1999.05.07 16:53 "Re: lines in PostScript", by Helge Blischke
I'm converting a 1bit bw TIFF file into an eps, using tiff2ps, and then embedding it into a ps file. When this gets ripped on a film output device I find that there are some very feint horizontal (or vertical, if I play at rotating things) lines which appear on some of the graphics. I can get these lines to move/disappear by rotating the images so I don't think that they are in the image. I can get several different RIPS to produce the lines, my suspition is that it is a problem with the core
thanks for the sample files. I think I've settled down the problem: The tiff image consists of 141 strips. The tiff2ps program defines a PostScript loop rendering an image strip of height 64 (which comes from the rows per strip tag), and for each strip the image matris y offset adds up by this amount. This means in essence: the PostScript interpreter renderes 141 different images patched one underneath the other. This approach will lead to rouning errors in device space if the device resolution does not match the resolution of the original image. I suppose this is the source of your faint lines (I saw this effect even with Ghostscript and a screen resolution of 72 dpi (the image has 1200 dpi)).
The best way to avoid these artefacts is to render the complete image as a whole. I'll send you an EPS I created that way from your TIFF that way by separate e-mail: it should not show any faint lines or so when rendered on your film setter. If you are interested, you could get the PostScript procsets I used to do the conversion. It's free under the GPL and requires a PostScript interpreter to run (Ghostscript or Acrobat distiller etc.)