[sane-devel] canon 2710 slide highlight burnout

[Laurie Moye]

Hi,

I'm using a Canon FS2710 with sane-frontends-1.0.11,
sane-backends-1.0.13, normally running Sane from gimp-1.2.5 under
RedHat-9.1.

I've been using it happily to scan negatives, but now I try to scan slides, I
find that the highlights are always burnt out.

Typically, I have a nice blue sky with puffy white clouds in the slide, but in
the scan, the whole of the sky (and any brightly lit light-coloured clothing)
is max white. The histograms show peaks one slot wide at the top end as you
would expect with over exposure (saturation of the scanner's A/D converter).

My understanding is that the 2710 adjusts its exposure when you turn it on
(and I think also when there is a SCSI reset). Certainly, if I turn it on with
an opaque slide in it, it knows that something is wrong and flashes it's
little light; so I don't think the scanner itself is broken.

Has anybody else experienced this?
Does anybody know any more about exposure adjustment on this scanner or other
Canon negative scanners?

I note that the last mention I can find in the archive of the 2710 is a
posting from Ulrich Deiters on Fri, 24 Aug 2001 asking if anybody had: "tried
the raw scanning mode for a FS2710 and obtained a picture containing random
data?".

There seems to have been no response to this even though the "raw" mode
produces scrambled bits on an i686. This suggests to me that there is not much
interest in the 2710 any more, so perhaps anybody with any knowledge or
suggestions about slide burnout should contact me directly and if I reach any
conclusions I'll post them.

Others need not bother to read further!!

Ulrich Deiters question about endianness is answered by looking in
/usr/include/endian.h; it is not the bytes, but the 16-bit parts of a 32-bit
word that are involved (apart from the DEC PDP architecture). So the byte
reversal at lines 1919-1924 of canon-sane.c should be removed. Correcting this
and getting non-scrambled "raw" mode only confirms that the scanner is
saturating on slide highlights.

My assumption about the exposure control of the 2710 (and any other scanner
that adjusts exposure with an empty negative carrier on startup) is that it
works out the exposure needed to just achieve saturation level and then turns
the exposure UP by some predetermined amount to compensate for the lack of
complete transparency in negatives. For transparencies, which can have close
to 100% transparency in the whites, I think it should not turn the exposure up
at all. Does the Windoze driver work OK with transparencies? Does it have to
tell the scanner to adjust its exposure? (I don't allow any Micros**t OS
in the house, so I can't try these things).

-- 
		      Laurie Moye, Upton-upon-Severn, UK