Package: loupe
Version: 49.1-1
Control: affects -1 glycin-loaders
A traditional example of a JPEG created with arithmetic encoding is
https://issues.chromium.org/action/issues/41288624/attachments/53088388
https://issues.chromium.org/action/issues/41288624/attachments/53088388. On
this example, loupe complains with “Could not Load Image” followed by “Either
the image is unsupported or it contains unsupported elements”. Clicking “More
Information” yields “Remote error: org.gnome.glycin.Error.LoadingError:
glycin-loaders/glycin-image-rs/src/main.rs:307:54: Format error decoding Jpeg:
"Parsing of the following header `DAC` is not supported,cannot continue"
stderr: Setting process memory limit”.
A reader might wonder, why care about the arithmetic encoding? Recently, I
tried to compress a rather big scan (private content, an A4 page at 1200 dpi
with black foreground text and a nontrivial green-white background) into a JPEG
file of a possibly small size. I ran:
cjpeg -quality 0 -arithmetic -dct float -outfile output.arithmetic.jpg -report
-strict -verbose input.pnm
cjpeg -quality 0 -optimize -dct float -outfile output.optimized.jpg -report
-strict -verbose input.pnm
cjpeg -quality 0 -dct float -outfile output.unoptimized.jpg -report -strict
-verbose input.pnm
(Quality 1 would do as well. The real file names have been replaced by “input”
and “output” for privacy here.)
The sizes are this:
output.arithmetic.jpg: 239224 bytes
output.optimized.jpg: 807963 bytes
output.unoptimized.jpg: 1928235 bytes
The foregrounds of the three files are readable and have visually comparable
quality. The savings in size between output.arithmetic.jpg and
output.optimized.jpg are a factor exceeding 3.3. (And NOT a tiny one-digit
percentage, as reported in “A Review on JPEG2000 Image Compression” by Maini
and Mehra and in the Wikipedia article on JPEG.) These savings do make a
difference if you use more images of this kind and your storage or bandwidth is
restricted.
An aside. I was wondering about whether to add the control line “Severity:
wishlist” and decided against it, as, strictly speaking, the files on which the
error occurs, though uncommon, strictly conform to the JPEG standard. The
corresponding patents expired, so the only limiting factor is human work hours
to implement this.
