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RE: filmscanners: image samples of digital artifacts



At 10:08 PM 7/19/01 +0000, Lynn Allen wrote:
>Hi, Dan--
>
>That looks like "Posterization" to me (at least, tha's whut ah calls it! :-) 
>--cf definitions (-:|:-) ). I'd say it's probably a result (in this case, 
>anyway) of pushing the sizing and JPEG compression too far. A good reference 
>is Larry Berman's Compression Comparisons (BermanGraphics--You can look it 
>up--I can't access the URL without losing my link on this service).


I'm willing to bet that Dan Honemann has his video
set to 256 colors ("indexed" color.)

JPG doesn't produce "topo maps"  Topo maps are a result 
of extreme posterization (loss of intermediate tones.)
Indexed color is, by definition, a severely posterized 
working space.  *Entirely* unsuitable for any graphic 
arts work.

To see posterization in Photoshop, go to 
Image->Adjust->Posterize, and select a small integer,
say 10 or so.  Some of the effects are quite nice, 
in fact, but hardly "photographic."

Amazingly, if the integer is over 50-100 on a well-
adjusted image, you won't see the posterization at 
all.  Which is one reason that I think all this 
talk about needing 48-bit color is... well, missing 
the point somehow.  16 million colors seems to do 
the trick for me.

256-color (indexed color) associates 256 triplets 
of RGB values, with the integers 0..255.  Those 
256 triplets are called a "pallette."  The video 
card can switch between pallettes quickly, and may 
be able to store several pallettes in its memory.
But it can only *use* one pallette at a time.

This is how color video was done, typically, about 
10 years ago, before "True Color" became the norm.

JPG doesn't cause "topo map" or posterization effects.
The typical "signature" of JPG is little blocks (8x8 
pixels) that are clearly discernable in the image.


rafe b.





 




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