# Re: image data format polarity

From: Ingo Wilken (Ingo.Wilken@Informatik.Uni-Oldenburg.DE)
Date: Mon Sep 11 2000 - 00:37:03 PDT

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  > [David Paschal]
  > > On the other hand, for 1-bit depths (lineart, halftone), 0 means
  > > white and 1 means black. Is there any particular reason for this,
  > > especially since xsane and xscanimage both end up having to invert
  > > it anyway in order to display a preview?
  > This sounds like a bug to me. The only partly sensible reason I can
  > make up on the fly to use a encoding like this is to make sure black
  > writing on white paper give mostly 0 bits.

  That's not a bug, it's the PBM image format, see pbm(5):

  # It was originally designed to make it reasonable to mail
  # bitmaps between different types of machines using the typical stupid
  # network mailers we have today.
  [...]
  # - Width * height bits, each either '1' or '0', starting at the top-left
  # corner of the bitmap, proceeding in normal English reading order.
  #
  # - The character '1' means black, '0' means white.

  And yes, it obviously was designed that way, so a human can see the
  image without using a graphics display program, just by looking at
  the ASCII representation (assuming a large screen and proper formatting,
  the manpage has a tiny example).

  Regards,
  Ingo

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