re:Digital vs Analog (and where to get a cheap scanner)

[P3D Peter Davis <pfd@xxxxxxxxxxxxxxxx>]

At 07:22 PM 12/12/96 -0600, you wrote:
>> 
>> I don't know how to explain it any clearer.  Halftoning works because
>> at some level, someone (e.g. a human applying a screen) or something
>> (an algorithm) knows that you can approximate a grey level using only
>> black/white "pixels" by trading off against resolution (you spread out
>> your virtual, grey, pixel among some number of physical black/white only
>> pixels, at the cost of a much larger "virtual" pixel).  In this
>> arrangement, the real pixels that comprise a virtual pixel are related;
>> a function related to the desired grey level that says which ones will
>> be on and which will be off.
>
>I agree with the purpose you had in saying what you did, however I'll
>hold to the notion that dithering/halftoning "actions" are still what
>they are even if produced by random-chance and that "knowledge" as such
>can be used, but isn't required.

I've been trying to stay out of this, but I had to point out that the
practice of halftoning was originally done by projecting a photograph
through a piece of glass with a finely etched grid of lines.  The
transmission properties of that grid resulted in the halftoning effect.  The
piece of glass had no intelligence or algorithm to accomplish it.

In signal processing terms, halftoning is really a form of "error
dispersion."  At any given point, the image we're trying to reproduce may be
some shade of gray, but we have only black and white with which to represent
it at that point. So we make that point black or white, resulting in, say, a
50% error, which we distribute among surrounding points, so that their
blackness or whiteness is partly a result of the image at that point, and
partly a result of the accumulated error.  The point is that there are some
phenomena in which this behavior occurs naturally (e.g., the grid-etched
glass) and some in which it is a human contrivance.

So, quantization alone does not make something analog or digital.
Unfortunately, my image processing books are buried in the basement, so I
don't have a good definition of "digital signal" handy.  It should be easy
to locate one, though.

-pd
--------
                                Peter Davis
                   URL:  http://www.ziplink.net/~pfd/

                 "Nondescript -- the one word oxymoron."


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