[Koha-bugs] [Bug 21987] New: Local cover 'thumbnail' size is bigger than 'imagefile' size in biblioimages table

[bugzilla-daemon at bugs.koha-community.org]

https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=21987

            Bug ID: 21987
           Summary: Local cover 'thumbnail' size is bigger than
                    'imagefile' size in biblioimages table
 Change sponsored?: ---
           Product: Koha
           Version: master
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Architecture, internals, and plumbing
          Assignee: koha-bugs at lists.koha-community.org
          Reporter: arouss1980 at gmail.com
        QA Contact: testopia at bugs.koha-community.org

We use local cover images in our setup, and also resize the covers
we scan to a width of 160px before uploading and attaching them to
a Koha bibliographic record (we upload one cover per biblio).

While using phpMyAdmin to view the contents of the 'biblioimages'
table (relevant screenshot here: https://imgur.com/a/cSEVLBI), we
noticed that the size of the BLOBs for the 'thumbnail' column was in
some cases twice as big as that of the 'imagefile' column. In fact,
this happens for more than 50% of uploaded covers as you can see
from the output of the SQL queries below:

mysql> SELECT COUNT( * ) AS count
         FROM biblioimages
        WHERE LENGTH( thumbnail ) > ( LENGTH( imagefile ) * 2 ) ;
+-------+
| count |
+-------+
|  1356 |
+-------+

mysql> SELECT COUNT( * ) AS count,
              SUM( LENGTH( imagefile ) ) AS images_size,
              SUM( LENGTH( thumbnail ) ) AS thumbnails_size
         FROM biblioimages ;
+-------+-------------+-----------------+
| count | images_size | thumbnails_size |
+-------+-------------+-----------------+
|  2347 |    68323933 |       115839686 |
+-------+-------------+-----------------+

It would appear that for each 160px-wide JPG with 24-bit depth that
we have uploaded, the 'imagefile' column has been populated with an
PNG of 8-bit depth with the same dimensions as the uploaded file,
whereas the 'thumbnail' column contains a 24-bit PNG image with a
width reduced to 140 pixels.

I've tracked down the creation of resized 24-bit PNGs to this code:
https://github.com/Koha-Community/Koha/blob/master/C4/Images.pm#L182-L183

Do thumbnails for covers need to be true colour (24-bit) images?

Also, why is it that _scale_image() returns an image of 8-bit depth
if the source image's dimensions are less than 600x800? (i.e. when
no resizing is performed)

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