Calibrating Wide-Angle Cameras with Pano-robots

[tksharpless]

Many panoramic photographers now use motorized robot rotators that can position a camera with milli-degree accuracy. These devices turn the camera around a single fixed point, so are not suitable for acquiring images for photogrammetry. However they do make possible a new way of calibrating cameras and lenses, that is both more convenient and more accurate than traditional methods based on special targets. In brief, one takes a series of photos of any suitably detailed large subject, turning the camera by a small angle between shots, and processes them with a panorama stitcher set up to estimate just the camera parameters, the pointing angles being given.
Fanotec Nodal Ninja Mecha robots and PTGui, the professional stitching software, are very suitable tools for this purpose. PTGui works similarly to an sfm tool, identifying matching keypoints and aligning them by a kind of bundle adjustment. The estimation problem is simpler because the photos all have the same viewpoint; consequently the camera parameters are usually very accurate -- especially so when accurate view angles are provided a priori.
Unfortunately PTGui's lens model differs considerably from any of the ones supported by colmap, and there is no simple way to convert its parameters. It is actually rather better at modeling the fish-eye and ultrawide lenses commonly used in panography, and no worse for longer lenses that have no tangential distortion. So I propose that the PTGui lens model be added to the roster of models that colmap supports: only for precalibrated cameras, there would be no need to optimize it in colmap. I have developed independent C++ code that duplicates PTGui's distortion/undistortion perfectly, and would be happy to make that available.