| CISRG research
into algorithmic sketching seeks to revive interest in the lost art of landscape drawing
as practised by Holmes, Lobeck, Raisz and others. Dowson (1994) and Visvalingam and
Dowson (1998) developed the P-stroke style for sketching. Visvalingam’s
algorithm for line filtering (published in Visvalingam and Whyatt, 1993) served to rank
the DEM cells. This ranking was independent of the view direction.
Approximately 5% of grid cells, called core cells, lying on the major curvatures in the
terrain data were filtered using four tolerances. These tolerances were varied to
suit the view and convex and concave shape of forms. Fragments of row
profiles, across the DEM, containing the core cells were filtered to provide P-stroke
sketches [1]. The sketches consist of about a third of the height values and are
thus still in the realms of minimal simplification. Visvalingam and Williamson (1995) found that the Douglas-Peucker
algorithm (1973) was better than Visvalingam's algorithm for minimal simplification of 2D
lines. Since the Douglas-Peucker algorithm is more readily available, Visvalingam
and Dowson (1999) investigated its utility for P-stoke sketching. The comparison of
filtered terrain profiles were confusing and inconclusive. When these filtered points were
plotted on contour maps, the Douglas-Peucker algorithm seemed to be better since it
provided more connected runs of core cells. However, the sketches abstracted with
the Douglas-Peucker algorithm lacked Gestalt qualities of coherence and pregnance. Whereas
the P-strokes generated with Visvalingam’s algorithm were perceived to be on a single
terrain surface, those derived with the Douglas-Peucker algorithm did not. The visual
system appears to have a tendency to project anomalous marks into the foreground where
they may be ignored or scrutinised. In places, these projections seem to be dependent on
interpretations of the pose of the graphic primitives, abstracted by the early visual
system. The results suggest why extreme point methods for abstracting TINs from grid DEMs
may not yield a set of optimal points for plateaus.
The results indicate that P-stroke sketching provides
another approach to evaluating line-filtering algorithms. While the sketches output
by the Douglas-Peucker algorithm may be instantly perceived as incoherent, the reasons for
this had to be deduced. The need for reflection and deduction suggests that the visual
system, having grouped marks in the early Gestalt stages of perception, may be performing
some knowledge-based comparisons at a subconscious level. It also seems as if
emotional judgements of the aesthetic quality of drawings may be based on qualitative
spatial reasoning. Variations in conceptual and procedural knowledge make such
qualitative evaluations subjective and unreliable. However, the notion that
subconscious computation is triggered in a knowledgeable mind affords a methodology for
computer evaluations based on cognitive criteria. The talk will therefore
focus on sketches for cognitive evaluation of line filtering algorithms.
[1] http://www2.dcs.hull.ac.uk/CISRG/projects/Royal-Inst/images/posters/cisrg18+19.htm
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