[sane-devel] Plustek and calibration

[Ralph Little]

Hi,

On Thu, Dec 3, 2020 at 1:34 PM Charles Lindsey <chl at clerew.man.ac.uk> wrote:

> I have an Epson Perfection 1250/Photo which uses the plustek backend. It
> works
> fine for color and gray modes, but for lineart, every time you scan a page
> it
> introduces more black noise along the right hand edge of the page. It
> turned out
> to be the result of calibration. I still don't know why repeated
> calibration
> causes that problem, but that is how I cured it.
>
> There was a problem with the calibration routine which I fixed some time
in the past, you may remember.
It's not impossible that there may be other issues :D


> Obviously, on startup (more specifically on warmup), any scanner has to be
> calibrated; in fact if the warmup is set to 'auto' it repeats the
> calibration
> until the result remains unchanged three times. Examination of the code
> for the
> Plustek Backend shows that it does much more; indeed every time you start
> a new
> scan, it does a new calibration (except for automatic document feeders and
> the
> like). This wastes time (typically six times until the obligatory three
> identical results), and it causes my black noise in lineart mode (for
> reasons
> yet unknown).


Yeah, I know. It's annoying. Some of the older scanners seem to take
*forever* to do calibration.
Other scanners that I have used do regular calibrations, but they do not do
it on every scan.
I would assume that other parameters like temperature might affect
calibration (as the machine warms up with use perhaps) so it would be
reasonable to do at least a quick calibration to ensure that the scanner
was still within spec every now and then.

It is for you guys to decide whether this is worth making permanent. I
> understand Ralph has a similar Epson scanner, so perhaps he could check
> whether
> that has this lineart problem, and give this patch a try.
>
>
I will have a look over the weekend if I get a chance.


> Just a few more oddities that I encountered on the way.
>
> Even after the patch, it sits for 13 seconds in
> usb_IsDataAvailableInDRAM()
> before the scan data starts to become available, even though after that it
> comes
> out steadily even if several buffers-full have to be downloaded. I think
> this is
> a problem with the scanner hardware which is too lazy about getting the
> carriage
> out of its garage.
>

:D

Cheers,
Ralph
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