At 11:17 AM 1/15/99 +0100, Omer Nezih GEREK wrote:
[snip]
>As for the second sentence, actually it is JPEGs weak point not
>to keep the general trend of colour change accross the blocks. Therefore,
>usually smooth images are unintentionally compressed too much and the
>8x8 blocks become visible. I would like to suggest that you produce an
>artificial very smooth "wave" image and a very high contrast "busy"
>image, and code at 40etting. You will see that at the same setting,
>the smooth image has a smaller JPEG but the JPEG image is unbearable. Low
>pass (smooth) images and sharp transitions of smooth regions (graphics
>art images) are bad for JPEG.
This has been my experience working with JPEGs of certain subjects with
continuous shading, particularly photos with large areas of blue sky with
gradual transitions from light to dark areas of blue. Too much JPEG
compression "blocks" the sky, producing weird stepping. Thus, in the
Unofficial Olympus Web Photo Gallery, Dr. Matthew J. Cordery's Australia and
New Zealand submissions have areas of sky with this stepping artifact, which I
find unpleasant. Unfortunately, to try and keep the file sizes somewhat down,
I had little choice.
Compare these to a picture by Branko Turk in the Gallery. This is a photo of
an older brick office building which I originally tried to save at quality
level "5" in Photoshop. Observing the result, I found that the image was
indistinguishable from the original submission, and the file size had hardly
changed at all. So then I tried saving it at "1". Surprise! Almost no change
in quality, and very little change in file size (it was still relatively large).
I've received complaints from people who say that some of the images in the
Gallery are too large, and that I should compress them more. Respectfully,
I've had to disagree -- even the compression I've done on some of the photos
has done harm to them, in my opinion. Matthew Cordery's original scans were
breathtaking -- the stuff I was forced to post in the Gallery, while nice, was
nothing compared to the originals. JPEG forced compromises between size of
file and quality of image.
I hate compromises like that.
Garth
"A bad day doing photography is better
than a good day doing just about
anything else."
The Unofficial Olympus Web Photo Gallery at:
http://www.taiga.ca/~gallery/
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