Question re jpeg compression

I am aware of the issue with repeated editing and re-saving of jpeg image
files, and the diminishing quality this can cause. I mostly work in jpeg all
the time as my little humble p&s Fuji only makes jpegs anyway. I notice in
the options settings of my editing software there is the ability to set the
default compression of jpeg files. In this package it defaults to 90 (on a
scale from 1 to 100), but I have seen similar settings in a few other
programs also. The help says the higher the setting the higher the quality
and the larger the file. What happens if I set the default to 100? Does that
mean no compression at all? Without recompression would this do away with
the slow drop in quality over repeated saves, in effect making all editing
and saving lossless?

Probably not all that relevant to the question but I am using Microsoft
Digital Image Standard Edition 2006 (Library and Editor). I would love to
know if this is possible as I love fiddling with my images, and storage
space is really not an issue for me.

Thanks for any thoughts out there.

--
Peter in New Zealand. (Email address is fake)
Collector of old cameras, tropical fish fancier, good coffee nutter, and
compulsive computer fiddler.

and the larger the file. What happens if I set the default to 100? Does
that mean no compression at all? Without recompression would this do away
with the slow drop in quality over repeated saves, in effect making all
editing and saving lossless?

Probably not all that relevant to the question but I am using Microsoft
Digital Image Standard Edition 2006 (Library and Editor). I would love to
know if this is possible as I love fiddling with my images, and storage
space is really not an issue for me.

Thanks for any thoughts out there.

--
Peter in New Zealand. (Email address is fake)
Collector of old cameras, tropical fish fancier, good coffee nutter, and
compulsive computer fiddler.


The professional photo labs I use say it is only necessary to save the final
image to a compression of only 8 (out of 10) in Photoshop. They have tested
this and guarantee no noticeable loss (to the human eye) in quality between
8 and higher. I have had many large and small images printed to this
compression and they all appear very well detailed.
It does help however prior to processing the image to convert it to a
lossless format (eg TIFF) and then only convert to jpeg (8 compression) when
ready to send out to get printed. At least allows you to save as many times
as you like during the processing stage without losing quality etc.

Benny

On Sat, 24 May 2008 22:02:15 +1200, Peter in New Zealand wrote:

I am aware of the issue with repeated editing and re-saving of jpeg
image files, and the diminishing quality this can cause. I mostly work
in jpeg all the time as my little humble p&s Fuji only makes jpegs
anyway. I notice in the options settings of my editing software there is
the ability to set the default compression of jpeg files. In this
package it defaults to 90 (on a scale from 1 to 100), but I have seen
similar settings in a few other programs also. The help says the higher
the setting the higher the quality and the larger the file. What happens
if I set the default to 100? Does that mean no compression at all?
Without recompression would this do away with the slow drop in quality
over repeated saves, in effect making all editing and saving lossless?

No. It will still do lossy compression. The difference on any one save
will not be noticeable, but it will accumulate. Best to either always
start with the original file or change to a lossless format (png is good).



Probably not all that relevant to the question but I am using Microsoft
Digital Image Standard Edition 2006 (Library and Editor). I would love
to know if this is possible as I love fiddling with my images, and
storage space is really not an issue for me.

Thanks for any thoughts out there.

On Sat, 24 May 2008 22:02:15 +1200, Peter in New Zealand wrote:
I am aware of the issue with repeated editing and re-saving of jpeg image
files, and the diminishing quality this can cause. I mostly work in jpeg all
the time as my little humble p&s Fuji only makes jpegs anyway.

To add to the other followups: My camera(s) only do JPEG, as well. In
my image editing software (gimp), I always pick up the original camera
image (a JPEG) and immediately save it as a PNG: xxximagexxx_00.png.
Thereafter, I work on it and continue to save intermediate work as PNGs:
xxximagexxx_01.png, xxximagexxx_02.png, etc.

I will save the final copy as xxximagexxx_99.png _and_ as a JPEG with
('just the right') compression as xxximagexxx_99.jpeg. Then I copy the
xxximagexxx_99.jpeg to the final destination -- assigning the final file
name I desire.

