Non-Photorealistic Computer Graphics Library

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Proceedings Sketchy Drawings

Author(s): Marc Nienhaus, Jürgen Döllner.
Proceedings: AFRIGRAPH '04: Proceedings of the 3rd international conference on Computer graphics, virtual reality, visualisation and interaction in Africa, pp. 73--81, Stellenbosch, South Africa, ACM Press, 2004.
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In non-photorealistic rendering sketchiness is essential to communicate visual ideas and can be used to illustrate drafts and concepts in, for instance, architecture and product design. In this paper, we present a hardware-accelerated real-time rendering algorithm for drawings that sketches visually important edges as well as inner color patches of arbitrary 3D objects even beyond the geometrical boundary. The algorithm preserves edges and color patches as intermediate rendering results using textures. To achieve sketchiness it applies uncertainty values in image-space to perturb texture coordinates when accessing intermediate rendering results. The algorithm adjusts depth information derived from 3D objects to ensure visibility when composing sketchy drawings with arbitrary 3D scene contents. Rendering correct depth values while sketching edges and colors beyond the boundary of 3D objects is achieved by depth sprite rendering. Moreover, we maintain frame-to-frame coherence because consecutive uncertainty values have been determined by a Perlin noise function, so that they are correlated in image-space. Finally, we introduce a solution to control and predetermine sketchiness by preserving geometrical properties of 3D objects in order to calculate associated uncertainty values. This method significantly reduces the inherent shower-door effect.

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