Pixel hits reconstruction is steered my the routine TPD2R, separate routines TPD2R_BAR and TPD2R_FB are used for the barrel and forward pixels. The reconstruction is done in 2 steps. First, clusters of pixels corresponding to one track are identified, second, the x and z coordinates of each cluster are determined.
The cluster finding involves a 2-dimensional search. Initially a standard jet-finding algorithm has been adopted. This algorithm is not optimal and will have to be replaced in the future.
Pixel clusters in the barrel can have very "non-round" shapes (due to charge sharing) and reconstructing their position is not straightforward. The PIX_AV_MODE parameter (see 5.6.1) controls the method for calculating the cluster position. It can be equal to 0 or be a 2 digit number and its meaning is the following :
xz - 2 digits, where x and z define separately the method
used in the x(r-fi) and z coordinates
(e.g. 11, 22, 31).
x and z can be :
1 - linear analog average
2 - digital average
3 - middle point ( max+min)/2.
4 - "smart" analog (nonlinear) average
5 - "edge" method (used for long clusters, z only).
0 - Best method.
The best algorithm depending on the region of z is used.
The choice is hard-wired in the code (TP_FIT_CLU).
This is the default setting for the barrel.
For the forward pixels clusters are usually composed out of 1 or 2 pixels and therefore it is easy to calculate their position by just using the simple analog or digital average.