Sketching with projective 2D strokes

Freehand sketching has long had appeal as an artistic medium for conceptual design because of its immediacy in capturing and communicating design intent and visual experience. We present a sketching paradigm that supports the early stages of design by preserving the fluidity of traditional freehand drawings. In addition, it attempts to fill the gap between 2D drawing programs, which have fixed views, and 3D modeling programs that allow arbitrary views. We implement our application as a two-dimensional drawing program that utilizes a projective representation of points — i.e. points that lie on the surface of a unit sphere centered at the viewpoint. This representation facilitates the production of novel re-projections generated from an initial perspective sketch and gives the user the impression of being immersed in the drawing or space. We describe a method for aligning a sketch drawn outside the system using its vanishing points, allowing the integration of computer sketching and freehand sketching on paper in an iterative manner. The user interface provides a virtual camera, projective grids to guide in the construction of proportionate scenes, and the ability to underlay sketches with other drawings or photographic panoramas.