Passing duplex data from reader process

From: Frederik Ramm (frederik@remote.org)
Date: Fri Mar 02 2001 - 10:20:45 PST

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    Hi,

       we've discussed how to handle duplex in a backend; the almost-
    consensus was to have the scanner scan both sides when the first
    "sane_start" is issued, but only return the front page, and then when
    a second "sane_start" follows, return the back side.

    As mentioned earlier, my driver uses a separate process for reading
    from the scanner; that process also decodes the image in front and
    back, and caches the back page. It writes its output to a pipe,
    which is read from the other process, within "sane_read". "sane_read"
    normally reads until the other end closes the pipe. Currently, for
    testing, I just write the front and back pages to the pipe, resulting
    in an image that is twice as high as requested.

    I have to change that somehow, to let sane_read know when to stop
    reading *although there's still data available* (and return that
    additional data only after another sane_start has been called).

    There's a myriad of ways to possibly do it and I am not sure how to
    proceed. Some ideas:

    + Let sane_start calculate the number of bytes expected, and
      let sane_read stop reading after that number of bytes. This
      would be easy to implement but what if e.g. the paper in the
      scanner is shorter than expected?

    + Instead of sending raw image data over the pipe, let the reader
      process send a header first - something that describes the
      nature and amount of data to follow. However, such a header
      would be most useful if it was already available when the
      front-end calls "sane_get_parameters", wouldn't it?

    + Open a second pipe between the backend and its reader process
      and use that pipe to transfer control information - either FROM
      the reader process ("image 01 follows, 123456 bytes") or TO
      the reader process ("send image 01 now").

    + Use IPC to communicate between reader process and backend...

    Any suggestions?

    Thanks
    Frederik 

    -- 
Frederik Ramm  ##  eMail frederik@remote.org  ##  N57°48.10' W005°40.32'

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