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AA Extension Lines
==================

With some dimensions, such as inner, are drawn between the vessel walls, it is
obvious what they are measuring, but there other dimensions drawn outside
of the vessel where the actual measuring point has to be explicitly shown.
Use extension lines to aid the user. Generally these are perpendicular to
both the object (vessel wall) and the dimension line.

.. figure:: ../figures/aadims/extender_aa.png
      :width: 260
      :height: 240
      :alt: extension lines on outer dimension
      :align: center
      
      Extension lines to outer dimension
      
      Extension lines do not touch object, but pass through the dimension line

Using an outer vertical dimension as an example, the vessel wall is shown as 
a thick
blue line running vertically. The dimension, also vertical, will show the 
length of the
vessel, and lie to the left of the vessel wall. The extenders 
will be horizontal and show where on the wall the dimension is 
referring.

The extender almost touches the vessel wall, but intersects the dimension 
line and extends slightly past it.

On a dimension that is always vertical or horizontal one might calculate the
dimension and extender positions using two dictionaries with the sine and
cosine values of the relevant horizontal and vertical angles::

   sin0 = {-180: 0, -90: -1, 0: 0, 90: 1, 180: 0, 270: -1, 360: 0}
   cos0 = {-180: -1, -90: 0, 0: 1, 90: 0, 180: -1, 270: 0, 360: 1}

then calculate the next position with something like this::

   ptC = ptA[0] + abs(extaO) * cos0[angle-90], ptA[1] + abs(extaO) * sin0[angle-90]
   
One could equally have used the sine and cosine values directly::

   ptC = ptA[0] + abs(extaO) * cos(angle-90), ptA[1] + abs(extaO) * sin(angle-90)

The problem is that the values are real numbers and the program 
requires 
integers. When using the dictionaries this problem falls away.

In the end the function polar2cart was used, since the splitting
of the points into x and y coordinates, and providing integers for the result
was taken care of in one step::

   ptC = polar2cart(ptA, angle-90, abs(extaO))

.. container:: toggle

    .. container:: header

        *Show/Hide Code* extender.py

    .. literalinclude:: ../examples/aadims/extender.py