 APEX/application, it is the mechanism for implementing clicking and erasing the initial version of the boundary. An <tt>image</tt> object with CData and AlphaData both set to the current state of the layer (the points which are selected) is used, as is a 2D matrix called the "stack", which records the order in which points have been clicked. This mechanism is important for the interpolation process, which is detailed within the documentation for <a href="InterpolationLayer.html"><tt>InterpolationLayer</tt>