MIT Sketching Understand
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<p>C O V E R F E A T U R E</p> <p>Magic Paper: SketchUnderstanding ResearchRandall DavisMassachusetts Institute of Technology</p> <p>Sketches are hand-drawn informal gures often created as a way of thinking about or working through a problem. Sketch-understanding systems let users interact with computers by drawing naturally, offering a freedom not available with traditional CAD systems.</p> <p>S</p> <p>ketching is ubiquitous: We draw as a way of thinking, solving problems, and communicating in a wide variety of elds, for both design (such as sketches of conceptual designs) and analysis (such as sketches drawn to help puzzle through problems in physics or electronic circuits). Unfortunately in todays technology, sketches are deadtheyre either graphite on slices of dead trees, or, if captured on a PDA or tablet computer, simply pixels of digitized ink. The Sketch Understanding Group at MIT has been working toward a kind of magic paperthat is, a surface thats as natural and easy to draw on as paper, yet that understands what you draw. What does it mean for the paper to understand? One example, in Figure 1, shows some of our earliest work. We use Assist (A Shrewd Sketch Interpretation and Simulation Tool) to sketch simple 2D physical devices, then watch them behave.1 Assist understands the raw sketch shown in Figure 1a in the sense that it interprets the ink the same way we do (Figure 1b), that is, as an inclined plane with a wheeled cart. As Figure 1c shows, it hands this interpretation to a physics simulator, which animates the device, giving the user the experience of drawing on intelligent paper. One detail helps illustrate the sense in which the system understands the sketch in a manner similar to a human observer. The wheels (blue circles) are attached to the cars body with pin joints (pink circles), yet the user draws both the wheels and pin joints with the same geometric shapea circle. The system interprets a</p> <p>circle differently based on contexta circle can be a pin joint only if its drawn over two bodies that are already overlapping (in this case the car body and the wheel). Weve built sketch-understanding systems for a variety of domains, including the simple physics sketcher shown in Figure 1, Unied Modeling Language diagrams (Figure 2), analog circuits (Figure 3), and chemical structure sketches2 (Figure 4).</p> <p>TERMINOLOGY AND FOCUSSketches differ from diagrams. By diagrams we mean the more formal, at times draftsman-like figures that CAD systems produce, while sketches are the hand-drawn informal figures people create on paper, whiteboards, napkins, and, more recently, tablet computers. There is a significant body of work on understanding diagrams such as the International Conferences on Document Analysis and Recognition, but the task of understanding a sketch is significantly different. Research on sketch understanding also differs from the sizable body of work on handwriting understanding. Sketch-understanding work proceeds largely from an attempt to recognize the shape of the objects drawn, using the same notion of shape that people use. Handwriting recognition, on the other hand, doesnt attempt to recognize each letter by its shape, and has successfully used machine-learning techniques that derive distinguishing features that might or might not correspond to what people attend to.0018-9162/07/$25.00 2007 IEEE</p> <p>34</p> <p>Computer</p> <p>Published by the IEEE Computer Society</p> <p>(a)</p> <p>(b)</p> <p>(c)</p> <p>Figure 1. Assist system. (a) The initial sketch, (b) the sketch as cleaned up by Assist, and (c) the simulation, showing the consequences.</p> <p>//Source le: c:\\video\\Deck.java public class Deck extends Hand implements Dealable { public Card theCard; public Game theGame; /** * @roseuid 3C21348C0257 */ public Deck() { } }</p> <p>Figure 2. A UML sketch recognized then turned into code using RationalRose.</p> <p>WHY SKETCH?Given the prevalence and power of design-automation tools, its reasonable to ask, why sketch when we have tools to create far more precise and polished drawings? Two things motivate the desire to enable sketching.</p> <p>First, picking up a pen is still more natural than using a keyboard and mouse, and free-form sketched gures seem more intuitive than the formal shapes a CAD program generates. Theres a freedom of thought that seems to go with the ease of sketching, an intuition supported by eviSeptember 2007</p> <p>35</p> <p>Figure 3. An analog circuit sketch recognized, then analyzed by Spice.</p> <p>Figure 4. A chemical structure sketch recognized, then redrawn by ChemDraw.</p> <p>dence from cognitive science research, which showed that designers who sketched produced more design alternatives than did those who were using a drafting tool.3 Second, and perhaps more important, CAD systems typically require commitments that the sketcher might not want to make, particularly at the early conceptual design stage, or when sketching a rough picture for an analysis problem. It is, for example, impossible simply to draw a line in, say, a mechanical CAD system: Every line has a precise dimension, angle, and so on. Yet, when you want to dash off an idea as to how a device might work, the need to specify precise sizes, angles, radii, and so on gets in the way.