JPEG Quality Issue

[d0m1n1k]

Hi gimp users,

I got JPEG images from a multifunction copier. I really like the quality per size of these images!
I would also like to use this jpeg quality with gimp exports. But this doesn't work as expected. First I opened an image generated by the copier. Then see attached 3 screenshots from export dialog:

1 - these are the standard values. It produces a very big size jpeg. This is not what I want.

2 - these settings are automatically choosed, if I just check the "Use quality setting from original image" checkbox.
The exported image with this settings is very good. The size is even a bit smaller than original. This is what I want! I like to use this quality settings for default at all images!

3 - But if I uncheck the "Use quality setting from original image" and keep all other settings changed before, I got an image with horrible quality.

Why can I not use this quality level by choosing the settings gimp choosed? It's absolutely insane: The only workaround I found to use this quality level is:
1. Open my own image (not generated by multifunction copier)
2. Copy the whole image to clipboard (Ctrl-C)
3. Open any image generated by the copier
4. Chance canvas size of this image to the image size from my own image
5. Paste my own image from clipboard (Ctrl-V)
6. Export with "Use quality setting from original image"

[Ofnuts]

If you don't use the image settings, you get Gimp's default values. So what you probably want to do is to saved these values as default (button at bottom of dialog).

Note that settings that are good for an image from a multifunction copier may not be good for another image type (from a camera; etc...). So you should better understand what the settings do and pick settings explicitly.

[d0m1n1k]

(06-29-2024, 04:21 PM)Ofnuts Wrote: If you don't use the image settings, you get Gimp's default values. So what you probably want to do is to saved these values as default (button at bottom of dialog).

No, "Dialog1" are the default values. If I check and check "Use quality setting from original image" the values are changed automatically. (e.g. quality to 5, and rhe subsampling setting - I didn't changed it by hand).

I also explicitly tried different quality levels by hand. But none has even come close to the quality and size of Checkbox "Use quality setting from original image".

[Ofnuts]

Q5 is usually terrible, so there is something amiss or your image has some very specific content (monochrome?) that makes Q5 palatable, but your changes require some better quality.

Q90 (left)  vs Q5 (right)

       

Can you share your original image?

[d0m1n1k]

Yes indeed, Q5 is terrible like your example! But not when using "Use quality setting from original image". This is a 400DPI Image A3-Format with a Resolution of 4672x6624. Not monochrome, RGB color 8bit Gamma integer. I don't know what Gimp does when this setting is selected. It seem to be magic . Only the developer should know how it works.
If I select manually Q15-Q20 instead of Q5, I got nearly the same file size as the source-jpeg. I tried it with some other pictures and the quality level is always detected correctly when the "Use quality setting from original image" checkbox is clicked.

[Zbyma72age]

(06-29-2024, 09:48 PM)Ofnuts Wrote: Q5 is usually terrible, so there is something amiss or your image has some very specific content (monochrome?) that makes Q5 palatable, but your changes require some better quality.

Q90 (left)  vs Q5 (right)

Can you share your original image?

It's a shame that no one here raises the topic of the Jpegli format
Here is an example of encoding the above RMdefault image as Q20 and Q5
Please note the size of the files
XL Converter program used.

published on the PL pages:

https://1drv.ms/u/s!Ao2sT6WskMABhL5qoQIJ...Q?e=X4gOmA

[Ofnuts]

JpegLi isn't a new format (otherwise Gimp wouldn't read it without additional plugins), but a new library implementation, published by Google in April 2024 (so, fairly recent...)

If you look at the number of colors, it is easy to guess how they achieve the result:

• Q90 image: 39600
  
• Q5 image: 2077
  
• JpegLI Q20 image: 7753
  
• JpegLI Q5 image: 1655
  

The image RGB histograms are also quite telling. Additionally, ImageMagick's identify can't find a quality setting in the JpegLi files).

So JPegLi is perhaps better at finding colors that mitigate the blocky look, but if you want to keep some image quality, you have better stick to good quality settings where it is not going to be much better than the "classic" library. And don't mistake the intent: Google has been in a crusade to reduce the internet bandwidth usage and has in the past provided many tools to reduce image size. But this is for distribution/publication, not archival nor editing...

[Zbyma72age]

(06-30-2024, 04:51 PM)Ofnuts Wrote: JpegLi isn't a new format (otherwise Gimp wouldn't read it without additional plugins), but a new library implementation, published by Google in April 2024 (so, fairly recent...)

If you look at the number of colors, it is easy to guess how they achieve the result:

• Q90 image: 39600
  
• Q5 image: 2077
  
• JpegLI Q20 image: 7753
  
• JpegLI Q5 image: 1655
  

The image RGB histograms are also quite telling. Additionally, ImageMagick's identify can't find a quality setting in the JpegLi files).

So JPegLi is perhaps better at finding colors that mitigate the blocky look, but if you want to keep some image quality, you have better stick to good quality settings where it is not going to be much better than the "classic" library. And don't mistake the intent: Google has been in a crusade to reduce the internet bandwidth usage and has in the past provided many tools to reduce image size. But this is for distribution/publication, not archival nor editing...

I use the cjpegli.exe file included with the jxl format implementation reference library 
(https://github.com/libjxl/libjxl/releases/ v0.10.2 08-03-2024r)
Google did not influence me.

After all, when processing a photo, I save as Q100, and only then use the XL Converter program.
What was the point of lossy re-encoding.
I believe that
We should like the results more by eye, which are more important than the various indicators.

[Ofnuts]

(06-30-2024, 05:41 PM)Zbyma72age Wrote: We should like the results more by eye, which are more important than the various indicators.

Yes, and no. yes if you publish, no if you archive/edit. You can process images to make them more pleasing (which what most smartphones do), however once they are processed you have lost a lot of data and that makes them rather difficult to edit.