Visualization Model: Four-Layer Model. The workflow of the Four-Layer model: (1) The Input Layer registers the cursor movement and the drawing tool selected by the user, thereby calling functions for user guidance or data acquisition. (2) The Mask Layer defines the drawing surface. (3) The Drawing Layer represents the drawing elements resulting from layers 1 and 2. (4) The Background Layer contains schematic diagrams with graphical descriptions. The red lines in the Mask Layer are defined by attributes beginning with the word "Border" (see table 1). These red lines show the boundary of a drawing surface on which shapes and symbols can be drawn.