Christian Nassau wrote:
> To be more concrete: I wonder if we could make the following
> setup legal:
> - the file descriptor handed over by the backend signals
> readable if raw data from the scanner is available to
> the backend, but
> - the backend does some postprocessing and would rather
> keep some of the data until more data is available
> (take chroma correction, for example, where the backend
> doesn't want to send some lines at the beginning
> which have incomplete color components. The initial
> call to sane_read() might just put some lines into an
> internal buffer, which will be (partially) transferred
> when sane_read() is called the next time)
sane_read is a command that shall read image data from a buffer.
sane_read should not be used to get the image data from the scanner,
the backend should start a second process (reader_process) for
reading the image data from the scanner and put it into a buffer.
Then you also can do postprocessing with the image data without
that sane_read will see it.
> Basically, this is a question about the specification of
> sane_read(). There seem to be two competing ideas of what a
> successful call to sane_read() is:
> - The last line on page 29, allows sane_read() to transfer
> 0 bytes but still return SANE_STATUS_GOOD
Yes, this is allowed in non blocking mode.
> - On page 30 (blocking I/O mode) sane_read() has to transfer
> at least one byte if it wants to return SANE_STATUS_GOOD
> Wouldn't it be better to allow a backend to transfer 0 bytes,
> as long as it considers this a success for some internal reasons?
Blocking mode means that sane_read has to wait until there is at
least one byte of image data!
> Actually, I've got another question about sane_read():
> It might be convenient for a backend to internally store only
> complete lines of image data (not fractions of a line). So:
No. That is not allowed. The fronend may ask for any number
> - Does it have to be legal (for the frontend) to request
> less than bytes_per_line bytes?
scanimage -T does test exactly that. The frontend may ask for any number
of bytes it likes to do.
> This could easily lead to a deadlock if the backend transfers
> 0 bytes successfully in such a case. (Which it may do in
> non-blocking mode.)
Non Blocking mode means that the backend may return with 0 bytes
transfered (because it has no data in the moment and it returns
to give the frontend the chance to react to user action, refresh display
-- Homepage: http://www.wolfsburg.de/~rauch sane-umax: http://www.wolfsburg.de/~rauch/sane/sane-umax.html xsane: http://www.wolfsburg.de/~rauch/sane/sane-xsane.html E-Mail: mailto:Oliver.Rauch@Wolfsburg.DE
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This archive was generated by hypermail 2b29 : Wed Jan 10 2001 - 08:57:28 PST