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Pixel brighteness 
  
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1b 
Fig.1 Examples of a section of the digital image brightness diagram produced by the computer. The section 
perpendicular to the arm of the image of a cross shows the profile line connecting individual pixels 
which: 
a) serves as a basis for linear interpolation of the profile symmetry axis, 
b) is approximated by the parabola, and a point on the axis of the cross is identified with the 
parabola axis 
coordinates of centers of crosses on a digital 
image with original test the projective transfor- 
mation fitting all 9 crosses of image and test 
according to the rules of the least square adjust- 
ment (tab.1) was applied. At the beginning two 
types of test images were analyzed : one taken 
with the objective Tevidon 1.9/35 mm from such 
distance that the test image was of the CCD 
512x512 frame size, and second - which gave 
image 2.4 times smaller. Due to proper test lighting 
the images were very contrast: the background 
had the luminosity number around 210 while the 
black non-transparent crosses had only luminosity 
number 50. A contrast edge of the cross is imaged 
in each of the image scales with 3 pixels of 
relatively proportional increment of luminosity 
number. The diagram of luminosity of subsequent 
pixels along the image line or column (section of 
cross perpendicular to the arm axes) gives the 
polygon of the luminosity brightness distribution 
along the profile (fig.1). It is worth while to 
underline that the 3 pixel width of the transition 
392 
stripe from the bright background to the dark cross 
surface does not depend on the image scale. It gives 
good opportunity to create an universal algorithm 
of the cross center determination on digital images. 
3. PRECISE DETERMINATION OF CENTERS 
OF CROSSES ON A DIGITAL IMAGE. 
There are several filters, which can be applied to 
determine image contours as a sharp lines (eg.- 
tresholding), but they do not offer the accuracy high 
enough to be used for precise determination of the 
cross center. 
Therefore the more laborious methods based on the 
analysis of luminosity diagram along the digital image 
lines and columns were developed during 1991 - 92 
period at our laboratories. 
There are three methods which gave the accurate 
enough results to be considered as a solution to 
the problem :