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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