Chagall Reference   Convert picture       12.5.93
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   This function converts various picture types
   from one to another.
   
   More exactly, it deals with conversion of
      
      Monochrome,
      Halftone and
      Colour pictures
      
   into one of the other formats in each case.
   
   The source format will be set by the type of
   the current picture and can therefore not be 
   altered by the user. For the goal you can choose 
   one of the other two formats.
   
   The 'Raster matrix' is significant for converting
   a monochrome picture into a greyscale one. It
   determines how many monochrome pixels will be
   collected together to form one greyscale pixel.
   Many monochrome pixels forming one greyscale 
   pixel will produce many different grey-steps,
   but the picture will be greatly shrunk in size.
   Using fewer pixels results in a relatively large
   picture, in which however the grey-steps have a
   coarser gradation. As long as the size of the
   greyscale picture is unimportant, then a matrix
   size of 16 x 16 pixels is recommended, since with
   this the maximum displayable number of grey-steps
   (256) is reached.

   If, for instance, you use a 32 grey-step hand-scanner,
   then you should set 'dX' and 'dY' in 'Raster matrix' 
   to 6, since these scanners generally work with a 6 x 6
   matrix. You can check the rastering process used by 
   your scanner in its operating instructions.
      
   Furthermore, when converting to a monochrome picture
   you can select a rastering process and a zoom step. 
   This allows you to prepare halftone pictures for 
   printing or photo-setting. These settings are
   independent of the selected display dither.

   The effective range of this function can be limited
   to a cut out portion. The result of a picture
   conversion will naturally always be displayed in a 
   new window, so that option can not be switched off.

