[sane-devel] scanning negatives using HP4850 and genesys backend

[Ralph Little]

Hi,
There are other backends that support transparency but the genesys chips 
are pretty much a world of their own. On the upside, the GL646 is pretty 
well documented.
I believe the main ways in which the transparency support would be 
invoked would be via the source and options related to capture type. I 
don't see any specific image capture type-related options in the backend 
but some would have to be added I suspect (e.g. for selecting between 
positive and negative, IR etc).

The main challenge with genesys-based devices is that every model is 
different to every other in implementation. The Genesys chips are very 
flexible and offer all sorts of driving options and they need to be set 
up for the particular hardware that they are matched with. The main way 
to tackle this task is to observe Windows driver behaviour through 
capturing traffic and looking at the register values that are set. It's 
a little more involved but that is the main gist.

I have had some dealings myself with looking at a GL646 device before 
although it was a fairly superficial look at motor stepping tables but I 
may be able to look at it in the future sometime.
One thing you *can* do (if you have the inclination) is to make some USB 
captures on Windows and post them to an issue at our GitLab issue 
tracker (https://gitlab.com/sane-project/backends/-/work_items), if you 
don't intend to do the implementation yourself. That way someone could 
pick up the issue at some point. Ideally a developer needs the hardware 
at hand, but a rough stab could be made by looking at the traffic.

Cheers,
Ralph

On 2026-07-31 10:01, 7xsuazlhhtls at gmail.com wrote:
>
> Ralph,
>
> Thanks for the reply.
>
> I looked at the Genesys Datasheet for the chip (GL646USB) that is used 
> in the HP4850.  I see the register that appears to control the slide 
> scanning lamp.
>
> Is there another backend that has support for a similar feature? 
> Possibly I could use that as a template to better understand how 
> support for this feature might be implemented in the genesys backend.
>
> Thanks again,
>
> Jon
>
>
> On 7/31/26 11:21 AM, Ralph Little wrote:
>> Hi,
>> According to our Supported Scanners list 
>> (http://www.sane-project.org/sane-supported-devices.html) the genesys 
>> backend doesn't yet support the slide scanning feature of this 
>> device. It requires additional code to drive the extra hardware involved.
>> We don't have an active maintainer for the genesys backend at the 
>> moment unfortunately, so I don't know when someone would be able to 
>> get to this task in the future. :(
>>
>> Regards,
>> Ralph
>>
>> On 2026-07-30 20:58, 7xsuazlhhtls at gmail.com wrote:
>>>
>>> |I recently acquired an HP Scanjet 4850 scanner.  The document 
>>> scanning works with no problems.  This particular scanner has a 
>>> negative/slide scanning component built in to the lid of the 
>>> device.  This includes a separate lamp to illuminate the film for 
>>> scanning.  The lamp in the lid comes when I run a hardware self test 
>>> procedure but I have not discovered any way to have the lamp come on 
>>> when scanning using sane.
>>>
>>> Is this capability supported?
>>>
>>> info about my setup ---
>>>
>>> HP Scanjet 4850
>>>
>>> $ scanimage -V
>>> scanimage (sane-backends) 1.4.0; backend version 1.4.0
>>>
>>> genesys backend
>>>
>>> Fedora Linux 44 (Workstation Edition)
>>> Linux 7.1.5-200.fc44.x86_64|
>>>
>>
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