re:Digital vs Analog (might be backwards!)

[P3D Gregory J. Wageman <gjw@xxxxxxxxxx>]

Mike K. responds:

>Why?  Does an acorn have to understand cell division to grow a leaf?  If
>it acts like it's halftoning, then it is, whether it knows it or not.
>The CCD cell in a digital camera is an "analog" device, so why isn't
>that digital camera called "analog"? 

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.

The grain in film has no such mechanism.  Each silver salt crystal acts
on its own without regard for what its neighbor is doing.  The film
CANNOT be binary as you suggest because there is no mechanism instrinsic
in the film to distribute the virtual pixel information around to the
physical grain structure.

While it's true that a CCD imager is an analog device, the circuitry in
a digital camera divides the continuous voltage changes from each pixel
into a distinct series of steps (256 for an 8-bit pixel).  Any and all
gradations between these steps that the analog CCD may be able to
distinguish are lost, and the intensity vs. output graph becomes a
stair-step, instead of a smooth curve.  So digital cameras are properly
called digital, despite using an analog imaging element, because their
output behaves in a digital way.

>With digital, each pixel can be completely stark-black or completely
>100% white at full resolution .... or some exact value inbetween.  I
>don't believe that is true for each line in your resolution test as
>it is usually measured (correct me if I'm wrong on that).

Not true.  Digital pixels do not change in intensity in a linear fashion,
that's an analog function.  Digital pixels change in a step-wise fashion,
with the amount of change being determined by the number of bits available
to encode the range.  If your digital circuit encodes white as 1.0 volts
and black as 0 volts, with 8 bits you have 254 intermediate values between
these extremes.  If you want a grey that's somewhere between two of these
values, you simply cannot represent it, without increasing the number of
bits of resolution.  When you have "enough" bits of resolution, a digital
system can equal or exceed the performance of an analog system.  "Enough"
resolution is defined such that a one-bit change in value produces an
output change that is just slightly smaller than a human can perceive.
In practice, however, cost compromises are usually made before this
point is reached.

The line pairs per millimeter measure of resolution specification
requires that you be able to *resolve* the lines as lines, which means
that there is some minimum separation of intensity between them.  It
certainly won't be 100%, as you say.  In reality, a digital system
won't produce 100% separation either, since a monitor isn't usually
capable of going from black to white within one dot time without some
smear or ringing or other artifact.  Inks spread.  Etc.


>Consider the situation where a picture is taken and the background is
>*COMPLETELY* white.   If *one* pixel-width circle of the image is only 
>*slightly* darker... say 1% darker.  Will that film faithfully resolve
>that one pixel with the correct 99%-brighness value?  The digital camera
>could.  And even more-bits-per-pixel are available now in scanners so
>the one-pixel dot could be even less different.

Well, all I can say is consider this:  a hologram, which is recorded on
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photographic emulsion, is the representation of the interference pattern
produced by two coherent wavefronts.  In order for a hologram to work,
the emulsion must be able to record with a resolution slightly greater
than the wavelength of the light used.  That puts it down into the
hundreds of nanometers.  There ain't no digital imagers with pixels
that small yet.  (This is black-and-white film, and extremely slow film
at that.  I don't believe there is a color film with anything near
this resolution.  I doubt it would be possible to make one.)

>I can't see that the comparison of pixels is one-for-one.  I'd guess that
>each digital bit  is "worth" perhaps eight film-bits, so using your numbers, 
>35mm film is  equivalent to about 4 million 24-bit pixels.  That's
>just a SWAG, but I think it's less than the full 30 million.

It's certainly a bit more complicated for color films, because with color,
the silver crystals aren't directly producing the image.  There are
multiple layers in the emulsion, each sensitive to a different wavelength
of light (sort of; in reality, there is cross-talk).  Dyes are attached
to the exposed crystals using "dye couplers" which are chemically specific
to the particular emulsion layer and the dye.  Unexposed crystals do not
get coupled and are washed away during developing.  Since the layers
physically overlap, and the placement of the grain is random, the size
of a color film "pixel" is generally much larger than for B&W.  Color
film is such an exercise in chemistry and compromise that I am amazed
it works at all!

        -Greg W.



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