JPG compression does not work

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LeanderAT
Posts: 4
Joined: Sat Mar 14, 2020 10:27 pm

JPG compression does not work

Post by LeanderAT »

Hello!

JPG compression does not work in XNView Classic. I use 80 quality and batch processing. As the file size was too big, I tested different quality settings and always get the same file size, regardless of the setting (100 quality, 80, 60, 5...).

Is this a bug? What am I doing wrong?
cday
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Re: JPG compression does not work

Post by cday »

LeanderAT wrote: Sat Mar 14, 2020 10:31 pm Hello!

JPG compression does not work in XNView Classic. I use 80 quality and batch processing. As the file size was too big, I tested different quality settings and always get the same file size, regardless of the setting (100 quality, 80, 60, 5...).

Is this a bug? What am I doing wrong?
Sounds as if you have Use estimated quality (when possible) set in the save settings...

From memory, as I spend most of my time in Linux, on the Save as... dialog there is a button to bring up detailed save options.
LeanderAT
Posts: 4
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Re: JPG compression does not work

Post by LeanderAT »

Thanks! However, I haven't ticked this setting.
cday
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Re: JPG compression does not work

Post by cday »

LeanderAT wrote: Sun Mar 15, 2020 11:49 pm Thanks! However, I haven't ticked this setting.
Sorry, first thing in the morning I missed that you were referring to batch processing, but there should be an equivalent setting there if you look...
LeanderAT
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Re: JPG compression does not work

Post by LeanderAT »

Not really. Image

Progressive
optimaze Huffman...

Keep exif data
keep ...
keep ipc profile
renew embedded thumbnail
use estimated quality..
cday
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Re: JPG compression does not work

Post by cday »

I have now tested using XnView 2.49.2 for Q91 and Q61 with the settings below, and the resulting file sizes vary as expected.

XnView_Classic_2.49.2.jpg

I'm afraid currently I can't suggest any explanation for the behavior you obtained, but possibly if you examine the detailed properties of the attached output files you identify an explanation.

If you are unable to resolve the issue you could test again using XnView MP, which in any case can be regarded as the future of XnView, or the stand-alone batch conversion software XnConvert...
Attachments
Examples_Q61_Q91.zip
(319.56 KiB) Downloaded 18 times
LeanderAT
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Joined: Sat Mar 14, 2020 10:27 pm

Re: JPG compression does not work

Post by LeanderAT »

It does not work. It does with the other two programs (XnView MP and Converter).

What quality setting would you recommend? How big should a photo be (file size, 880px height)?
cday
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Re: JPG compression does not work

Post by cday »

LeanderAT wrote: Mon Mar 16, 2020 10:43 pm It does not work. It does with the other two programs (XnView MP and Converter).
You have tested using the above exact settings with 2.49.2, although I'm not aware of any issue in other recent versions?

If it works for me and not for you there must be a reason, but I've no idea what it could be, possibly some confusion somewhere...

Note: I tested using the ZIP full download but that shouldn't be relevant.

What quality setting would you recommend? How big should a photo be (file size, 880px height)?
It really depends on the purpose for which the image will be used, and how important file size is when there is a large collection of images.

For example, whether the image will be printed at a large size, or just be used as a small image on the screen, and whether a small number of images will be used, or the image will be part of an archive of thousands of images that must be distributed or downloaded.

Also, in my experience images with large pixel dimensions can often be saved with a surprisingly low quality setting without noticeable visible loss.

I would suggest that unless you have a reason to do otherwise, you save at Q80 rather than a higher setting.
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j7n
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Re: JPG compression does not work

Post by j7n »

LeanderAT wrote: Mon Mar 16, 2020 10:43 pmWhat quality setting would you recommend? How big should a photo be (file size, 880px height)?
JPEG is close to being perceptually lossless in a photo at about q 95 and no color subsampling (1x 1x 1x). Very clean, artificial images might require q 98. The useful range of the quality slider is between 90 and 99. The IJG scale is rather misleading with so many steps of extreme quantization, and subsampling being enabled by default in most programs. Photoshop only goes as low as about 60 on its 1-12 scale.

If the image already has low sharpness (due to being upscaled in the camera) or contains noise, then the additional degradation from JPEG might not be noticable at around q 90. At a height of only 880 pixels, your image will likely be viewed at a close distance and can't tolerate any quality reduction.

The file size at a given quality depends on the amount of detail (texture, noise) and whether that detail varies greatly in color.
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