[Koha-bugs] [Bug 23984] 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=23984

            Bug ID: 23984
           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: martin.renvoize at ptfs-europe.com
        QA Contact: testopia at bugs.koha-community.org
                CC: arouss1980 at gmail.com,
                    jonathan.druart at bugs.koha-community.org,
                    lucas at bywatersolutions.com, magnus at libriotech.no,
                    martin.renvoize at ptfs-europe.com,
                    nick at bywatersolutions.com, tomascohen at gmail.com
        Depends on: 21987
            Blocks: 23963

The original resolution to this issue, bug 21987, caused a nasty regression and
has since been reverted.  Opening this bug so we may pursue a new route to
goal.

+++ This bug was initially created as a clone of Bug #21987 +++

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)


Referenced Bugs:

https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=21987
[Bug 21987] Local cover 'thumbnail' size is bigger than 'imagefile' size in
biblioimages table
https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=23963
[Bug 23963] Visible reduction in image quality
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