Halftone - October, 1986

          Robert B. Goldstein and E. Peli
          Eye Research Institute of Retina Foundation
          20 Staniford St
          Boston, MA 02114
          (617) 742-3140

          This is an update of a previous DECUS submission


               This program takes images that are  in  greyscale  form
          (ie,  pixel  values  0-255), and converts to halftone (pixel
          values 0,1).  It does it  by  an  error  propagation  method
          published  in  a paper by Saghri, Hou and Tescher.  The size
          of the output image is only dependant on the characteristics
          of  the target device, not on the input image.  It therefore
          tries to take fullest advantage of the available  pixels  on
          the  output device.  Output can be on a sixel device, a text
          only CRT, or even on the system lineprinter.

               Some useful things you should know if you want  to  try
          to get this program to run:

          1.  Adage output device.

                   We have  an  Adage  3000  image  processor  on  our
              system.   Since  most sites do NOT have this hardware, I
              have removed all support for output to the adage.  I did
              this  because  some  of  that software is proprietary to
              Adage Corp, and also to eliminate the Undefined  Globals
              linker message.

          2.  PIX logical

                   A logical, PIX, must be defined  to  point  to  the
              directory  which  holds your images.  An image, GIRL, is
              provided here.  To use halftone to operate on GIRL,  you
              must issue the command
                                  $ assign [] pix:

          3.  Image Format

                   Probably most sites will  have  their  own  way  of
              storing  images.  The image is pulled into memory by the
              routines getimage and imlod_2.  Re-write these  routines
              so  it  is  appropriate  to your own image formats.  The
              routine imlod_2 that is provided is appropriate to  read
              the  image  GIRL  that is included with this submission.
              GIRL is an image of the face of a girl.  We store images
              with  an  associated header file, with an extension .phd
              that gives info to the program about  the  image  (size,
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              etc).   Images  can be read without the .phd header, but
              the operator is queried for the missing info.  The  main
              item is the size, which, for GIRL, is 128x128.

          4.  Compilation and linking instructions

                   On our system the following command lines work:
                $ for halftone
                $ for half_subs
                $ link halftone,half_subs
              You  might  have  to  put  for/check=nobounds,  if  your
              Fortran  compiler  has  /check=bound  as  a  default.  I
              appoligize for this.  For reasons I wont go into, I have
              dummy  dimensioned  the image buffer (buf) as having the
              dimension 1 in some routines.

          5.  Enhancements from previous DECUS submission
              a.  Output of the processed image to a file.
              b.  Re-input of a processed image to just display it.
              c.  Factor of 3 speed-up for  the  case  of  a  2x2  'C'
                  matrix (most cases)
              d.  Override of default device dimensions


          6.  Files in this subdirectory
              a.  AAA_HALFTONE.MEM, RNO - This file
              b.  HALFTONE.COM  -  Command  file  for  compiling   and
                  linking
              c.  HALFTONE.FOR,OBJ,LIS - Fortran  source,  object  and
                  listing
              d.  HALF_SUBS.* - Subroutines needed by Halftone
              e.  GIRL.DAT, GIRL.PHD - Original GIRL image with header
                  file
              f.  GIRL_240.HFT - Already halftoned GIRL.  Read  in  by
                  chosing  H  for  image  type.  Target device was the
                  VT240.
              g.  GIRL_ALPHA_CRT.LIS - hafltoned girl.  Can  be  typed
                  to ANY device.
              h.  GIRLLA50.SXL - Sixel file for LA50.  Can  be  copied
                  to  an LA50 (or LA100?).  However, you must have set
                  the  width  of  that  device  to  511  (the  maximum
                  allowed)