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

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2012/3/23 Liam R E Quin <liam@xxxxxxxxxxx>:
> On Fri, 2012-03-23 at 21:52 +0100, Olivier wrote:
>
>> Considering the quality rating in JPEG as a percentage would mean that
>> a quality equal to 100 would be perfect, i.e. no loss at all.
> Nonsense. A "quality" of 100% means you have chosen 100 out of a
> possible 100. "per cent" means "out of 100" in Latin. It says nothing
> about secondary values. A value of 75% means 75% of the way from 0 to
> the maximum allowed value of 100. I agree that popular usage is to infer
> more than is stated from such an assertion. Note that in general (but
> not in this case) percentages can be greater than 100.

A percentage is a way of expressing a ratio. 20% means the same as
20/100 or 1/5. It's used to express how large/small one quantity is,
relative to another quantity. Here, what is the other quantity?

In degrees centigrade, the temperature of liquid water ranges from 0
to 100. Does that mean that water at a temperature of 30 degrees is
30%? 30% of what?

Notice that most descriptions of the quality factor of JPEG carefully
avoid speaking of percentages. See for example
http://docs.gimp.org/2.8/en/gimp-images-out.html#idp11992944 or
http://en.wikipedia.org/wiki/JPEG.

-- 
Olivier Lecarme
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