Will saving a JPEG with 4:2:0 chroma subsampling again cause extra quality loss?
Asked 11/9/2019
125 views
2 answers
0
I’m assembling a larger image from smaller source images that are already JPEGs saved with 4:2:0 chroma subsampling. If I export the final combined image as JPEG again, will choosing 4:2:0 cause an additional loss similar to recompressing the file multiple times? Would using 4:4:4 for the final save avoid a “double” chroma-subsampling effect?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
3
When not using chroma subsampling, JPEG generational losses are pretty much limited to rounding error as long as the same compression settings are used. (Majority of compression losses occur during the first compression.)
Here is what happens when an image is recompressed until convergence at q=90 without chroma subsampling (original, 1, n, difference):
However, when chroma subsampling is enabled, additional color information is thrown away, which probably also affects quantization, so that losses are much more visible when steady state is reached. Here is what happens with chroma subsampling enabled.
If you're concerned about single-digit resaves, the effect isn't so pronounced.
See also
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
6y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—saving again as JPEG can add generational loss, and using 4:2:0 again can discard additional chroma detail. JPEG loss is usually greatest on the first save, and if you re-save with the same settings and no chroma subsampling, later losses are often relatively small and mostly due to rounding. But with chroma subsampling enabled, each save can throw away more color information, so repeated 4:2:0 encoding is more visibly damaging over time.
That said, for just one or a few re-saves, the extra loss is often not dramatic. Saving the final image as 4:4:4 won’t restore chroma detail already lost in the source JPEGs, but it can avoid throwing away even more color information on the final export.
If quality matters, do your editing/compositing in a lossless format and save to JPEG only at the end, ideally at high quality and with 4:4:4 if your workflow supports it.
Recommended products
UniqueBot
AI6y ago
Your Answer
Related Questions
What causes JPEG generational loss when a file is re-saved multiple times?
Does changing JPEG metadata recompress or degrade the image?
Does opening and closing a JPEG reduce image quality, or only saving it?
Does resizing and resaving a JPEG add noise or just compression artifacts?
Why does Luminance HDR produce a gray, banded sky from my bracketed exposures?