Shape from Positional-Contrast: Characterising Sketches with Qualitative Line Arrangements
Graphical queries for the purpose of searching for pictorial information are of growinginterestinareaswherepicturesprovidevaluableinformation, including, for instance, design, architecture, and engineering. Sketching graphical queries is a natural way of revealing the visual appearance of objects one has in mind. The problem which arises is identifying necessary shape properties of sketches, thatis,thosepropertieswhicharenotaccidentalbutarenecessaryforspecifying a particular object property. This problem arises in particular because sketches are imprecise, and often distorted by the artistic limitations of the sketcher. From the theoretical point of view the concept of pictorial space applies. In this context, new concepts are required, in particular for dealing with imp- cise shape information in the plane. Taking into account constraints imposed by pictorial space, a relation algebra of intersection-free relations is proposed, which allows reasoning about qualitative line arrangements in the plane. The theory is further developed for characterising polygons, by deriving a number of qualitative properties which aid in describing polygons qualitatively. Applying this theory, only line arrangements which are both readily sketched and easily perceivable are considered to be di?erent. The notion of positional-contrast is introduced, which points out that the particular arrangements of line segments, i. e. their positions relative to each other, provide an expressive means of ch- acterising necessary shape properties. The method developed in this work is applied in using graphical queries to search for historical objects.
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The imprecision of sketches 13
Qualitative line arrangements
Characterising polygons qualitatively 75
stances the numbers refer to the TLT12 relations
with respect to line segment x
Zwei Jahre dauernde Renette
B Queries and reference sets
The first query
The second sketch and its precision and recall
The third sketch and its precision and recall
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ab ba bb algorithm angles ba bb bbbb bb ab ba bb ab rq bb bbbb rrr bbb bbbb bb bbbb bb ab Bl Bl bq bb brrr brr characterise Co(a coarse concave consider convex hull convex polygon course curvature curve defined described distinguish endpoints example global properties granularity level holds imprecise line segments line tracks Moratz neighbourhood graph number of lines objects perceptually aided Precision Precision Precision Recall precision-recall primary interval qr rq rr qualitative approach qualitative line arrangements Qualitative matrix quantitative ra bb ra br bbrr ra rr reference line reference segment reference system relation algebra relevant images representation result set rq bb bbbr rq rr rrrr rr qr rq rr rrrr rrr rrr bbb bbbb rrrr rr qr rrrr rrr bbb Schlieder scope shape information shape properties shows side of Figure sketches spatial stalk straight Table topological
Page xi - FIG. 2. Subjects attempted to approximate the closed figure shown above with a pattern of 10 dots. Radiating bars indicate the relative frequency with which various portions of the outline were represented by dots chosen. Evidence from other and entirely different situations supports both of these inferences. The concentration of information in contours is illustrated by the remarkably similar appearance of objects alike in contour and different otherwise. The "same...