Gamal Seedahmed 
  
equation does the tracing, and every pixel in the edge image satisfying these equations will be reported 
as a potential pixel candidate. Based on the detected FM centers, a window is opened in the edge image 
to restrict the search area for linear structural elements of the fiducials. The Hough space is generated 
for the linear elements followed by peak detection and pixel tracing. Since the linear elements are 
known with specified orientation, the peak search in its Hough space is restricted to a predetermined 
range of orientation, namely, 35-55 degrees. and 125-145 degrees. The linear elements of the fiducials 
support only the identification process and not the precise localization of the FM. Precise localization 
of the fiducial centers is based on least-squares adjustment using the two radii and the pixels along the 
circumferences of the two circles as observations. | 
When two fiducial marks have been successfully recognized, a similarity transformation can be 
performed to establish an approximate relationship between the pixel coordinates and the camera 
coordinates. This approximate transformation is generally very helpful in defining the search area for 
the third fiducial if we have a difficulty in its identification using the HT. By using the similarity 
transformation with the developed techniques we benefit from the available strategies to come up with 
an intelligent solution. | 
The peak searching in the Hough space is implemented via histogram searching, and this casts the HT 
as a fast version in terms of peak detection. Also the histogram provides an elegant mechanism of 
multiple peak detection without any further search. The histogram memory allocation is determined 
during the run time based on the expected feature size. 
6 EXPERIMENTS 
  
Fig. 3: a, b, c, and d are fiducial patches belong to the upper left corner (UL), upper 
right (UR), lower left (LL) and lower right lower (LR) corners of 8k x8k image. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 
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