Filmscanners mailing list archive (firstname.lastname@example.org)
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: filmscanners: image samples of digital artifacts
At 10:08 PM 7/19/01 +0000, Lynn Allen wrote:
>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
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.