</p> <p>DIFFICULTIES WITH SKETCH INTERPRETATIONSketch understanding seems so quick and intuitive when we do it that we may wonder why creating soft36Computer</p> <p>ware to do the same thing is so difcult. One way to see the difculties in the task is to view the problem as one of signal interpretation (interpreting a time-stamped sequence of points). Many of the standard signal interpretation issues arise, along with some novel ones. Sketch interpretation in general is difficult in direct proportion to the users allowed degree of freedom the less constrained the drawing style, the more difcult the interpretation task. At one end of the spectrum lie techniques such as Grafti, which prescribes a specic set of gestures to be drawn in a specied manner. In these circumstances, recognition is considerably easier, and the computational demand relatively modest. The tradeoff is the necessity of learning and using a particular drawing style. An intermediate position is taken by work like that in Silk,4 which suggests doing minimal interpretation of the strokes, in part because the system is</p> <p>intended as a design platform, and in part to avoid interfering with the creative process. Our work lies closer to the unrestricted end of the spectrum. We want people to be able to draw as they would normally, without specifying the number, order, or direction of strokes in a symbol, yet still have the system understand. Figure 5 illustrates several difculties that arise from this. First, our task is incremental. That is, we want the system to interpret users strokes as theyre drawn. This lets the system provide continuous feedback about its understanding of the sketch, so the user can make corrections when a misunderstanding arises. But it also means that interpretation must be continually revised. In Figure 5, for example, the user has not yet drawn the line connecting the circle and rectangle in the upper part of the sketch that will indicate a married couple. In addition, as in any signal interpretation problem, there is noise. In this case, noise arises from our inability to draw with mechanical precision: Lines wiggle, curves intended to be closed might have a gap or an overlap, and so on. Next, the drawing conventions in many domains permit variations. In analog circuits, for example, you can draw capacitors as two straight parallel lines or as a straight line and a curved line. Individual styles also vary, across users and even within a sketch. In Figure 5, the user drew the three successive rectangles in the lower part of the gure, yet drew them in three different ways (using a single stroke, a squared C-shape closed by a line, and two L-shaped strokes). Another issue is the difficulty of segmentation. Consider the arrow pointing toward the rectangle at the bottom left in Figure 5. The arrowhead touches the rectangle, but isnt connected to the shaft. Determining which strokes might belong together requires understanding the domain. Next, difculties arise when the user lays down extensive amounts of ink, as when strokes are overtraced or when gures are lled infor example, the lled-in triangles used in drawing some arrowheads. The problem here is that the fundamental view of the sketch as a set of strokes or time-stamped points breaks down. A lled-in shape is a particularly clear example: Its of little use to represent the ink in the interior of the shape as a sequence of strokes. It might have been laid down that way, but the intent was to create a 2D area of continuous ink; the particular set or sequence of strokes used is irrelevant. Overtracing is similar. Its the inks appearance that is intended to create the line. The numerous (possibly zigzagging) strokes used to lay down the ink are irrelevant. This calls for a change of representation, from the sketch as a sequence of strokes to an area of ink. Finally, and perhaps most interesting, the signal is both 2D and nonchronological. Its 2D in the obvious sense that, unlike normal handwriting, it spreads across the</p> <p>Figure 5. Snapshot of a family tree as a user is drawing it. Squares indicate males, circles indicate females, arrows indicate offspring, and a straight line (not yet drawn) indicates a marriage. Numbers have been added to indicate stroke order.</p> <p>page. In this its similar to but more unrestricted than handwritten mathematical formulas. The signal is nonchronological in the sense that we dont require each object to be nished before the next is started, so a user might add strokes to a sketch to complete something started earlier. For example, as the numbers in Figure 5 indicate, the user drew the arrows shafts in sequence, and only later added the heads. This differs from other signal interpretation problems, such as speech recognition. When talking you might restate something, but you cant go back in time and change the sounds you made earlier. Yet in sketching, newly added strokes can change the interpretation of strokes made earlier.