diff --git a/.todo b/.todo
index 5e7b7d7..2a63452 100644
--- a/.todo
+++ b/.todo
@@ -1,2 +1,3 @@
 add audio capabilities
 add gradient functionality
+fix blending of overlay drawing routines
diff --git a/doc/index.html b/doc/index.html
index 82f5603..612591c 100644
--- a/doc/index.html
+++ b/doc/index.html
@@ -81,7 +81,12 @@ The Z axis extends positively into the "air" above the XY plane, and the
 default gravity vector pulls along the negative Z axis.
 </p>
 <p>
-FIXME: 2D coordinates
+When 2D coordinates are used (with mouse cursor positions and
+overlay drawing routines), the origin is in the lower-left corner of
+the screen.
+The X axis increases positively toward the right edge of the screen.
+The Y axis increases positively toward the top of the screen.
+The upper-right pixel on the screen has coordinates (width-1, height-1).
 </p>
 
 
diff --git a/tests/managed_objects.lua b/tests/managed_objects.lua
index 1a2991b..4cbec08 100644
--- a/tests/managed_objects.lua
+++ b/tests/managed_objects.lua
@@ -80,4 +80,9 @@ function update_overlay_event(width, height)
     end
     ag.fillArc(1, 1, 0, 50, 50, 30, 30, 330)
     ag.fillCircle(1, 0, 1, width - 50, height - 50, 40)
+    ag.drawRect(1, 0.5, 0, width, height, width/2, height/2)
+    ag.drawPoint(1, 0, 1, 0, 0, 0)
+    ag.drawPoint(1, 0, 1, 0, width-1, height-1)
+    ag.drawPoint(1, 0, 1, 1, -1, -1)
+    ag.drawPoint(1, 0, 1, 1, width, height)
 end