Many powerful and expressive strokes have been proposed before in SIGGRAPH and other literature. Smith and Whitted proposed the use of antialiased Z-buffer images to create strokes with a 3D appearance. Bleser et. al. used a lookup table of bitmaps indexed by the pressure and tilt reading of a 5D digitizer stylus. They effectively recreated a digital charcoal stick. Another system models graphite pencils in a similar way. Greene's drawing prism [2 4] even allows the capturing of the path of virtually anything which is being dragged over the prism surface. However, strokes created by these systems suffer from the same problem as those provided by paint programs: after being drawn, they are difficult to edit, especially if they are antialiased.
Other physical models exist. Strassmann modelled the ink-laying processes of bristle brush on paper. Guo and Kunii extended them to include ink-diffusion through the paper fabric mesh. Their results are attractive despite the relatively slow computation speed (1 to 2 minutes per stroke in Strassmann's prototype system). To achieve ultimate authenticity, Pang et.al. even attached real bristle brushes to plotters and defined the strokes by the paths and pen up/down control parameters.
These strokes did reproduce on the digital computer, to a certain extent, the effects made by a brush or pen. Unfortunately, the dexterity demanded of the artist in the handling of the brush or pen is also transferred to the user at the computer, often in an unnatural way. The control parameters (speed of movement, pressure, tilt) of the individual strokes are either dependent on both a powerful input device and a skilful user, or they have to be specified explicitly. If digital strokes are no less difficult to control than real ones, it makes sense to do the drawings on paper and let the computer do the retouching and other post-processing jobs on the scanned-in version. A further problem is that simulation often takes too long for interactive use on small machines.
Figure 3: A picture drawn using nine different types of skeletal strokes after the Lithograph The Scream by Edvard Munch (18631944).
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