Hi Samuel!
26 Apr 2002, "Samuel W. Heywood" <[EMAIL PROTECTED]> wrote:
SH> I have heard some stories about a phenomena called
SH> "lossy compression".
Its no phenomena.
There are 2 types of compression:
1) lossless compression
2) lossy compression
The difference is that you can extract an exact copy from losslessy
compressed data.
Lossy compression is eg used in MP3, jpg, MPEG ....
you don't get an exact copy, but something that is close to the original.
(eg sounds or looks almost like the original data)
SH> According to these stories it is OK to zip images in some formats, but
SH> not others.
It's OK to ZIP everything.
But with already compressed data its simply useless ...
you can't compress it any further.
(eg it makes sense to compress BMP as this is raw uncompressed data ...
it makes no sense to compress jpg, because this is already compressed)
SH> If you zip a JPG and then unzip it, the resolution will not be
SH> as keen as seen in the original before it was zipped.
NONSENS !!!
ZIP is lossless ...
eg when you zip data und than unzip it it will be EXACTLY the same as the
original data.
SH> If it is true that the phenomena called "lossy compression" does in
SH> fact exist, is the loss of resolution noticeable to the naked eye?
There's no "phenomena" but there are some algorithms that compress better
than lossless algorithms, by leavin out some details, that the human eye
(or ear) is mostly not capable to detect.
SH> 1. If there is such a thing as "lossy compression", could it be
SH> mimimized by using ARJ, ARC, DWC, LHA, TAR, or some other alternative
SH> to PKZIP?
Those programs all use lossless compression.
Otherwise you would not be able to compress binaries, or texts.
Lossy compression is only acceptable if you don't need a 100% original copy
of the data. (images, videos, sounds ...)
SH> 2. Of the many image formats that we know of, and if it is true that
SH> so called "lossy compression" is a problem in the case of some image
SH> formats, which types should not be zipped?
Some formats are already compressed. (it is *NOT* important weather it was
a lossy (eg JPG) or lossless (eg GIF) algorithm)
If you try to compress these again, the resulting compressed file will not
be smaller than the original file.
If you zip a zip file you don't get a smaller file.
SH> It could be that all this talk I have heard about "lossy compression"
SH> is just a bunch of bull, but I tell you that a lot of people believe
SH> it.
Why don't you google for it befor asking the list ????
SH> I won't believe it unless I can find an authoritative source on
SH> the subject which confirms the existence of the reported phenomena.
It's no phenomenon ...
it simply a class of compression algos.
SH> Sam Heywood
CU, Ricsi
--
|~)o _ _o Richard Menedetter <[EMAIL PROTECTED]> {ICQ: 7659421} (PGP)
|~\|(__\| -=> Merry Christmas and Happy New Year! <=-
