Introduction

Xsigno is a photo measurement software for interior planning. Using your digital camera, you take a set of pictures of the room or object to be measured and process the pictures to measure coordinates, lengths or angles. Using photography for measurements is ideally suited if it is necessary to forward and explain your measurements to a third party. As you are measuring spatial coordinates it is very versatile, from the coordinates one can measure lengths, angles or compute areas. Xsigno allows to extract planar textures from photographs. For simple objects composed of planar surfaces, it can be used as a modeling tool.

The principle idea of a photo-measurement is illustrated below. For known camera location, each pixel of a camera image spans a ray through space. By marking an object point in two or more camera perspectives, its spatial coordinate can be located from the intersection of the vision rays. This principle is well known to us as our human vision uses a stereoscopic view for the spatial perception of our environment.

Coordinate measurement by triangulation from multiple camera perspectives..

To apply this principle to conventional photography, it is necessary to accurately determine the location and orientation of the camera. Additionally, it is necessary to characterize the image distortion characteristics of the camera. The determination of the cameras internal parameters (distortion and focal length) and external parameters (location and orientation) is done here by using a reference or orientation board. The orientation board is a checkerboard like grating, which uses ring-markers for easy processing.

The measurement by photography is known as “Photogrammetry” – an innovative but well established science. In geodesy, architecture and manufacturing it is a well known and highly accurate measurement technique. One key challenge of photogrammetry is the use of a stable and well characterized camera. With consumer grade cameras, the stability of the camera parameters (like focal length and image distortion) from picture to picture is the main limiting factor to the measurement accuracy. The accuracy which can be expected from a consumer grade camera for distances between 3~5 m is in the mm range.

This software is meant as an entry-level solution to photogrammetry where measurement accuracy is not the main objective. It offers a simple radial distortion model and assumes the principle point to be in the image center. For planar faces, textures can be extracted and saved into the Collada 3D format, the resulting models can then be further processed by CAD modeling tools.

Size measurement of a side table.