Compressing images... loss?

On 25 Jul 2004 18:37:47 -0700, realone27 DeleteThis @hotmail.com (Jeff) wrote:

 >I have ~1.5 GB of jpg and tif files, should I compress them when
 >backing them up or will the compression program compromise the image?
 >
 >Are there any compression programs anyone likes? (winzip... pkzip...?)
 >
 >Thanks.


Winrar has a large following. No quality lost when using it of
winzip.
--

- Charles
-
-does not play well with others<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

"Jeff" <realone27 RemoveThis @hotmail.com> wrote in message
news:12ecaca4.0407251737.59a4712b@posting.google.com...
 > I have ~1.5 GB of jpg and tif files, should I compress them when
 > backing them up or will the compression program compromise the image?
 >
 > Are there any compression programs anyone likes? (winzip... pkzip...?)

JPG and gif files are already compressed. If you use winzip on them they
will usually be slightly bigger due to the added data winzip will need fo
uncompress the files. You can use the jpg program to reduce the size of a
jpg file but it is a lossey reduction and you will loose some of the image
data.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

"Jeff" <realone27.DeleteThis@hotmail.com> wrote in message
news:12ecaca4.0407251737.59a4712b@posting.google.com...
 > I have ~1.5 GB of jpg and tif files, should I compress them when
 > backing them up or will the compression program compromise the image?
 >
 > Are there any compression programs anyone likes? (winzip... pkzip...?)

I like WinRAR myself, as it's used widely and also supports ZIP files, plus
can view inside CAB and ISO files.

Regular compression tools, including the compression used by backup
software, is designed to be lossless, so you won't lose any quality.
However, the level of compression depends on repetitive data (a text file of
all one letter will compress a LOT; a text file containing the number PI
will not). Multimedia files in general - audio, video and image - tend to
be extremely random data, and as such will compress very little (1-2% at
best) and in some cases you may even end up with a larger file once you add
in the compressor overhead. Meanwhile, compression does take time to
perform; the higher the compression you set, the longer it takes. It
usually isn't worth the time for the minimal amount of space you save.

About the only way you might see a benefit is to archive numerous images
into one ZIP or RAR file - the more files moved into a solid archive, the
more you save in lost cluster space. But again, the savings would be
minimal until you get a LOT of files in there, and with the cost of storage
and backup space today, it really isn't worth the time it would take.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

On 2004-07-26, Matt Ion <soundy RemoveThis @moltenimage.com> wrote:
 > Regular compression tools, including the compression used by backup
 > software, is designed to be lossless, so you won't lose any quality.
 > However, the level of compression depends on repetitive data (a text file
 > of all one letter will compress a LOT; a text file containing the number
 > PI will not). Multimedia files in general - audio, video and image - tend
 > to be extremely random data, and as such will compress very little (1-2%
 > at best) and in some cases you may even end up with a larger file once you
 > add in the compressor overhead. Meanwhile, compression does take time to

Actually, audio compresses quite well with the right lossless compressor.
FLAC, for example, will typically cut a WAV file in half.

It's more accurate to say that compression depends on *predictable* data,
rather than repetitive data. If an algorithm, given the data up to any
given point in the file, can predict the next byte better than by guessing,
you can turn it into a compression algorithm. The more accurate the
prediction, the better you can compress. Repititive data is predictable, so
compresses well, but other data can be predictable, like audio.

--
--Tim Smith<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

Tim Smith wrote:
 > On 2004-07-26, Matt Ion <soundy.RemoveThis@moltenimage.com> wrote:
 >
  >>Regular compression tools, including the compression used by backup
  >>software, is designed to be lossless, so you won't lose any quality.
  >>However, the level of compression depends on repetitive data (a text file
  >>of all one letter will compress a LOT; a text file containing the number
  >>PI will not). Multimedia files in general - audio, video and image - tend
  >>to be extremely random data, and as such will compress very little (1-2%
  >>at best) and in some cases you may even end up with a larger file once you
  >>add in the compressor overhead. Meanwhile, compression does take time to
 >
 >
 > Actually, audio compresses quite well with the right lossless compressor.
 > FLAC, for example, will typically cut a WAV file in half.
 >
 > It's more accurate to say that compression depends on *predictable* data,
 > rather than repetitive data. If an algorithm, given the data up to any
 > given point in the file, can predict the next byte better than by guessing,
 > you can turn it into a compression algorithm. The more accurate the
 > prediction, the better you can compress. Repititive data is predictable, so
 > compresses well, but other data can be predictable, like audio.
 >


Predictable data?