This-a-way I greatly reduce the ravages of JPEGism and I can go back a
ways in my editing process and pick it up and futz with it again if I
determine my original work was sub-par -- pretty much every time. :-)

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
*** Killfiling google posts: http://jonz.net/ng.htm

ray wrote:

On Sat, 24 May 2008 22:02:15 +1200, Peter in New Zealand wrote:

I am aware of the issue with repeated editing and re-saving of jpeg
image files, and the diminishing quality this can cause. I mostly work
in jpeg all the time as my little humble p&s Fuji only makes jpegs
anyway. I notice in the options settings of my editing software there is
the ability to set the default compression of jpeg files. In this
package it defaults to 90 (on a scale from 1 to 100), but I have seen
similar settings in a few other programs also. The help says the higher
the setting the higher the quality and the larger the file. What happens
if I set the default to 100? Does that mean no compression at all?
Without recompression would this do away with the slow drop in quality
over repeated saves, in effect making all editing and saving lossless?

No. It will still do lossy compression. The difference on any one save
will not be noticeable, but it will accumulate. Best to either always
start with the original file or change to a lossless format (png is good).

This brings up a question I've pondered. If png is lossless (and I'm not
arguing that point), then why does it offer levels of compression? If
it's lossless, then quality will be the same for least *and* most
compression; so why not compress maximally?


--
Blinky
Killing all posts from Google Groups
The Usenet Improvement Project: http://improve-usenet.org
NEW -- Now evaluating a GG-free news feed: http://usenet4all.se

In article 1211622886.390240@ftpsrv1,
Peter in New Zealand wrote:

I am aware of the issue with repeated editing and re-saving of jpeg image
files, and the diminishing quality this can cause. I mostly work in jpeg all
the time as my little humble p&s Fuji only makes jpegs anyway. I notice in
the options settings of my editing software there is the ability to set the
default compression of jpeg files. In this package it defaults to 90 (on a
scale from 1 to 100), but I have seen similar settings in a few other
programs also. The help says the higher the setting the higher the quality
and the larger the file. What happens if I set the default to 100?

Take a look at the FAQ document at http://www.faqs.org/faqs/jpeg-faq/
and read the answer to Question 5.

Briefly:

- JPEG is lossy, even at the highest-quality setting.

- If you're going to be editing the image repeatedly, you'll be
better off using a lossless format for intermediate storage steps,
and re-compressing to JPEG only as the final step.

- There are a *few* image-editing operations you can do to a JPEG
which won't result in any further image degradation - rotation and
flipping, some types of cropping, some types of margin insertion.
To be lossless, these operations *must* be done using software
designed specifically to use the lossless-JPEG manipulation
techniques which bypass the decompression/recompression steps and
work directly on the existing compressed representation.

- If you decompress a JPEG image, and then recompress at the *same*
quality setting *without* changing the image data, "relatively
little" further degradation occurs - the recompression operation
will be working with an image color palette which has already been
quantized, and little additional color quantization error will
occur. This "relatively little further degradation" rule doesn't
apply if you edit the image in ways that change the colors of the
pixels (sharpening, blending, blurring, level-setting, etc.), or if
you recompress with a different "quality" setting.

- The meaning of the "quality" settings is not standardized, although
since a lot of products use JPEG compressors based on the
Independent JPEG Group (IJG) libraries, the "quality value goes up
to 100" scale tends to have the IJG meaning.

- Quality settings above 95 generally have little or no additional
benefit, but do come with a penalty in size. "Q 100 is a
mathematical limit rather than a useful setting. If you see a file
made with Q 100, it's a pretty sure sign that the maker didn't know
what he/she was doing."

- Even at the highest quality settings, you may see some color
fringing at edges. This can be reduced or eliminated (again at the
cost of file size) by turning off "chroma downsampling". Some
programs have a separate setting/switch for this, others turn off
downsampling automatically at higher quality levels.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Blinky the Shark wrote:
[]
This brings up a question I've pondered. If png is lossless (and I'm
not arguing that point), then why does it offer levels of
compression? If it's lossless, then quality will be the same for
least *and* most compression; so why not compress maximally?

There is a trade-off between the degree of compression, and the CPU time
that compression (and decompression) takes. Fast but less compression.
Slow with more compression. Your choice. There are also some options in
PNG to take the image line-to-line similarity into account when
compressing. Again, this takes more time, but may produce better
compression. The quality is the same in all cases, but more compression
may reduce the file size.

