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Resizing Images

Posted: Sat Jun 30, 2018 1:55 pm
by core
Hi,
I tried resizing some jpeg images from the camera on my phone, and I chose the "original size" preset, curious what would happen. Each time the file size was drastically reduced. I checked the exif data, and everything is the same except for the filesize. The quality seems the same (although maybe it's not?). I used the lanczos method.

The filesize was reduced from 2.38 MB to 832.86 KB. In another case I went from ~4.4 MB to 1.79 MB.

Can someone help me to understand....What information did I lose in those several Mb?

Thanks,

Bryan

Re: Resizing Images

Posted: Sat Jun 30, 2018 2:25 pm
by cday
core wrote:
Sat Jun 30, 2018 1:55 pm
I tried resizing some jpeg images from the camera on my phone, and I chose the "original size" preset, curious what would happen. Each time the file size was drastically reduced. I checked the exif data, and everything is the same except for the filesize. The quality seems the same (although maybe it's not?). I used the lanczos method.

The filesize was reduced from 2.38 MB to 832.86 KB. In another case I went from ~4.4 MB to 1.79 MB.

When images are saved, the compression level used is the value set, rather unusually due to a limitation in the Qt framework, in File > Format settings... in the viewer.

Edit: You mean Use estimated quality (when possible) ? In that case I'm not sure, unless it wasn't possible to determine the original setting...

What information did I lose in those several Mb?

Quite possibly no useful information: the JPEG format is designed to compress images efficiently with little or no visible loss of quality, dependent on the quality setting used and the particular image. Ultimately it's a subjective judgement depending on all the factors involved, and images with large pixel dimensions can often be compressed with quite low settings without noticeable loss of quality, with a corresponding substantial reduction in file size.

Re: Resizing Images

Posted: Sat Jun 30, 2018 2:54 pm
by core
cday wrote:
Sat Jun 30, 2018 2:25 pm
When images are saved, the compression level used is the value set, rather unusually due to a limitation in the Qt framework, in File > Format settings... in the viewer.
Thanks!
It seems to be tied to the quality setting in Format Settings, as you said. It seems it's tied to saving in general rather than using the resize tool.

I think that helps a lot, thanks so much!
Edit: You mean Use estimated quality (when possible) ? In that case I'm not sure, unless it wasn't possible to determine the original setting...[
Is that possible to estimate?

Re: Resizing Images

Posted: Sat Jun 30, 2018 3:28 pm
by cday
core wrote:
Sat Jun 30, 2018 2:54 pm
cday wrote:
Sat Jun 30, 2018 2:25 pm
When images are saved, the compression level used is the value set, rather unusually due to a limitation in the Qt framework, in File > Format settings... in the viewer.

Edit: You mean Use estimated quality (when possible) ? In that case I'm not sure, unless it wasn't possible to determine the original setting...[
Is that possible to estimate?
I have no information on that, but Googling 'jpeg use estimated quality' produces some possible sources of further information...

Re: Resizing Images

Posted: Sat Jun 30, 2018 3:47 pm
by core
cday wrote:
Sat Jun 30, 2018 3:28 pm
I have no information on that, but Googling 'jpeg use estimated quality' produces some possible sources of further information...
Thanks!