Am 26.04.2010 22:29, schrieb Kenneth:

> I always thought that .jpeg compression quality was set within a simpler
> 1-through-10 scale --just those and no in-between values. (That's how my old
> version of Photoshop does it, anyway...which is about the extent of my
> knowledge.)

No, basically the JPEG compression quality is a non-discrete value; from 
what I known, theoretically it should be in the range from 0.0 to 1.0, 
but software may use a pretty arbitrary scale in its UI. POV-Ray uses 
percents, for that matter.

> How was 95 arrived at for the 3.7 default (vs. 100)? They're so close.

Well, someone figured that raytraced images should be stored at pretty 
high quality, and decided that 95% made for a nice trade-off between 
size and quality. (File size does not grow linear with the quality 
value; in a quick test I just did, going from 95% to 100% more than 
doubled the file size.)

 > More
> importantly, can all image-viewing apps decode .jpegs created with such a
> 'fine-scale' 1-to-100 compression choice? I base this question on problems I've
> encountered (in Photoshop again, v5.0): strangely, even within its own 1-to-10
> scale, there are several values that produce an image which isn't viewable in
> some other apps I have. Maybe that's strictly a problem with the older
> Photoshop--but it makes me wonder about the 1-to-100 variation in 3.7. Could the
> 'wrong choice' of a particular interim value produce an image-decoding problem?

I don't think this has anything to do with the compression quality value 
/per se/. Probably it's some bug in Photoshop 5.0's encoder that just 
doesn't show at low compression settings (e.g. an overflow somewhere in 
the math).

(My versions of Photoshop (6.0) happens to use a 0..100% range as well.)

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