add note about mysterious -15 in the code

common/lib/capa/capa/inputtypes.py

@@ -510,7 +510,8 @@ class ImageInput(InputTypeBase):
         # if value is of the form [x,y] then parse it and send along coordinates of previous answer
         m = re.match('\[([0-9]+),([0-9]+)]', self.value.strip().replace(' ', ''))
         if m:
-            # TODO (vshnayder): why is there a "-15" here??
+            # Note: we subtract 15 to compensate for the size of the dot on the screen.
+            # (which supposedly has size 30).
             (self.gx, self.gy) = [int(x) - 15 for x in m.groups()]
         else:
             (self.gx, self.gy) = (0, 0)

common/lib/capa/capa/tests/test_inputtypes.py

@@ -400,6 +400,7 @@ class ImageInputTest(unittest.TestCase):
         self.assertEqual(context, expected)
 
     def test_with_value(self):
+        # Check that compensating for the dot size works properly.
         self.check('[50,40]', 35, 25)
 
     def test_without_value(self):