, height;
   gdouble        x;
   gdouble        y;
+  gdouble        point_x;
+  gdouble        point_y;
   gint           closest_point;
   gint           i;
 
@@ -812,8 +816,6 @@ gimp_curve_view_button_press (GtkWidget      *widget,
       view->leftmost = -1.0;
       for (i = closest_point - 1; i >= 0; i--)
         {
-          gdouble point_x;
-
           gimp_curve_get_point (curve, i, &point_x, NULL);
 
           if (point_x >= 0.0)
@@ -826,8 +828,6 @@ gimp_curve_view_button_press (GtkWidget      *widget,
       view->rightmost = 2.0;
       for (i = closest_point + 1; i < curve->n_points; i++)
         {
-          gdouble point_x;
-
           gimp_curve_get_point (curve, i, &point_x, NULL);
 
           if (point_x >= 0.0)
@@ -839,7 +839,17 @@ gimp_curve_view_button_press (GtkWidget      *widget,
 
       gimp_curve_view_set_selected (view, closest_point);
 
-      gimp_curve_set_point (curve, view->selected, x, 1.0 - y);
+      gimp_curve_get_point (curve, view->selected, &point_x, &point_y);
+
+      if (point_x >= 0.0)
+        {
+          view->offset_x = point_x         - x;
+          view->offset_y = (1.0 - point_y) - y;
+        }
+      else
+        {
+          gimp_curve_set_point (curve, view->selected, x, 1.0 - y);
+        }
       break;
 
     case GIMP_CURVE_FREE:
@@ -865,6 +875,9 @@ gimp_curve_view_button_release (GtkWidget      *widget,
   if (bevent->button != 1)
     return TRUE;
 
+  view->offset_x = 0.0;
+  view->offset_y = 0.0;
+
   view->grabbed = FALSE;
 
   set_cursor (view, GDK_FLEUR);
@@ -885,6 +898,7 @@ gimp_curve_view_motion_notify (GtkWidget      *widget,
   gdouble         x;
   gdouble         y;
   gdouble         point_x;
+  gdouble         point_y;
   gint            closest_point;
 
   if (! curve)
@@ -899,6 +913,9 @@ gimp_curve_view_motion_notify (GtkWidget      *widget,
   x = (gdouble) (mevent->x - border) / (gdouble) width;
   y = (gdouble) (mevent->y - border) / (gdouble) height;
 
+  x += view->offset_x;
+  y += view->offset_y;
+
   x = CLAMP (x, 0.0, 1.0);
   y = CLAMP (y, 0.0, 1.0);
 
@@ -909,12 +926,19 @@ gimp_curve_view_motion_notify (GtkWidget      *widget,
     case GIMP_CURVE_SMOOTH:
       if (! view->grabbed) /*  If no point is grabbed...  */
         {
-          gimp_curve_get_point (curve, closest_point, &point_x, NULL);
+          gimp_curve_get_point (curve, closest_point, &point_x, &point_y);
 
           if (point_x >= 0.0)
-            new_cursor = GDK_FLEUR;
+            {
+              new_cursor = GDK_FLEUR;
+
+              x = point_x;
+              y = 1.0 - point_y;
+            }
           else
-            new_cursor = GDK_TCROSS;
+            {
+              new_cursor = GDK_TCROSS;
+            }
         }
       else /*  Else, drag the grabbed point  */
         {
diff --git a/app/widgets/gimpcurveview.h b/app/widgets/gimpcurveview.h
index 25a056ca2f..49b43ebba6 100644
--- a/app/widgets/gimpcurveview.h
+++ b/app/widgets/gimpcurveview.h
@@ -48,6 +48,8 @@ struct _GimpCurveView
   gint               grid_columns;
 
   gint               selected;
+  gdouble            offset_x;
+  gdouble            offset_y;
   gdouble            last_x;
   gdouble            last_y;
   gdouble            leftmost;