Re: TECH-3D digest 178

[T3D Eric Goldstein <egoldste@xxxxxx>]

John O sez:

> You have to compare density (dynamic range).

Eric sez:

If you are an engineer conducting an experiment, yes.

If instead you are shooting a picture to be viewed with a pair of
eyeballs in a particular context, no.

> For example, say we shoot a typical film which can record 12 fstops with
> a density range of 3.0.  Now we substitute a CCD back with 12 bits per
> channel.  It too will record 12 fstops with a dynamic range of log12 =
> 4.8 density, ie linear.  

(Note: CCDs have non-linearities too, btw.)

> Obviously, all you need is a density of 3.0 for
> the film to display what it recorded, ie 10 bits per channel, but
> because the film is nonlinear you may miss subtle detail in certain
> level ranges.  And we just keep going in circles.

Not from a creative users POV, we don't.

There are non-linearities both in how we perceive the world and 
in each and every media we use to record it. The density range of
photographic print paper does not match that of negative stock, and an
f/stop on the negative is not the same as an f/stop on the paper. You
can squeeze about 10 f/stops of negative contrast onto a paper print,
and there will roughly be the *appearance* of ten stops of contrast in
the print (if it is well done), but the actual measured density range
will be a fraction of that of the negative, and will not necessarily be
linear in the very light or dark regions of the print. If you then make
a printed reproduction of this paper positive, the half tone may
*appear* to have the full range of the print (again, if skillfully
done), but you will actually loose a couple
more stops in measured density. Photographers (and recording engineers,
incidently) have been dealing with these issues since man began to
record sights and sounds.

Similarly, our eyes do not have a flat response to all frequencies of
light. Neither do photographic lenses and coatings. Neither do CCDs or
d/a converters.

Nothing translates perfectly, but there is typically a useful expression
of
contrast (in f/stops) between two visual media which represents a
working equivalence. The human perceptual apparatus does not mirror the
measurements of a densitometer or db meter, and perceived contrast or
dymanic range is very situationally specific and "non-linear."

> You have to decide whether the film is to be presented in its current
> artistic transfer function, or in a linear transfer function.  Only then
> can you determine the dynamic range (density) appropriate for the
> output.

These decisions are made every instant of every day in the creative
world. Because nature and our ability to perceive it is notably
non-linear and has a vastly greater dymanic range than our recording
media, we begin making those decisions with our first recording of the
sight or sound, and continue making them for each subsequent media
transfer. