Archived from groups: alt.comp.periphs.dcameras (
More info?)
"Gymmy Bob" <nospamming@bite.me> wrote in message news:<JOOdnfxKnIzEQQrcRVn-iA@golden.net>...
> Compression does ***NOT*** reduce quality.
?? Are you suggesting that there is some sort of lossless compression
available on your typical consumer camera? If we are talking about
compression on a digicam, we are talking about JPEG compression. JPEG
compression is LOSSY compression. In other words, as you said
yourself, 'lossy compression techniques will throw away certain
resolution/qualities of an image'. Actually, JPEG compression works
on the basis of breaking an image up into 8x8 blocks, and then using
various algorithms to mimic the original image. Even at 100% quality
(or 'Fine' as Sony calls it), the JPG file is of slightly less quality
than a TIF.
> Compression, by itself, only
> eliminates repeating of patterns that are not needed.
No. That is how compression works with ZIP files and certain TIFF
files, but it is NOT correct as it applies to JPG files. I suggest
you do some research on how JPG compression works. It is quite
different.
> Upon decompression the
> exact same image with the exact same bits are reproduced.
It is when the file is saved as a JPEG file, that there are losses of
quality. If you don't believe that, try setting the quality down to
say 50%, and then compare the saved file to the original. (At 100%
quality, the file is ALMOST indistinguishable from the original and
will require very close examination to spot any differences, but they
ARE there.) If you re-open a JPEG file, make small changes and then
resave it at the SAME quality level, you generally will not get any
*further* degradation in unedited areas.
> Only lossy compression techniques will throw away certain
> resolution/qualities of an image.
Which is what JPG files are. JPEG compression, as used for digital
camera image files, is LOSSY. (There *are* forms of JPEG compression
that are lossless but they are NOT used in the type of digital cameras
discussed here.)
On recent Sony cameras, FINE quality level means a JPG of around 100%
quality. My guess is that STANDARD is somewhere around 85%-90%,
possibly with a small added 'smoothing' factor. Most folk will
struggle to spot the difference between them unless enlarged and
closely inspected. But they are different.
> > >I would imagine if you set the JPG compression to low (or 'standard' I
> > >think), then you will get 100 or more images on your memory card at
> > >4Mp - offload them to your computer, and then off you go again. And
> > >given that memory cards, and memory on your computer, are NOT that
> > >expensive...
> >
> > Don't ... either 'imagine' or do this!
> >
> > Fine or Very fine are the compressions, otw just use the next lower pixel
> > resolution. The artifacts are far worse than fewer pixels!
Firstly, are you sure about this? On my Sony, FINE means highest JPG
quality, and STANDARD means lower JPG quality. It has the actual
numbers for the pixel count/resolution, eg 5Mp, 3Mp, etc, and it does
NOT use the words Fine/Very Fine for resolution.
Secondly, as I mentioned above, the JPG artefacting, even at the lower
('Standard') compression level, is almost undetectable. JPG artefacts
do not get worse as you increase the size of the image, say for
printing, but of course there comes a point when you simply do not
have enough pixels-per-inch and the image 'pixellates' - also known as
Game Over. That seems to be the issue referred to in the OP.
So I stick to my comments..
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