</p> <p>SKETCH UNDERSTANDINGTwo basic assumptions ground most work in sketch understanding. First, the work is done in domains where theres a reasonably well-established graphical lexicon and grammar. The lexicon is the set of shapes used in a domainfor example, the standard graphical notation for digital circuit components. The grammar describes interrelations among shapes, indicating, for example, that a transistor symbol should be connected to three other components. Many scientific and engineering domains have such a graphical lexicon and syntax, but this is clearly not universal. The sort of impressionistic sketches drawn by architects early in the design process, for example, are not describable in these terms. Second, much like work in speech understanding, sketch-understanding systems are built for a specific domain. Unrestricted sketch understanding is currently out of reach, for many of the same reasons that unrestricted speech understanding isnt particularly successful.September 2007</p> <p>37</p> <p>(a)</p> <p>(b)</p> <p>(c)</p> <p>(d)</p> <p>Figure 6. Characteristics of a drawing. (a) A hand-drawn square with corresponding graphs showing the drawings (b) direction, (c) curvature, and (d) speed.</p> <p>Representing a sketchAt the most basic level, a sketch is a collection of timestamped coordinate points, grouped into individual strokesthat is, from pen down to pen up. As with handwriting recognition, online and offline sketching differ. Offline sketches must be scanned and lines located, introducing additional noise, while the raw data of online sketches is composed of strokes whose locations and widths are known precisely.</p> <p>Recognizing shapesThe variety of information available in online sketches enables three distinctly different representations, with three correspondingly different approaches to recognition: how the shape is denedthe set of geometric constraints the shape must obey to be an instance of a particular object; how the shape is drawnthe sequence of strokes used; and what the shape looks likethe traditional concept of image appearance. Weve built recognition systems to explore each of these representations individually and are working to understand how to combine them. How its defined. We represent a shapes denition in a language called Ladder.7 As Figure 7 shows, Ladder descriptions list the basic components making up the shape and the geometric constraints that must hold between them. Recognizing individual shapes is then a process of matching the strokes drawn against a set of shape descriptions for a domain.8 The description is significant both for what it constrainsfor example, to be an arrow the shaft must be longer than the headsand for what it doesnt constrainfor example, it doesnt specify the lines orientation or length, letting the system recognize arrows of any size and in any orientation. How its drawn. When drawing the standard symbol for an AND-gate (Figure 8a), most people start with the vertical stroke, add the arc, then draw the connecting wires. This sequence, along with just a few others, accounts for most of the routinely encountered stroke orders. This is true for a wide variety of shapes, not just those as simple as in Figure 8. This observation provides a foundation for recognition by observing stroke sequence, using a variation on a hidden Markov model (HMM) called a dynamic Bayes net. We train the DBN to recognize shapes based on the sequence of primitive geometric elements such as lines and arcs found in the strokes.9</p> <p>Finding primitivesOne basic task common to much sketch-recognition research involves reinterpreting the raw data as primitive shapeslines and arcs. This low-level processing is an interesting task by itself, given the noisiness of the data. One common approach to finding primitives uses the datas temporal character, based on the observation that, when drawing by hand, people routinely slow down at corners, without consciously attempting to do so.5 This lets the system locate corners more precisely by combining information about curvature and speed, looking for points that combine high curvature and low speed. Although combining information from these two sources helps, the problem is still difficult, due to the fine-grained noisiness of the data (Figure 6). In response, we have explored the use of scale-space filtering to remove the noise, relying on the basic intuition (common to all scale-space approaches) that the signal and noise have different spatial frequencies.6 As those frequencies are not generally determinable a priori, that work uses dynamic selection of the appropriate scale from the data itself. After locating the corners, we represent strokes as piecewise linear approximations, with additional processing merging nearly collinear segments, and using arcs (for e...</p>
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