[sane-devel] Colour problem with Canon Maxify MB5150

[Dr Rainer Woitok]

Ralph,

On Wednesday, 2021-05-19 18:21:57 -0700, you wrote:

> ...
> Some of the pixma devices seems to have a completely different engine
> for generating scans from the ADF compared to the flatbed.
> For ADF, they return JPEG data to SANE instead of the normal raster data
> that we get from flatbed.

This would be in accordance with my own observations that the quality of
the flatbed scans is better than set of the ADF scans.

> I have no idea why that would be the case but they do use the same sensors.

As Aisha reminded me in another reply to my mail  my scanner does duplex
ADF scans in a single shot,  so there should be  at least one additional
sensor for scanning the reverse side simultaneously.

> For a number of pixma devices that do this, we tend to see a very good
> image from the ADF, and a poor one from the flatbed.

Shouldn't that be the other way round?  At least that's what I'm observ-
ing here and also what I would expect when JPEG is involved.

> ...
> Presumably their WIndows drivers do some post-processing to the flatbed
> raster data to rebalance it.

I can confirm this.   Using the scanner  from Windows results in no sig-
nificant differences between  flatbed and ADF scans.   In this case both
are looking like having been postprocessed  with ImageMagicks's "convert
-normalize", that is, under Windows both scans are lower quality.

Well, am I summing this up correctly: the quality of Canon scanners when
using the ADF has dropped considerably?  Bummer :-(

Sincerely,
  Rainer