Old Gimp 2.6.6 vs 2.10.18: Differences in Scale Image dialog behavior and files coming in from scanning

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Old Gimp 2.6.6 vs 2.10.18: Differences in Scale Image dialog behavior and files coming in from scanning

Jay Smith-6
We are struggling with having upgraded to new hardware, new Ubuntu linux
o/s, and a new Epson scanner (using VueScan for control), new VueScan,
all at the same time.  (We had no other choice.)

Now using Gimp 2.10.18 (old was 2.6.6) on desktop Ubuntu Linux 20.04 --
everything right up to date as far as I can determine.

Now using Scanner Epson Perfection V600 Photo (old was also a MUCH older
Epson model of similar type).  Now using the most current version of
VueScan.

Please excuse if my terminology is not exactly correct.  And I realize
that I am probably failing to fully understand something fundamental to
this process, even though we have been doing this activity for 20 years.

We have been using Gimp "forever" to make scans of *postage stamps* for
images that are 1) exported to TIFF (300 dpi for offset press printing)
and 2) Those TIFF images are then are each made (by Perl / ImageMagick
scripts) into multiple sizes of JPEGs.

However, using the new scanner, what we used to scan at 300 dpi (100% of
actual size) without a problem, now on the NEW scanner results in a
serious moire effect if scanned at 300 dpi.  So, we have had to start
scanning (using VueScan) at 1200 dpi *input* and 300 dpi printed
*output* setting.  That is saved from VueScan as a TIFF file.  That
creates wonderful image quality.

But...

1) In *new* Gimp, when we import the TIFF image files saved from
VueScan, the Gimp Scale Image dialog states "printed size" values that
are ****double**** the actual size of the item.  Nowhere are we aware
that we have specifically set anything to double the size.  We are
unable to determine if the "double actual size" is a VueScan problem, a
Gimp problem, or if we are misunderstanding something.

2) In *new* Gimp, when we import the TIFF saved from VueScan, the Scale
image dialog states a "dpi" value of 600 -- instead of 300.  Nowhere are
we aware that we have specifically set anything to 600.  We expected
that dialog to say 300.  We are unable to determine if the "600 vs 300"
is a VueScan problem, a Gimp problem, or if we are misunderstanding
something.  [Given TIFF related problems with Gimp in 2.6.6, I would not
be surprised if there was a Gimp problem or some sort of mismatch
between what VueScan is putting out and Gimp is reading on import.]

As a result of one or both of the above factors, our image sizes are
coming out with *tremendously* larger file size than would have happened
using the same process with the old scanner, old VueScan, and old Gimp.

3) In *old* Gimp, if, in the Scale Image dialog, we changed the dpi from
a higher number to a lower number (for example 600 to 300), the printed
size setting would also *automatically* instantly change.  In *new*
Gimp, the printed size value does not automatically change when the dpi
value is changed.  Apparently that whole way of doing things has
completely been altered, but I cannot find using web searches and
reading Gimp Help anything like "If you *were* doing that, *now* this".

4) In *new* Gimp, if instead of changing the dpi, if I now change the
physical size to be equal to the actual item (instead of double, as
stated above)... a) The dpi displayed value does not automatically
change, but b) the resulting exported TIFF image file size *does* get
down much closer to the file size we were accustomed to.

5) But, since we do also need to print-on-paper those TIFF images (not
just use them to make JPEGs), I don't know if I should change the 600
dpi that they are being imported as (TIFFs made from VueScan), down to
300.  I understand that for most printing purposes anything over 300 is
wasted.  *However* in Gimp changing from 600 to 300 (without changing
the printing size) does not change the actual TIFF image file size, thus
there is apparently no change in the amount of information contained int
he TIFF image file.

I would very much appreciate learning...

- How the new Gimp model is intended to work in the kind of situation I
have described.  What are we doing "wrong"?  What should we be doing
differently?

- Does Gimp seem to be correctly working in what I have described?  Or
is there a possibility that Gimp is incorrectly reading what is coming
in from VueScan.  (And if anybody is familiar with VueScan, if there is
a possibility of a problem on the VueScan side of things.)

- Any ideas of why the image size (dimensions as shown in the Scale
Image dialog) is coming in *doubled* from VueScan-made TIFF images (with
the settings I have described).

- Any ideas of why the dpi (as shown in the Scale Image dialog) is
coming in *at 600 dpi instead of 300* from VueScan-made TIFF images
(with the settings I have described).

- Does it make a difference (downline in printing on paper) if I have a
dpi value of 600 vs 300 for since the actual dimensions are not being
changed and the resulting file size is the same?

I know experienced users of *new* Gimp and the developers are surely
tired of people like me not understanding changes in the fundamentals
and questioning if something is wrong or has a bug.  I thank you for
your patience.  Please be gentle.

Thank you very much.

Jay Smith
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