Draw and order - modeless interactive acquisition of outlines

The representation of an object's outline with polygons or more specific shapes is the first choice for interactive outline acquisition. Usually, an operator has to select appropriate tools to capture an object's outline according to his or her notion of the shape with its underlying construction principles. We propose a modeless approach guided by geometric reasoning, i.e., the user does not have to select operation modes according to his or her model. To cope with modern input devices such as touch pens, styluses, or fingers, we consider strokes as input in arbitrary order. By approximating theses strokes with straight line segments, we obtain estimated levels of noise accounting for the uncertainty of the acquisition. These straight line segments are the basis for the subsequent hypothesis generation to recognize geometric relations. After identifying sets of consistent and nonredundant constraints, we enforce these constraints in an adjustment process. To demonstrate the usability of the proposed approach, we capture the outlines of buildings and roof areas in true orthophotos of a benchmark data set.