app: add configurable nonlinear mapping (gamma) to spinscale widget

The default value is 1.0 which is linear and the old behavior, values above
1.0 gives finer control in the lower portions of the range, the lower half of
the widget behaves like before doing small relative adjustments.

app/widgets/gimpspinscale.c

@@ -23,6 +23,7 @@
 #include <gtk/gtk.h>
 
 #include "libgimpwidgets/gimpwidgets.h"
+#include "libgimpmath/gimpmath.h"
 
 #include "widgets-types.h"
 
@@ -54,6 +55,7 @@ struct _GimpSpinScalePrivate
   gboolean     scale_limits_set;
   gdouble      scale_lower;
   gdouble      scale_upper;
+  gdouble      gamma;
 
   PangoLayout *layout;
   gboolean     changing_value;
@@ -459,6 +461,7 @@ gimp_spin_scale_change_value (GtkWidget *widget,
       gdouble fraction;
 
       fraction = x / (gdouble) width;
+      fraction = pow (fraction, private->gamma);
 
       value = fraction * (upper - lower) + lower;
     }
@@ -582,6 +585,7 @@ static void
 gimp_spin_scale_value_changed (GtkSpinButton *spin_button)
 {
   GtkAdjustment *adjustment = gtk_spin_button_get_adjustment (spin_button);
+  GimpSpinScalePrivate *private = GET_PRIVATE (spin_button);
   gdouble        lower;
   gdouble        upper;
   gdouble        value;
@@ -590,8 +594,9 @@ gimp_spin_scale_value_changed (GtkSpinButton *spin_button)
 
   value = CLAMP (gtk_adjustment_get_value (adjustment), lower, upper);
 
+
   gtk_entry_set_progress_fraction (GTK_ENTRY (spin_button),
-                                   (value - lower) / (upper - lower));
+                  pow((value - lower) / (upper - lower), 1.0/private->gamma));
 }
 
 
@@ -632,10 +637,36 @@ gimp_spin_scale_set_scale_limits (GimpSpinScale *scale,
   private->scale_limits_set = TRUE;
   private->scale_lower      = lower;
   private->scale_upper      = upper;
+  private->gamma            = 1.0;
 
   gimp_spin_scale_value_changed (spin_button);
 }
 
+void
+gimp_spin_scale_set_gamma (GimpSpinScale *scale,
+                           gdouble        gamma)
+{
+  GimpSpinScalePrivate *private;
+
+  g_return_if_fail (GIMP_IS_SPIN_SCALE (scale));
+
+  private = GET_PRIVATE (scale);
+
+  private->gamma = gamma;
+}
+
+gdouble
+gimp_spin_scale_get_gamma (GimpSpinScale *scale)
+{
+  GimpSpinScalePrivate *private;
+
+  g_return_if_fail (GIMP_IS_SPIN_SCALE (scale));
+
+  private = GET_PRIVATE (scale);
+
+  return private->gamma;
+}
+
 void
 gimp_spin_scale_unset_scale_limits (GimpSpinScale *scale)
 {
@@ -671,4 +702,3 @@ gimp_spin_scale_get_scale_limits (GimpSpinScale *scale,
 
   return private->scale_limits_set;
 }
-

app/widgets/gimpspinscale.h

@@ -58,5 +58,9 @@ gboolean    gimp_spin_scale_get_scale_limits   (GimpSpinScale *scale,
                                                 gdouble       *lower,
                                                 gdouble       *upper);
 
+void        gimp_spin_scale_set_gamma          (GimpSpinScale *scale,
+                                                gdouble        gamma);
+
+gdouble     gimp_spin_scale_get_gamma          (GimpSpinScale *scale);
 
 #endif  /*  __GIMP_SPIN_SCALE_H__  */