Developing 3D Models from Photos
In 1330 the church burned down and was left unused for the rest of that century. When the reconstruction of the cathedral began, the Gothic style had reached Denmark. Around 1500 the cathedral was completed in the form we see today.
At 305 feet (93 meters) long, it is the longest church in Denmark. Its height is also 305 feet (93 meters). It can seat approximately 1200 people. Aarhus Cathedral is dedicated to St. Clement, the patron saint of sailors.
The Photogrammetry Process
The first step was to calibrate a digital camera using the PhotoModeler's calibrator program. This software computes the camera's parameters including focal length, lens distortion, image aspect ratio, and principle point.
Then I took a series of 16 overlapping photos of the cathedral from street-level using a wide-angle (24-millimeter) lens because the narrow space around the church made it impossible to take pictures from a normal distance.
I then imported the photos into PhotoModeler. Within the software, I marked and referenced easy-to-identify features such as building corners and peaks, signs, doors, and other elements that appear on the multiple photographs.
The next step was just a matter of clicking the "process" button and waiting for the software to do the computations. The procedure optimizes all the data to determine, first, the position and angle of the camera at time of exposure for each photograph and, second, the positions of the marked points.
PhotoModeler's "bundle adjustment" algorithm provides both relative and absolute camera adjustment, features that are normally offered only by packages that cost 10 times more.
When the first stage of the model was completed, I evaluated its accuracy by checking how closely it was able to match the target points in adjacent photographs. (In applications that require greater accuracy than is obtained on the first iteration, I find weak 3D points using the software's audit tools. I can then adjust, delete, and add points until the accuracy reaches an acceptable level.)
When the 3D model of the church was satisfactory, I added to its surface photographic textures taken from the original project photos. PhotoModeler then updates the fully textured model and displays it in the interactive 3D Viewer. The software keeps track of the bitmap that was selected for each surface and displays it in the proper position regardless of the perspective from which the model is viewed.
In the views shown, notice that the rendered images included unintended pictures of street traffic. This project was an experiment to determine if it was possible to make a 3D model from photos taken with a wide angle lens. To take pictures optimized for texture rendering I would have had to use a lift or some other means of avoiding the unwanted elements.
The result is a detailed VRML model that can then be exported for use in animation, CAD, or other applications. Those interested in history, architecture, tourism, or religion can then view and interact with the model via the Web.
Much of my work has been to model and measure existing buildings to create architectural documentation for renovation and expansion projects. I have also modeled a wide range of interesting objects, such as archeological artifacts and museum pieces, which can be placed on Web sites to give visitors a chance to experience them from a new perspective.
This kind of 3D modeling clearly holds benefits for many companies and nonprofit organizations. The applications for photogrammetry using only a digital camera and inexpensive, easy-to-use PC-based software are virtually unlimited.
Jens Vedel is an architect in Hoejbjerg, Denmark. His company, 3D Photo, creates 3D models for architects, archaeologists, and museums.