Estimating Gothic facade architecture from imagery

This article proposes a method for estimating the shape of masonry elements present in the facade of a Gothic building from a single image. Our approach takes as input a rectified image of a Gothic building facade and user-specified side information and provides a 3D model estimate of structural elements, e.g., doorways, windows, arches and cornices, within the facade as output. Facade estimation proceeds in two steps: (1) estimation of arches and rectangular openings and (2) estimation of the masonry, i.e., mortar and bricks, surrounding these structures. Arches and rectangular facade elements are detected and extracted using a 2-pass algorithm. Pass 1 detects and estimates individual facade elements using active contours with shape-preserving constraints. Pass 2 groups elements based on their shape similarity, proximity, and horizontal and vertical positions. Pass 1 and 2 are iterated multiple times to extract hierarchical arrangements, i.e., arches within arches that are typical to Gothic architecture. Those pixels not included as part of the architectural elements are considered masonry and are segmented into two classes: (a) mortar and (b) bricks. While current techniques use 3D scans or over-simplify facades using generic 3D models and texture-on-plane methods, the proposed work establishes promising initial steps towards estimating a brick-and-mortar model from imagery alone, i.e., a model of the actual facade components. Such models can expedite preservation efforts by providing detailed records of the geometry of these structures which may collapse or require repair and provides quantitative measurements of building components for use in research on the methods and tools used to construct these buildings.