I don't know if this is true of JPEG algorithms, but often there are
a *several* parameters than can (programatically) be assigned values.
Such parameters probably do not have names or descriptions that
easily map to something like a person's perception of the "quality"
of an image.
How those multiple possible settings are translated into a single
"slider" control is up to the programmer. For example, let's say
that there's a parameter named "aggressiveness," that accepts input
values from 0 to 99. Higher values result in smaller files. Smaller
files are compressed more and so probably have more artifacts, so
maybe we could think of "aggressiveness" as "quality" but with the
scale reversed. :)
Whatever. The middle value "must be" halfway between the worst
quality and the best quality. So... we (the programmer) hooks the
(reversed) scale of "aggressiveness" to a LINEAR slider control in
the interface. The algorithm has "5" other parameters, but the
programmer doesn't really know what they do, so just hard codes them
at their defaults.
There, done! Uhhhh, probably not. :(
(Mind you, I'm inventing the details here, but you can see how this
sort of thing could happen!)
eo
On Jan 12, 2008, at 12:55 PM, Keith Martin wrote:
Sometime around 12/1/08 (at 17:01 +0100) Hans Nyberg said:
I have complained about this for the last 4-5 years. First
Panocube and then Pano2QTVR.
Now it is even a larger problem as we now also have Pano2VR for Mac.
As far as I know RealViz uses the same standard as we always had
on Mac also for Windows.
And as this standard is very close to Photoshop save for web it
should be the one which should be used.
But there are many other ways of calculating the level of
compression! Photoshop's is not the only one worth using; in fact,
it is slanted towards quality rather than compression; a particular
percentage value in it has a different quality/compression balance
than in some other tools. There's no reason why everything should
use that.
There's never been a single standard, except perhaps back when the
JPEG format was a new, experimental format that couldn't be read by
anything directly other than the compression tools. And even then,
I seem to remember different tools would produce different results
with apparently-similar options. But certainly since JPEG files
have been more widely used, there's not been a single true industry-
wide standard for compression settings, not that I'm aware of.
Should there be a single standard? It makes a lot of sense. But who
gets to decide what that is? And should it be slanted more towards
compression or quality? I do suspect Adobe's approach isn't
entirely linear, and others may not be either.
k
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