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Re-saving JPG


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adrjork

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Hi everyone, newbie here.

Does re-saving JPG changing DPI value (no re-samp) get worse the image quality? (Re-compresses JPG?)
I saved a JPG in Photoshop (quality 12 from an original PSD). It's 1200 DPI 5000x3000 px, without copyright infos saved.
Later I decide I want to add copyright infos but I haven't more the original PSD, so I open again my JPG in Photoshop, add the copyright, and save.
Later again, I decide I want to change the DPI value without resampling, so I re-open my JPG, set 266 DPI in Image Size maintaining 5000x3000 px, and save.

Question: since I re-saved my JPG twice, does it mean that I re-compressed my JPG twice as well? Does it mean the quality of the original JPG is now worsened?

Thanks in advance.
 
adrjork said:
Hi everyone, newbie here.

Does re-saving JPG changing DPI value (no re-samp) get worse the image quality? (Re-compresses JPG?)
I saved a JPG in Photoshop (quality 12 from an original PSD). It's 1200 DPI 5000x3000 px, without copyright infos saved.
Later I decide I want to add copyright infos but I haven't more the original PSD, so I open again my JPG in Photoshop, add the copyright, and save.
Later again, I decide I want to change the DPI value without resampling, so I re-open my JPG, set 266 DPI in Image Size maintaining 5000x3000 px, and save.

Question: since I re-saved my JPG twice, does it mean that I re-compressed my JPG twice as well? Does it mean the quality of the original JPG is now worsened?

Thanks in advance.
Click to expand...

Hi @adrjork
Good question and I did not know the answer.
I used the Save As command in PS and in both cases when starting with a saved JPEG at 1200, reloading and either saving at 300 dpi to resmaplng or just changing the info template (e.g. copyright), the reloading either back into Photoshop, it turns out there was a slight difference between them.
I used a random noise image as the base reference (good reference to cover a wide range of RGB values).
The differences were small yet measurable (up to a 4 bit difference in one of the RGB channels though most of the differences were only 1 bit different in a channel.
I was surprised by the result as a re-compression does not need to be done if the color numbers in the image have not changed at all and the save/compression settings have not changed at all. Neither changin DPI nor changing metadata changes the color numbers.
So bottom line, yes there is degradation when using Save As.
There are programs out there that do not recompress with such simple changes in the file yet I guess PS is not one of them (assuming I did the experiment correctly)
Note I did not check using the Save for Web functionality.
John Wheeler
 
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