CIPA 2005 XX International Symposium, 26 September - 01 October, 2005, Torino, Italy 
SOPHISTICATED USE OF VIRTUAL SHAPES OF ARCHITECTURE 
+ VISUALIZATION OF QUALITY 
W. Karel 
Institute of Photogrammetry and Remote Sensing Vienna University of Technology GuBhausstraBe 27-29 / 122, 1040 Vienna, 
Austria wk@ipf.tuwien.ac.at 
KEY WORDS: Accuracy, Adjustment, High Resolution, Information Systems, Virtual Reality 
ABSTRACT 
The object of this project was the creation of a precise photorealistic model of the Romanesque church of San Juan del Hospital in 
Valencia, Spain. The church building should be modelled with a geometric accuracy, a level of detail, and a texture resolution of 
5cm, the neighbouring pavilion even with 2cm, respectively. To reach this with economically reasonable efforts despite the complex 
geometry and difficult accessibility, virtual shapes of architecture (also called ‘fictitious observations’ or ‘Gestalts’) were integrated 
into the modelling process. The application of virtual shapes takes advantage of the fact that even ancient buildings were created on 
the basis of a plan, delimiting the object mainly with planes and straight edges. This means that in the simplest case, it can be 
presumed that points on the same face of a wall reside on a (vertical) plane. More sophisticated surfaces and curves can be applied to 
arcs and cupolas. The same virtual shapes were used for all objects built the same way (e.g. columns). Furthermore, symmetries were 
considered. Many areas on the building could only be pictured in one photograph. Thus, photogrammetric or polar spatial 
intersection was impossible there. Nevertheless, vertices within these areas could be determined by usage of shape assumptions. 
Additionally, the application of virtual shapes enhanced the determination of points on lines in poorly textured areas. The extensive 
use of virtual shapes is not the only innovation in this project, but also the visualization of the photorealistic model. Users may not 
only inspect the model itself, but also the modelling quality as a meta-information. Having activated a model vertex, the quality of 
point determination is visualized numerically by the three coordinate standard deviations and by the number of observations that 
were employed to determine the point. Moreover, quality is viewed graphically by the point's photogrammetric and tachymetric 
observation rays in space and by fading in the point's error ellipsoid. 
1. INTRODUCTION 
This publication deals with the two most interesting aspects of a 
project dedicated to the creation of a photorealistic model of the 
Romanesque church of San Juan del Hospital in Valencia, 
Spain. The task was to create a spatial model with rectified 
texture in the VRML97 format, complying with two 
preconditions: 
• Model Quality: the standard deviation of model verti 
ces, the level of detail, and the pixel size of the recti 
fied texture of the church model should be better than 
5cm. The according value for the model of the 
neighbouring pavilion amounted to even 2cm. 
• Efforts: by all means, the demanded model quality 
should be reached, but all available methods should 
be applied to keep efforts as little as possible. 
For photogrammetric data capture, there was a digital amateur 
camera at hand, with a resolution of 6.3MPx and a lens with a 
variable focal length of 15 to 30mm. Activities started with the 
calibration of the camera, using a known field of control points. 
This method was preferred to an on-the-fly calibration because 
of three matters: 
• Vegetation hindered the sight to tie-points, and the 
surrounding buildings did not offer access to neces 
sary points of view for a stable block. 
• Not only the mean interior orientation of the camera 
was of interest, but due to the variable, non-fixable 
objective, also the recoverability of the interior orien 
tation after transportation, etc. should be examined. 
• The application of virtual shapes was planned. This 
can complicate an on-the-fly calibration of a camera. 
All calibration photos were taken with the same setting of the 
focal length. After the capture of various images, the focal 
length was shifted and then restored again to the original 
setting. Subsequently, the rest of calibration photos was taken. 
Now, an interior orientation was computed for each of the two 
subsets of images. Statistical tests proved a significant 
difference between the focal lengths of the first and the second 
set. For that reason, the block of San Juan was stabilised 
globally by tachymetric measurements. To comply with the 
stated precondition of model texture pixel size, special rules 
were derived for the realisation of photogrammetric data 
capture. These rules allow for the computation of possible 
camera orientations that adhere to a minimum pixel size all over 
a model face. The second precondition of minimum efforts led 
to the application of virtual shapes of architecture, which were 
integrated into the modelling process with great success. Virtual 
shapes employ the fact that even historic buildings were 
constructed on the basis of a plan, wherefore edifices are 
delimited mainly by planes intersecting in straight lines, and by 
curved surfaces of higher degrees with curved lines as 
intersections. Virtual shapes of architecture are the idealized 
mathematical description of these object parts and they were 
integrated into the block adjustment. This way, observations 
could be saved to accelerate the creation of the photorealistic 
model. By combining and reusing them, and by stating 
interrelations between them, production could be accelerated 
further. Frequently, the spatial intersection of model vertices 
was impossible due to occluding vegetation or hindered access 
to necessary points of view. Nevertheless, the application of 
virtual shapes permitted the determination of these points. 
Virtual shapes were also used to ease the determination of 
points on edges in poorly textured areas by substituting the 
search for corresponding points in different images by 
measurements of arbitrary, non-corresponding points. Due to 
software limitations, it was necessary to model curved areas 
with planes. But the manual determination of these inflexion 
points would have resulted in evil appearance of symmetrical 
curves. This problem could be solved with virtual shapes, too. 
This first issue of applying virtual shapes of architecture in 
order to accelerate and enhance the creation of photorealistic 
models is discussed in section 2, and examples from the project