backends: Add function to apply pressure sensitivity to tablet tools

A bezier curve is created out of the 2 control points in settings, so
the pressure is made to follow the stablished curve between 0 and 1.

src/backends/meta-input-settings-private.h

@@ -110,6 +110,11 @@ GDesktopStylusButtonAction meta_input_settings_get_stylus_button_action (MetaInp
                                                                          ClutterInputDeviceTool *tool,
                                                                          ClutterInputDevice     *current_device,
                                                                          guint                   button);
+gdouble                    meta_input_settings_translate_tablet_tool_pressure (MetaInputSettings      *input_settings,
+                                                                               ClutterInputDeviceTool *tool,
+                                                                               ClutterInputDevice     *current_tablet,
+                                                                               gdouble                 pressure);
+
 #ifdef HAVE_LIBWACOM
 WacomDevice * meta_input_settings_get_tablet_wacom_device (MetaInputSettings *settings,
                                                            ClutterInputDevice *device);

src/backends/meta-input-settings.c

@@ -62,6 +62,7 @@ struct _ToolSettings
   ClutterInputDeviceTool *tool;
   GDesktopStylusButtonAction button_action;
   GDesktopStylusButtonAction secondary_button_action;
+  gdouble curve[4];
 };
 
 struct _MetaInputSettingsPrivate
@@ -921,6 +922,36 @@ lookup_device_settings (ClutterInputDevice *device)
   return settings;
 }
 
+static void
+tool_settings_cache_pressure_curve (ToolSettings *tool_settings)
+{
+  GVariant *variant;
+  const gint32 *curve;
+  gsize n_elems;
+
+  if (clutter_input_device_tool_get_tool_type (tool_settings->tool) ==
+      CLUTTER_INPUT_DEVICE_TOOL_ERASER)
+    variant = g_settings_get_value (tool_settings->settings, "eraser-pressure-curve");
+  else
+    variant = g_settings_get_value (tool_settings->settings, "pressure-curve");
+
+  curve = g_variant_get_fixed_array (variant, &n_elems, sizeof (gint32));
+  if (n_elems == 4)
+    {
+      tool_settings->curve[0] = (gdouble) curve[0] / 100;
+      tool_settings->curve[1] = (gdouble) curve[1] / 100;
+      tool_settings->curve[2] = (gdouble) curve[2] / 100;
+      tool_settings->curve[3] = (gdouble) curve[3] / 100;
+    }
+  else
+    {
+      tool_settings->curve[0] = tool_settings->curve[1] = 0;
+      tool_settings->curve[2] = tool_settings->curve[3] = 1;
+    }
+
+  g_variant_unref (variant);
+}
+
 static void
 tool_settings_changed_cb (GSettings    *settings,
                           const gchar  *key,
@@ -930,6 +961,14 @@ tool_settings_changed_cb (GSettings    *settings,
     tool_settings->button_action = g_settings_get_enum (settings, "button-action");
   else if (strcmp (key, "secondary-button-action") == 0)
     tool_settings->secondary_button_action = g_settings_get_enum (settings, "secondary-button-action");
+  else if (strcmp (key, "pressure-curve") == 0 &&
+           clutter_input_device_tool_get_tool_type (tool_settings->tool) !=
+           CLUTTER_INPUT_DEVICE_TOOL_ERASER)
+    tool_settings_cache_pressure_curve (tool_settings);
+  else if (strcmp (key, "eraser-pressure-curve") == 0 &&
+           clutter_input_device_tool_get_tool_type (tool_settings->tool) ==
+           CLUTTER_INPUT_DEVICE_TOOL_ERASER)
+    tool_settings_cache_pressure_curve (tool_settings);
 }
 
 static ToolSettings *
@@ -952,6 +991,7 @@ tool_settings_new (ClutterInputDeviceTool *tool,
     g_settings_get_enum (tool_settings->settings, "button-action");
   tool_settings->secondary_button_action =
     g_settings_get_enum (tool_settings->settings, "secondary-button-action");
+  tool_settings_cache_pressure_curve (tool_settings);
 
   return tool_settings;
 }
@@ -1278,3 +1318,47 @@ meta_input_settings_get_tablet_wacom_device (MetaInputSettings *settings,
   return info->wacom_device;
 }
 #endif /* HAVE_LIBWACOM */
+
+static gdouble
+calculate_bezier_position (gdouble pos,
+                           gdouble x1,
+                           gdouble y1,
+                           gdouble x2,
+                           gdouble y2)
+{
+  gdouble int1_y, int2_y;
+
+  pos = CLAMP (pos, 0, 1);
+
+  /* Intersection between 0,0 and x1,y1 */
+  int1_y = pos * y1;
+
+  /* Intersection between x2,y2 and 1,1 */
+  int2_y = (pos * (1 - y2)) + y2;
+
+  /* Find the new position in the line traced by the previous points */
+  return (pos * (int2_y - int1_y)) + int1_y;
+}
+
+gdouble
+meta_input_settings_translate_tablet_tool_pressure (MetaInputSettings      *input_settings,
+                                                    ClutterInputDeviceTool *tool,
+                                                    ClutterInputDevice     *current_tablet,
+                                                    gdouble                 pressure)
+{
+  ToolSettings *tool_settings;
+
+  pressure = CLAMP (pressure, 0, 1);
+
+  g_return_val_if_fail (META_IS_INPUT_SETTINGS (input_settings), pressure);
+  g_return_val_if_fail (CLUTTER_IS_INPUT_DEVICE_TOOL (tool), pressure);
+  g_return_val_if_fail (CLUTTER_IS_INPUT_DEVICE (current_tablet), pressure);
+
+  tool_settings = lookup_tool_settings (tool, current_tablet);
+  pressure = calculate_bezier_position (pressure,
+                                        tool_settings->curve[0],
+                                        tool_settings->curve[1],
+                                        tool_settings->curve[2],
+                                        tool_settings->curve[3]);
+  return pressure;
+}