How Many Gray Scales On Laserwriter II NTX?

[Tim_Buxton at UM.CC.UMICH.EDU]

         We are using the icut and tops commands to take grayscale images 
 from the screen and send 
 them to an Apple Laserwriter Plus NTX.
 We are getting  very "stair stepped" halftones.  That is,
 where a standard ramp is displayed on the screen varying
 smoothely from dark to light, the LaserWriter image will
 show a series of discrete bands.  The reason for this 
 evidently is the half-tone process. 
 
      As half-toning is explained in the "Red" PostScript
 Reference manual, each pixel you send is represented by
 "subpixel" dots, more of which are progressively turned on
 to darken the image pixel.  I had assumed that, having
 taken my eight bit pixel information on good faith, that
 PostScript would do its level best to give me 256 gray 
 levels.  This is not the case, as I proved by sending 
 a finely shaded image to the same IRIS screen which
 I had taken the 256 level image from using the psh <
  image.eps command.  The Postscript image showed banding
 instead of smooth gradations.  Doing an icut to produce
 an "rgb" image and doing the hist command on that file, I
 found that there were "spikes" at every tenth gray level.
 All of the level 30's were lumped into 30, the 40's to
 40, etc.  That meant in the 30 - 100 range I was looking
 at there were only 8 gray levels, and in the whole
 range there were only 25 real levels (~ 5bit resolution).
 
      The PS Reference Manual also indicates that the number
 of displayable grayscales is determined by the setscreen 
 command, which determines how many subpixels are in each
 pixel, and hence the scale resolution.  The first 2 
 arguments for the command, (angle? and density?) were
 doubled and halved by me with 
 no effect on the SCREEN display.
 There is an alternate "line" mode setscreen command 
 in the tops.c source code which I did not understand
 and have not tried.  Ways to change the resolution 
 were averred to be in the PS Cookbook, but what I
 saw did not seem to apply.
 
      So, help would be APPRECIATED from anyone who
 understands this, and (I hope) can tell me how to
 get 256 cool gray levels out of the LaserWriter Plus NTX
 or PostScript on the IRIS for that matter.  I don't
 mind if it takes longer to display, as long as we
 don't have to buy a Linotronic to get better pix.  Thanks.
 
                            -Tim Buxton
                             Tim_Buxton at um.cc.umich.edu
                             OptiMetrics, Inc.