Why does GIMP let me export a JPEG at a higher quality than the original?
Asked 7/15/2020
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2 answers
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In GIMP, when I open an existing JPEG and choose Export Image as JPEG, the quality slider defaults to 90. If I enable Use quality settings from original image, it changes to 75. What confuses me is that GIMP still lets me move the slider above 75, which makes it seem like I can increase the image quality beyond the original JPEG.
Since JPEG is lossy, I assume lost detail cannot be recovered. So what does a higher export quality actually mean in this case? Is the JPEG quality value an absolute measure, or just a setting for how much additional compression will be applied when saving again?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
5
After loading the image, Gimp is just working with the grid of pixels, which now include noise/artifacts from that first save (as well as noise/artifacts from the original sensor and any processing done in-camera).
When you go to save in Gimp, you're selecting how much more degradation you're willing to allow in saving the current batch of pixels (or to put it another way, how much fidelity you want to preserve of the current noise/artifacts).
If I Compress a JPEG at 70 Percent and Later Reopen It and Compress It at 90 Percent, the Final Image Will Be Restored to a Quality Setting of 90 Percent: False
The initial save at 70 percent introduces a permanent loss in quality that can't be restored. Saving again at 90 percent only introduces additional degradation to an image that has already had a considerable loss in quality.
Originally by user85767. Source · Licensed CC BY-SA 4.0
user85767
5y ago
0
Generated from our catalog & community — verify before relying on it.
You cannot restore detail that was already lost in the original JPEG.
In GIMP, once the file is opened, it is just a grid of pixels—including any JPEG artifacts already baked in. The export quality setting only controls how much additional JPEG compression is applied when saving that current pixel data.
So if the original file was saved around quality 75, exporting at 90 does not make it better than the original. It simply tells GIMP to preserve the current pixels more faithfully and add less new compression damage. Exporting at a lower value would usually add more degradation.
This can still make sense above the original setting if you edited the image—for example, adding text, graphics, or cropping. Those new or shifted pixels may benefit from being saved with less compression than the original file used.
So the slider is not an absolute “image quality score.” It is an encoder setting for the new save. “Use quality settings from original image” just estimates and reuses the original JPEG’s compression level; it does not mean that value is the maximum possible quality of the already-opened image.
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