Cheers,
David

On Sat, 24 May 2008 09:41:08 -0700, Blinky the Shark wrote:

ray wrote:

On Sat, 24 May 2008 22:02:15 +1200, Peter in New Zealand wrote:

I am aware of the issue with repeated editing and re-saving of jpeg
image files, and the diminishing quality this can cause. I mostly work
in jpeg all the time as my little humble p&s Fuji only makes jpegs
anyway. I notice in the options settings of my editing software there
is the ability to set the default compression of jpeg files. In this
package it defaults to 90 (on a scale from 1 to 100), but I have seen
similar settings in a few other programs also. The help says the
higher the setting the higher the quality and the larger the file.
What happens if I set the default to 100? Does that mean no
compression at all? Without recompression would this do away with the
slow drop in quality over repeated saves, in effect making all editing
and saving lossless?

No. It will still do lossy compression. The difference on any one save
will not be noticeable, but it will accumulate. Best to either always
start with the original file or change to a lossless format (png is
good).

This brings up a question I've pondered. If png is lossless (and I'm
not arguing that point), then why does it offer levels of compression?
If it's lossless, then quality will be the same for least *and* most
compression; so why not compress maximally?

Time.

In article .net,
Blinky the Shark wrote:

This brings up a question I've pondered. If png is lossless (and I'm not
arguing that point), then why does it offer levels of compression? If
it's lossless, then quality will be the same for least *and* most
compression; so why not compress maximally?

Maximal compression takes more CPU time during the compression
process. Going from "moderate" to "highest" compression quality for
PNG may increase the CPU time needed by a factor of several times (3x
to 5x I think) while decreasing the size of the compressed data by
only a few percent.

PNG uses the "deflate" version of the LZ77 lossless compression
algorithm. To greatly oversimply things, the compressed data consists
of either:

[1] The original bytes of data from the input, unaltered, or
[2] Special sequences of codes which mean "Hey, you've seen this
sequence of bytes before... you can find the next N bytes by
looking back in the data by a distance of XXXX and copying that
sequence."

In PNG compression, the furthest that the sequences can "look back" in
the data is 32k bytes... this is the size of the "data window" that
the decompressing software must keep buffered, so that it can "look
back" and copy the data pointed to by the compression sequence.

The job of the software which does the compression, is to look through
the image, find sequences of bytes which appear more than once, and
use this knowledge to create the compressed representation.

There will (almost certainly) be many different ways to compress the
data... numerous different "Hey, look back XXXX bytes and copy N
bytes" sequences which will accurately reproduce the original data.
The compressed sequences will vary in their total length... shorter is
better.

The compressing program gets to decide "how hard it wants to work"...
that is, how many different alternative compression sequences it wants
to try, to find the one which ends up being the shortest. That takes
time. In most cases, it's not "worth the effort" to spend a maximal
amount of CPU time looking for the Very Best Compressed Sequence...
it's usually possible to do 95% as well, with only 10-20% as much
searching effort.

Back in the days of 40 MHz 486 CPUs, this was a big issue. Nowadays,
with multi-gigahertz CPUs, using a higher compression level may be
more worthwhile for some users.

The decompressing software "doesn't care" - it's no more work (and in
fact may be a bit less) for the decompression program to handle an
optimally-compressed representation than it is to handle one that's a
bit more loosy-goosy.

As a practical example of the tradeoff: I sometimes burn filesystem
backups to CD-R for offsite storage, using a backup utility that
incorprates "gzip" compression (another LZ77 variant). I set the
compression level to one that's high enough to be useful, but low
enough (and fast enough) that it can compress the filesystem data
faster than the CD-R drive can burn it to the media. This choice
ensures that the CD-R drive doesn't suffer from a buffer underrun
while burning.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Peter in New Zealand wrote:
I am aware of the issue with repeated editing and re-saving of jpeg image
files, and the diminishing quality this can cause. I mostly work in jpeg all
the time as my little humble p&s Fuji only makes jpegs anyway. I notice in
the options settings of my editing software there is the ability to set the
default compression of jpeg files. In this package it defaults to 90 (on a
scale from 1 to 100), but I have seen similar settings in a few other
programs also. The help says the higher the setting the higher the quality
and the larger the file. What happens if I set the default to 100? Does that
mean no compression at all? Without recompression would this do away with
the slow drop in quality over repeated saves, in effect making all editing
and saving lossless?

Probably not all that relevant to the question but I am using Microsoft
Digital Image Standard Edition 2006 (Library and Editor). I would love to
know if this is possible as I love fiddling with my images, and storage
space is really not an issue for me.

Thanks for any thoughts out there.

Just to be clear, there is no damage from repeated saves unless you
close the file between saves. Of course you are saving the original, so
you can always start from scratch if you want to edit more another day.

--
Paul Furman
www.edgehill.net
www.baynatives.com

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