Ken<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

Jeff wrote:
 > I have ~1.5 GB of jpg and tif files, should I compress them when
 > backing them up or will the compression program compromise the image?
 >
 > Are there any compression programs anyone likes? (winzip... pkzip...?)
 >
 > Thanks.

Don't compress the images further (e.g. by using more JPEG compression).
Either accept that you will need to backup onto three CDs, or buy yourself
a DVD writer. I would not recommend putting all the images into one large
Zip file, as if that file becomes corrupt, all the images may be lost.
It's also possible that you may not be able to read a proprietary backup
format in the future, so stick with a straight copy.

Cheers,
David<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

"Tim Smith" <reply_in_group.TakeThisOut@mouse-potato.com> wrote in message
news:pC%Mc.16483$Qu5.4165@newsread2.news.pas.earthlink.net...
 > On 2004-07-26, Matt Ion <soundy.TakeThisOut@moltenimage.com> wrote:
  > > Regular compression tools, including the compression used by backup
  > > software, is designed to be lossless, so you won't lose any quality.
  > > However, the level of compression depends on repetitive data (a text
file
  > > of all one letter will compress a LOT; a text file containing the number
  > > PI will not). Multimedia files in general - audio, video and image -
tend
  > > to be extremely random data, and as such will compress very little (1-2%
  > > at best) and in some cases you may even end up with a larger file once
you
  > > add in the compressor overhead. Meanwhile, compression does take time
to
 >
 > Actually, audio compresses quite well with the right lossless compressor.
 > FLAC, for example, will typically cut a WAV file in half.

Granted, although that's not a "standard" compression tool. Plus, the
original post asked about "backing up", from which I got the impression he
was using backup software like Veritas or something.

 > It's more accurate to say that compression depends on *predictable* data,

Good point...

 > rather than repetitive data. If an algorithm, given the data up to any
 > given point in the file, can predict the next byte better than by
guessing,
 > you can turn it into a compression algorithm. The more accurate the
 > prediction, the better you can compress. Repititive data is predictable,
so
 > compresses well, but other data can be predictable, like audio.

This is true. And again, depends on the algorithm. Those like ZIP and RAR
and whatever is used by various backup programs are designed to be more
general-pupose, and generally assume that most computer files will be text
and text-based (word processor, spreadsheet, etc.), or program files of some
sort. For other file types, sometimes an algorithm designed specifically
for that file's structure is more effective.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

"David J Taylor" <david-taylor.DeleteThis@blueyonder.co.not-this-bit.nor-this-part.uk>
wrote in message news:%e2Nc.8169$8W1.83901600@news-text.cableinet.net...
 > Jeff wrote:
  > > I have ~1.5 GB of jpg and tif files, should I compress them when
  > > backing them up or will the compression program compromise the image?
  > >
  > > Are there any compression programs anyone likes? (winzip... pkzip...?)
  > >
  > > Thanks.
 >
 > Don't compress the images further (e.g. by using more JPEG compression).
 > Either accept that you will need to backup onto three CDs, or buy yourself
 > a DVD writer. I would not recommend putting all the images into one large
 > Zip file, as if that file becomes corrupt, all the images may be lost.
 > It's also possible that you may not be able to read a proprietary backup
 > format in the future, so stick with a straight copy.

Oh, and speaking of DVD writers... in my market at least (Vancouver, BC),
DVD is by far cheaper per-byte than CD now.

A CD-RW drive can be had around here starting at CDN$50-$60; CD-RW/DVD-ROM
combo for about $75, and a 4X DVD+/-RW for a hair over $100 (8X writer for
about $120-$130), which makes the drive itself very affordable.

CD-R media can be found as low as $30 for a spindle of 100 (very cheap)
discs; better quality disks still run about $50-$60/100... or 30-60 cents
per 700MB disc. Averaging that at 45 cents/disc, you get about 65 cents per
gigabyte.

DVD+R and DVD-R media can be had for $25 for a spindle of 25, or $1 per
4700MB disc... or less than 22 cents per gig.

