SANE & exposure times

Ewald R. de Wit (ewald@pobox.com)
Wed, 28 Jul 1999 20:38:55 +0200

Andreas Rick (rickand@gemse.fr) wrote:
> When I said "quick scan" I didn't mean the "prescan".
> The "quick scan" happens when you put in the film without
> sending anything to the scanner on the SCSI bus.
> I called it "quick scan" as it determines already something
> based on the images on the film - I'm not sure whether the
> scanner is actually scanning.
> As you said: "Prescan" - which you have to ask for by a SCSI command once
> you have positioned the film on one image - is to find the exposure time.

Ok, thanks for clearing this up for me.

> > Btw, is there anyway a frontend could extract the proper
> > RGB exposure times from you backend?
>
> Not in version 0.4.2 but I am working on the problem.
> Do you really need these values or would it be sufficient
> if you can change the exposure time in percent of the optimal
> value determined by the prescan as it was for the LS-20?

My concern is not changing the values but getting them from the
backend so the frontend knows how to make sense out of the RGB values.
I think we could use SANE_NAME_SCAN_EXPOS_TIME_R (defined in
sane/saneopts.h) and friends for that. That is, the frontend should
check these options and if they are present then it should divide the
RGB values by their corresponding exposure time.

The RGB exposure options that the backend exports need not be absolute
in any way; they can be relative to each other and the frontend should
be able to make sense of it.

An alternative way would be to do the dividing in the backends. Since
we're dealing with filmscanners here that will most likely use
16 bit transmit data, there is no loss of accuracy. For 8 bit data
there would be unacceptable loss of accuracy when scanning negatives.

So where should we put it, in the frontend or up the backend?

-- 
  --  Ewald

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