Allow to constrain color slider values by pressing Ctrl key

Currently
- steps of 16 (for sliders with 0-255 range) and
- steps of 10 (for all other cases)
are hardcoded but could be made a preference if there's demand for it.

3 files changed, 18 insertions, 5 deletions

diff --git a/src/ui/widget/color-scales.cpp b/src/ui/widget/color-scales.cpp
index cb1f36066..cb6349c4b 100644
--- a/src/ui/widget/color-scales.cpp
+++ b/src/ui/widget/color-scales.cpp
@@ -244,9 +244,18 @@ gfloat ColorScales::getScaled(const GtkAdjustment *a)
     return val;
 }
 
-void ColorScales::setScaled(GtkAdjustment *a, gfloat v)
+void ColorScales::setScaled(GtkAdjustment *a, gfloat v, bool constrained)
 {
-    gfloat val = v * gtk_adjustment_get_upper(a);
+    gdouble upper = gtk_adjustment_get_upper(a);
+    gfloat val = v * upper;
+    if (constrained) {
+        // TODO: do we want preferences for these?
+        if (upper == 255) {
+            val = round(val/16) * 16;
+        } else {
+            val = round(val/10) * 10;
+        }
+    }
     gtk_adjustment_set_value(a, val);
 }
 
diff --git a/src/ui/widget/color-scales.h b/src/ui/widget/color-scales.h
index 0b7cae47e..1fc465418 100644
--- a/src/ui/widget/color-scales.h
+++ b/src/ui/widget/color-scales.h
@@ -30,7 +30,7 @@ public:
     static const gchar *SUBMODE_NAMES[];
 
     static gfloat getScaled(const GtkAdjustment *a);
-    static void setScaled(GtkAdjustment *a, gfloat v);
+    static void setScaled(GtkAdjustment *a, gfloat v, bool constrained = false);
 
     ColorScales(SelectedColor &color, SPColorScalesMode mode);
     virtual ~ColorScales();
diff --git a/src/ui/widget/color-slider.cpp b/src/ui/widget/color-slider.cpp
index 761f2e748..ed034598e 100644
--- a/src/ui/widget/color-slider.cpp
+++ b/src/ui/widget/color-slider.cpp
@@ -157,7 +157,9 @@ bool ColorSlider::on_button_press_event(GdkEventButton *event)
         signal_grabbed.emit();
         _dragging = true;
         _oldvalue = _value;
-        ColorScales::setScaled(_adjustment->gobj(), CLAMP((gfloat)(event->x - cx) / cw, 0.0, 1.0));
+        gfloat value = CLAMP((gfloat)(event->x - cx) / cw, 0.0, 1.0);
+        bool constrained = event->state & GDK_CONTROL_MASK;
+        ColorScales::setScaled(_adjustment->gobj(), value, constrained);
         signal_dragged.emit();
 
 	auto window = _gdk_window->gobj();
@@ -213,7 +215,9 @@ bool ColorSlider::on_motion_notify_event(GdkEventMotion *event)
         Gtk::Allocation allocation = get_allocation();
         cx = get_style_context()->get_padding(get_state_flags()).get_left();
         cw = allocation.get_width() - 2 * cx;
-        ColorScales::setScaled(_adjustment->gobj(), CLAMP((gfloat)(event->x - cx) / cw, 0.0, 1.0));
+        gfloat value = CLAMP((gfloat)(event->x - cx) / cw, 0.0, 1.0);
+        bool constrained = event->state & GDK_CONTROL_MASK;
+        ColorScales::setScaled(_adjustment->gobj(), value, constrained);
         signal_dragged.emit();
     }