The P-stroke Sketch

The sketch on the right (see Visvalingam and Dowson, 1988; Visvalingam and Whelan, 1988) shows the P-stroke style of sketching. P-stroke stands for profile-stroke since this type of sketch is no more than a filtered subset of the conventional profile plot. This style was inspired by Robinson and Thrower’s and Dickinson’s adaptations of Tanaka’s inclined contours.

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This Figure comes from the exhibition mounted at the Royal Institution. It shows the main elements in the sketch. The DEM cells (points) on row and column profiles are assigned measures of significance by my line filtering algorithm.

These values can be used to filter the more important form defining cells. The filtering is view dependent and only 5% of cells are filtered.

Portions of the row profiles, called P-strokes, containing the filtered cells are then selected. Different rules are used for selecting the P-strokes for the concave and convex forms on column an row profiles.

The four layers are superimposed to give the P-stroke sketch, which consists of about 28% of DEM cells. Occluding contours may then be added to enhance depth perception as shown in the sketch above.

As it stands, the sketch is still in the realms of approximation. Many of the drawings by past masters are also verging on approximation although there are excellent examples of typification (see samples in exhibition).