In article <cja### [at] netplexaussieorg> , 
Christopher James Huff <cja### [at] earthlinknet>  wrote:

>> I thought this would be a good idea for a couple of reasons. The first is
>> that I'm under the impression a jpg can be compressed yet still maintain
>> the same quality as bmp type file.  This is because I'm assuming;
>> a) Default quality of a jpg is 80% (100% would be our bmp formated file)
>
> Even a 100% JPEG loses information. JPEG2000 does allow lossless
> compression, however.

Actually, the JPEG specification also allows a lossless compression, but
hardly anybody supports it.  Of course, there would be no gain in compressed
result size over other formats.  More to the contrary :-(

    Thorsten

____________________________________________________
Thorsten Froehlich, Duisburg, Germany
e-mail: tho### [at] trfde

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In article <web.3faef3a6513875ff5cd9832d0@news.povray.org> , "Tony LaVigne"
<ton### [at] xenomechanicscom> wrote:

> a) Default quality of a jpg is 80% (100% would be our bmp formated file)

The "percentage" most programs allows you to set says nothing.  It is just
there for users.  In any case, anything based on the Independent JPEP
Group's library (nearly all non-commercial programs, and many commercial
ones), the actually "default" giving you the quality JPEG was designed for
will be 75% iirc (it is documented in the source code docs of the library).

> b) each pixel of a jpg indexes a  rgb color map (assuming multi toned)

By default JPEGs are not stored in RGB format but in YUV 4:2:0 format.  I am
not aware of any program that allows you to write RGB, and there aren't many
which allow you to switch to 4:1:1, 4:2:2 or 4:4:4 format.  Practically JPEG
supports a lot of variations in this regard.  At least most programs can
actually read many of these variations.

> c) some pixels will have the same rgb value, so they use the same index to

The JPEG format is not indexed.  It uses a lossy compression, which has
nothing to do with indexing.

    Thorsten

____________________________________________________
Thorsten Froehlich, Duisburg, Germany
e-mail: tho### [at] trfde

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In article <3fb016bc$1@news.povray.org>,
 "Thorsten Froehlich" <tho### [at] trfde> wrote:

> Actually, the JPEG specification also allows a lossless compression, but
> hardly anybody supports it.  Of course, there would be no gain in compressed
> result size over other formats.  More to the contrary :-(

That's interesting...the pages I read about JPEG2000 implied that it was 
a completely new addition. Looks like it about halves file sizes...PNG 
is probably far better.
This page appears to have some good information:
http://www.faqs.org/faqs/jpeg-faq/part1/

Specifically, #13: "Isn't there a lossless JPEG?"

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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In article <cja### [at] netplexaussieorg> , 
Christopher James Huff <cja### [at] earthlinknet>  wrote:

> That's interesting...the pages I read about JPEG2000 implied that it was
> a completely new addition. Looks like it about halves file sizes...PNG
> is probably far better.
> This page appears to have some good information:
> http://www.faqs.org/faqs/jpeg-faq/part1/
>
> Specifically, #13: "Isn't there a lossless JPEG?"

Indeed, this summarises it very well.

And actually, what it doesn't say is that you can actually create a
"lossless"  baseline compliant JPEG.  You "just" have to use adequate
quantisation tables (ones that don't drop any data).  However, the catch is
that this would provide nothing but a very simple Huffman code based
compression...

    Thorsten

____________________________________________________
Thorsten Froehlich, Duisburg, Germany
e-mail: tho### [at] trfde

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