Drawing with skeletal strokes is similar to drawing with constant thickness strokes on other systems. To select a stroke to be used for a particular path, there is a repertoire of predefined or user-defined strokes to choose from in the menu. However, choosing from the menu is inconvenient, especially when artists are likely to use a variety of strokes interchangeably. To ensure quick switching between strokes, we therefore allow the binding of 40 different strokes to the numeric keys (together with a combination of the Shift and Control modifier keys). The system can of course take advantage of more powerful input devices like the pressure sensitive stylus. Skilful artists may want to modify the stroke attributes and to switch between strokes based on the pen pressure and speed. It is however worth pointing out that all the illustrations shown in this paper were created with just a modest 2-button stylus.
Besides varying the application path, a stroke application can be modified by specifying the stroke width and shear angle. This is done by interactively pulling out a rubber line from any one end of the stroke (Figure 21) or by specifying explicit numeric values. The length of the line indicates the stroke width and the angle the line makes with the path's normal would give the stroke the same amount of shearing. The direction of application can also be reversed by hitting a key. The colour and fill pattern of a stroke can also be changed without touching the objects in the original definition. Figures 20 and 22 are drawn using the same set of strokes used in Figure 2 and 3. After modifying an applied stroke, the system will immediately redraw on the screen the stroke resulting from the new application.
As is the case with PostScript, the stroking process (the application of a skeletal stroke along a path) is separated from the filling process. An area enclosed by a skeletal stroke path can be filled in the same way as polygons enclosed by constant thickness strokes are.
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