step 6/28 t 15/28 step 6/28 28 (a) easy-to-follow tutorial steps (only a few shown) (b) user sketch step 6/28 step 7/28 step 9/28 step 15/28 step 24/28 step 28/28 Figure 1 : (a) We present How2Sketch, a system that automatically generates easy-to-follow tutorials for drawing 3D models. Each generated tutorial presents a list of steps for drawing scaffolding primitives that help the user draw the object in correct perspective. To help the user draw the scaffolding, the tutorial shows how to

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... ruct guidelines that anchor object parts relative to one another. User study feedback on the tutorials indicates that users feel they are able to create more accurate drawings (b). Abstract Accurately drawing 3D objects is difficult for untrained individuals, as it requires an understanding of perspective and its effects on geometry and proportions. Step-by-step tutorials break the complex task of sketching an entire object down into easy-to-follow steps that even a novice can follow. However, creating such tutorials requires expert knowledge and is time-consuming. As a result, the availability of tutorials for a given object or viewpoint is limited. How2Sketch (H2S) addresses this problem by automatically generating easy-to-follow tutorials for arbitrary 3D objects. Given a segmented 3D model and a camera viewpoint, H2S computes a sequence of steps for constructing a drawing scaffold comprised of geometric primitives, which helps the user draw the final contours in correct perspective and proportion. To make the drawing scaffold easy to construct, the algorithm solves for an ordering among the scaffolding primitives and explicitly makes small geometric modifications to the size and location of the object parts to simplify relative positioning. Technically, we formulate this scaffold construction as a single selection problem that simultaneously solves for the ordering and geometric changes of the primitives. We generate different tutorials on man-made objects using our method and evaluate how easily the tutorials can be followed with a user study.