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RE: filmscanners: image samples of digital artifacts



Rafe wrote--

>>JPG doesn't produce "topo maps"

Ah, but it does! I'd refer you to the "Aniversary" picture on Larry Berman's 
Compression page. I found (as Larry did) that getting the original image 
below 120mb without posterizing was impossible. :-)

>Topo maps are a result of extreme posterization (loss of intermediate 
>tones.) Indexed color is, by definition, a severely posterized
>working space.

Using that "conventional wisdom," I was completely baffled when a picture I 
was working on in Photoshop suddenly posterized in a skin-tone area. I do 
not use a limited palette (except in Amiga graphics). The causes in that 
incident are still unknown--it was a program glitch of some sort that I 
corrected by using a different program to get the results I wanted. :-)

>[Indexed color is] *Entirely* unsuitable for any graphic arts work.

That's also a bit too "broad" to be true. Indexed color *does* have its uses 
in output applications. I'd refer you to the book "Real Life Photoshop." 
Limited color has limited applications, OTOH.

>The typical "signature" of JPG is little blocks (8x8
>pixels) that are clearly discernable in the image.

That's true enough. However, the little buggers are more recognizable by 
their "shimmerey" off-color than as patterns. The rule of thumb is to push 
the compression just that far, then back off a few clicks. You can only do 
this with a few programs, Picture Publisher 8 being one of them.

Best regards--LRA



>From: rafeb <rafeb@channel1.com>
>Reply-To: filmscanners@halftone.co.uk
>To: filmscanners@halftone.co.uk
>Subject: RE: filmscanners: image samples of digital artifacts
>Date: Thu, 19 Jul 2001 20:41:23 -0400
>
>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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