[sane-devel] Weirdness with Canon FS2720U

[Bertrik Sikken]

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Tabor Kelly wrote:
> What I would love for someone to take a look at is URB 289-300 in this
> file
> (https://projects.cecs.pdx.edu/~kellyt/SANE-FS2720U/index.cgi/browser/sniffs/just_preview_color_negative_bw.LOG).
>  It looks like the windows driver writes 24kB of data to the scanner.
> But look at the data, it isn't firmware, it is a pretty pattern. Any
> idea? Was the developer who wrote this driver just having a little fun
> at my expense?

I'm quite sure that these are the gamma or shading tables. They are
used to do gamma correction (usually to improve contrast in dark
areas and decrease it in light areas), but you can also do other
color corrections, like brightness and contrast.
They are uploaded to the scanner, so the scanner can do the
correction at a higher accuracy than the PC (because the scanner
usually has more bits to work with than the usual 8-bit that is
sent to the PC).

There is one table for each color component (R, G, B) although
your log shows four tables ... :)

> I think next I am going to write some libusb code (my first) and try
> cutting out some of these steps. The scanner has flash memory on the
> mainboard, I don't understand why it would need its firmware upgraded
> every time. Maybe Canon didn't build in a facility to actually upgrade
> the firmware and they are writing over active memory every time you turn
> it on?

Just a guess, it could be that the 'firmware' is needed for the
USB-SCSI bridge and that the scanner itself is not upgraded.

Regards,
Bertrik
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