[gimp] app: allow snapping GimpDial to 15 degree intervals

[Michael Natterer <mitch src gnome org>]

commit 2367e985a4c6c786ea6b90dd9ff6697f8831bb46
Author: woob <thetoastcaper gmail com>
Date:   Thu Dec 26 14:32:56 2019 -0500

    app: allow snapping GimpDial to 15 degree intervals
    
    The GimpDial widget can now be snapped to the nearest fifteen degree
    interval by holding the shift key while adjusting. When adjusting both
    an alpha and beta value, both will be snapped according to the alpha,
    preserving the angle between them

 app/widgets/gimpdial.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
---
diff --git a/app/widgets/gimpdial.c b/app/widgets/gimpdial.c
index 7d13436f10..90a297531d 100644
--- a/app/widgets/gimpdial.c
+++ b/app/widgets/gimpdial.c
@@ -41,6 +41,8 @@
 
 #define SEGMENT_FRACTION 0.3
 
+/* round n to the nearest multiple of m */
+#define SNAP(n, m) (RINT((n) / (m)) * (m))
 
 enum
 {
@@ -310,6 +312,9 @@ gimp_dial_button_press_event (GtkWidget      *widget,
       if (dial->priv->clockwise_angles && angle)
         angle = 2.0 * G_PI - angle;
 
+      if (bevent->state & GDK_SHIFT_MASK)
+        angle = SNAP (angle, G_PI / 12.0);
+
       dial->priv->last_angle = angle;
 
       switch (dial->priv->target)
@@ -349,11 +354,16 @@ gimp_dial_motion_notify_event (GtkWidget      *widget,
     {
       gdouble delta;
 
+      if (mevent->state & GDK_SHIFT_MASK)
+        angle = SNAP (angle, G_PI / 12.0);
+
       delta = angle - dial->priv->last_angle;
       dial->priv->last_angle = angle;
 
       if (delta != 0.0)
         {
+          gdouble alpha = dial->priv->alpha;
+
           switch (dial->priv->target)
             {
             case DIAL_TARGET_ALPHA:
@@ -365,6 +375,10 @@ gimp_dial_motion_notify_event (GtkWidget      *widget,
               break;
 
             case DIAL_TARGET_BOTH:
+              /* snap both by the alpha value */
+              if (mevent->state & GDK_SHIFT_MASK)
+                delta = SNAP (alpha + delta, G_PI / 12.0) - alpha;
+
               g_object_set (dial,
                             "alpha", gimp_dial_normalize_angle (dial->priv->alpha + delta),
                             "beta",  gimp_dial_normalize_angle (dial->priv->beta  + delta),