Doesn't take long at that rate to offset the cost of the writer.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

In message <cf%Mc.114562$ek5.110689@pd7tw2no>, Matt Ion
<soundy.DeleteThis@moltenimage.com> writes
 >
 >"Jeff" <realone27.DeleteThis@hotmail.com> wrote in message
 >news:12ecaca4.0407251737.59a4712b@posting.google.com...
  >> I have ~1.5 GB of jpg and tif files, should I compress them when
  >> backing them up or will the compression program compromise the image?
  >>
  >> Are there any compression programs anyone likes? (winzip... pkzip...?)
 >
 >I like WinRAR myself, as it's used widely and also supports ZIP files, plus
 >can view inside CAB and ISO files.

I usually use 7-Zip, which is free, though I usually stick tot he zip
format
--
Chris French<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

In message <gwZMc.13747$iK.3378@newsread2.news.atl.earthlink.net>, Ralph
Mowery <rmowery28146 DeleteThis @earthlink.net> writes
 >
 >"Jeff" <realone27 DeleteThis @hotmail.com> wrote in message
 >news:12ecaca4.0407251737.59a4712b@posting.google.com...
  >> I have ~1.5 GB of jpg and tif files, should I compress them when
  >> backing them up or will the compression program compromise the image?
  >>
  >> Are there any compression programs anyone likes? (winzip... pkzip...?)
 >
 >JPG and gif files are already compressed. If you use winzip on them they
 >will usually be slightly bigger due to the added data winzip will need fo
 >uncompress the files. You can use the jpg program to reduce the size of a
 >jpg file but it is a lossey reduction and you will loose some of the image
 >data.

Although it is just about possible for a JPEG file to be incompressible
with ZIP encoding it is more common for them to compress by about 1-5%.
This is usually not worth the effort. In fact a quick and dirty test for
badly damaged JPEG files is to see if they will compress by more than
5%.

TIFs may well compress by a sufficient margin 30-70% to be worth using
ZIP. Depending on how tight you are for space this may be worthwhile.

Regards,
--
Martin Brown<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

Jeff wrote:

 > I have ~1.5 GB of jpg and tif files, should I compress them when
 > backing them up or will the compression program compromise the image?
 >
 > Are there any compression programs anyone likes? (winzip... pkzip...?)
 >
 > Thanks.

JPG format is already compressed, and little will gained (space wise)
with further compression, and the loss has already happened. TIF files
aren't usually compressed but some compression methods can be applied to
them, and space savings varies. You can apply a lossless compression,
such as LZW and gain some space, on some pictures. Winzip and Pkzip
both do lossless compression.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss? 

Already compressed files, such as jpeg, don't compress well. Think about
it. If compression worked on _every_ file then run the data through the
compression program often enough and any data of any complexity could
eventually be compressed down to half a dozen bytes.

Needless to say it ain't like that. In fact you can prove with quite simple
mathematics that for every file which you can compress there must be at
least one file which would get bigger when subjected to the same compression
algorithm.

However the good news is that all general purpose data compression
applications (eg. ZIP) are lossless. Again, if you think about the
consequences were it not so, you will realise that they have to be. If what
you are compressing is a picture then you might not be too upset if it
expanded to something very close, but not identical, to the original. If it
was your last five years' accounts, you might be a trifle upset if the
compression program decided to change a few of the figures.

To return to your original question, should you compress images before
archiving? If they are in an already compress format, such as jpeg, then
no. If they are uncompressed (eg. tiff) then yes. If they are a mixture
then the only way really to tell is to try compressing them and see if the
result is bigger or smaller than the sum of the component parts.

Incidentally, the same argument applies to compressed folders within
Windows. If you habitually use Windows' compressed folders feature, as I
do, it is a good idea to make sure you turn it off for those folders where
you store jpeg images.

Keith
 >> Stay informed about: Compressing images... loss? 

c0smic wrote:

 > Jeff wrote:
 >
 >
  >>I have ~1.5 GB of jpg and tif files, should I compress them when
  >>backing them up or will the compression program compromise the image?
  >>
  >>Are there any compression programs anyone likes? (winzip... pkzip...?)
  >>
  >>Thanks.
 >
 >
 > It’s true that compressing JPGs will not save much space. But it will not
 > harm the file. The restored file will be identical to the original. TIFs
 > compress nicely, sometimes to a third the original size. If you are backing
 > folders with different file formats don’t worry about it. The files will be
 > identical to the originals. With a lot of TIFs and a good compression
 > program like Winrar you might be able to backup 1.5 GB on 3 CDs.
 >
Hummm. 1.5 GB and 3 times 600 MB. Yeah, I should think SO. Grin.
Probably on two....<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Compressing images... loss?