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bbradon  Create a black-box for a Radon transform.
   [BB,DIM2]=bbradon(DIM,THETA) creates an operator for a discretized
   Radon transform. That is, an intensity image is integrated along
   lines to form a set of projection lines (PL). Each projection
   occurs along a line of response (LOR), which is perpendicular to the
   projection line (PL).

   THETA is a vector of angles (in degrees) with the direction of the
   PLs (default is [0:179]). S=reshape(BB*IMG(:),DIM2) is a sinogram of
   the image IMG, where each column in S is the projection of an angle.

   The intensity image is modeled by a point mass in the middle of each
   pixel. The image has square pixels and is centered at the origin, with
   the longest axis ranging from -1 to 1.

   Options can be passed as an option followed by a value, or they can
   be passed as fields in a struct. The dimension may be specified using
   the field 'dim1', while the field for THETA is 'theta'. The syntax
   [BB,DIM2,OPT2]=bbradon(OPT) returns the actual options used.

   The options are:
     'len':   Length of projection lines.
     'samp':  Number of samples on the projection line.
     'roi':   A ROI, "region of interest", is an image of dimension DIM.
              Any zeros in the ROI are assumed to be zeros in the image.
              Disregarding points known to be zero may significantly
              increase the speed of the transform.
     'notol': Do not determine the accuracy of the operator. This saves
              some time, but limits the number of iterative algorithms
              that can be used.

   The struct returned in OPT2 contains these same fields, with the true
   values used if any quantity were modified. It also includes the following
   informational fields:
     'roi':   As input, but guaranteed to be an image
     'rect':  Location of the 4 extreme point masses: NE (north-east), NW,
              SW, SE. Each column holds a coordinate.

   Although bbradon is comparable to the function radon found in the image
   processing toolbox from Mathworks, it is not intended to be a
   replacement. The algorithms are quite different.

   The Radon-transform may be used to approximate some tomographs, e.g.
   CT-, SPECT-, and PET-scanners. An extended version that models some
   additional effects in SPECT and PET, is given in bbtomoemit.

   See also bbtomoemit, bbfbp,  (image toolbox).