Re: Why does saves as JPG default to quality 85?

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On Sat, 2012-03-24 at 02:08 +0100, Ofnuts wrote:

> >>> save a JPEG file the quality is reduced and information is lost.
> >> Not true... if nothing changes the algorithm is stable (decoded values get
> >> re-encoded to the same values). You lose quality if you recompute something
> >> different; changed settings, changed pixel values, changed 8x8 boundaries
> >> (image crop).

Interesting. Not 100% true - I made a sample image and saved it at 75%
(OK, 75 if you prefer) in gimp, then opened the jpeg and saved it again,
and reopened that; the 1st and 2nd generation files do have visible
differences, but the differences are massively smaller than those
between the original and the 1st generation image. The visible
differences were extra "fringes" around the jpeg-introduced fringes.
I've appended the (totally not scientifically constructed) sample image
I used.

You are right that the changes seem to get smaller each time.

In practice, however, someone opening the file and doing changes may
very well do global changes (e.g. unsharp, sharpen, curves) that
accentuate the jpeg artifacts; I still think it better to save as PNG
(or xcf.gz) until you're finished.

Thank you for the script.

Liam

-- 
Liam Quin - XML Activity Lead, W3C, http://www.w3.org/People/Quin/
Pictures from old books: http://fromoldbooks.org/

Attachment: 01.png
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