Checklist/Tutorial: Auto view direction to marker

This change automatically changes the view direction to look
at a <marker> if any is defined for a checklist item or tutorial
step.

Both the tutorial and checklist features support a <marker> element
which can be used by an aircraft developer to display a magenta
circle around an item of interest (typically a control in the
cockpit).

Previously aircraft developers had to add a <view> element to move
the viewpoint to look at the marker, while users of checklists had
to look for the marker manually.

Now:
- For checklists, pressing the "?" button on a checklist will pan the viewpoint
to the marker.
- For tutorials, if there is not a <view> element defined for a
tutorial step, the view will automatically pan to the marker.

Docs/README.checklists

@@ -21,7 +21,8 @@ with the following tags
                 hand side
   <marker>    - A tutorial marker (displayed when the user presses the ? button)
                 This can be easily placed using the Help->Display Tutorial Marker.
-                Contains x-m, y-m, z-m and scale tag.
+                Contains x-m, y-m, z-m and scale tag.  The view will automatically
+                change so the marker is in the center of the screen.
   <condition> - Optional standard FlightGear condition node that evaluates when the
                 checklist item has been completed.
   <binding>   - Zero or more bindings to execute the checklist item.  Allows the user

Docs/README.tutorials

@@ -356,8 +356,8 @@ copilot view etc. Default view-number is 0 (captain's view).
       <field-of-view>55</field-of-view>            default: 55; smaller zooms in
   </view>                                                       bigger zooms out
 
-
-
+If a <marker> is set by no <view>, the tutorial will automatically change the view
+direction so the marker is in the center of the screen.  
 
 
 -- <marker> -------------------------------------------------------------------
@@ -387,9 +387,8 @@ model in its animation xml file:
       ...
   </PropertyList>
 
-
-
-
+If a <marker> is set by no <view>, the tutorial will automatically change the view
+direction so the marker is in the center of the screen.  
 
 -- <message>/<audio> ----------------------------------------------------------
 

Nasal/tutorial/tutorial.nas

@@ -347,12 +347,43 @@ var remove_models = func {
 var set_view = func(node = nil) {
 	node != nil or return;
 	var v = node.getChild("view");
+	var m = node.getChild("marker");
 	if (v != nil) {
 		# when changing view direction, switch to view 0 (captain's view),
 		# unless another view is explicitly specified
 		v.initNode("view-number", 0, "INT", 0);
 		view.point.move(v);
 		return 1;
+	} elsif (m != nil) {
+		# We don't have a specific view, but there is a marker we should
+		# direct the viewpoint towards.
+
+        # Determine offset to the marker.
+        var vx = getprop("/sim/current-view/x-offset-m");  # Port/Starboard (+ve starboard)
+        var vy = getprop("/sim/current-view/y-offset-m");  # Up/Down (+ve up)
+        var vz = getprop("/sim/current-view/z-offset-m");  # Fore/Aft (+ve aft)
+
+        # BUT the marker has a different coordinate system!
+		var x = m.getNode("x-m", 1).getValue(); # Fore/Aft (+ve aft)
+		var y = m.getNode("y-m", 1).getValue(); # Port/Starboard (+ve starboard)
+		var z = m.getNode("z-m", 1).getValue(); # Up/Down  (+ve up)
+
+        # So we need to do a coordinate transformation as we calculate the angles
+        var hdg = math.atan2(y-vx, x-vz) * R2D + 180;
+        var pitch = math.atan2((z-vy), math.sqrt((x-vz)*(x-vz) + (y-vx)*(y-vx))) * R2D;
+
+        if (! ((hdg > 160) and (hdg < 200))) {
+          # Making the assumption we're in the cockpit, there are limits to where we can look
+          # that are enforced by the view system.  So we can't look into the region of heading
+          # 160-200.
+          var v = props.Node.new();
+          v.setValues({
+            "heading-offset-deg" : hdg,
+            "pitch-offset-deg": pitch,
+          });
+          view.point.move(v);
+		  return 1;
+        }
 	}
 	return 0;
 }

gui/dialogs/checklist.xml

@@ -325,8 +325,36 @@
 					"scale/value": scale,
 					"arrow-enabled": 1,
 				});
+
+        # Determine offset to the marker.
+        var vx = getprop("/sim/current-view/x-offset-m");  # Left/right (+ve right)
+        var vy = getprop("/sim/current-view/y-offset-m");  # Up/Down (+ve up)
+        var vz = getprop("/sim/current-view/z-offset-m");  # Forward/backwards (+ve aft)
+
+        # BUT the marker has a different coordinate system!
+        # x = fore/aft (+ve aft)
+        # y = left/right (+ve right)
+        # z = up/down  (+ve up)
+
+        # So we need to do a coordinate transformation as we calculate the angles
+        var hdg = math.atan2(y-vx, x-vz) * R2D + 180;
+        var pitch = math.atan2((z-vy), math.sqrt((x-vz)*(x-vz) + (y-vx)*(y-vx))) * R2D;
+
+        if (! ((hdg > 160) and (hdg < 200))) {
+          # Making the assumption we're in the cockpit, there are limits to where we can look
+          # that are enforced by the view system.  So we can't look into the region of heading
+          # 160-200.
+          var v = props.Node.new();
+          v.setValues({
+            "heading-offset-deg" : hdg,
+            "pitch-offset-deg": pitch,
+          });
+          view.point.move(v);
+        }
+        
       };
 
+
       var nextPage = func() {
         var currentPage = getprop("/sim/gui/dialogs/checklist/selected-page");
         var s = getprop("/sim/gui/dialogs/checklist/selected-checklist");