Studio-W: will multiple compression of images lead to a degradation of quality

[sergeleger]

This is a general question to discuss with the team, if we compress every time, won't image quality degradation be accelerated with each revision?

Originally posted by @deltork in #451 (comment)

An example scenario:

• create a new read along
• download the offline HTML

Then repeat these two steps:

• load the newly created file in editor
• download the offline HTML

[sergeleger]

Here's an example of a JPG file getting compressed multiple times

for f in title*.zip; do unzip -l $f; done | grep .jpg
  Length      Date    Time    Name
---------  ---------- -----   ----
   818590  08-15-2025 10:42   www/assets/image-title-20250815074212-0.jpg
   818182  08-15-2025 10:42   www/assets/image-title-20250815074234-0.jpg
   818154  08-15-2025 10:43   www/assets/image-title-20250815074303-0.jpg
   818049  08-15-2025 10:43   www/assets/image-title-20250815074318-0.jpg
   818049  08-15-2025 10:49   www/assets/image-title-20250815074933-0.jpg

I've run a few tests, it does seem to stabilize at some point. The image-conversion/compress() function does validate the file size of the newly compressed image: if it is larger than the original file, the original file is returned.

[dhdaines]

Intuitively, repeatedly compressing and decompressing JPEGs at the same compression level with the same implementation will only degrade quality due to numerical imprecisions. The information loss happens the first time the DCT coefficients are quantized, so re-quantizing them with the same parameters should just give the same values.

But I don't know enough about the details of the algorithm to know if this actually happens in practice!

Obviously it's a waste of cycles to be recompressing things all the time, so it would be nice to avoid this.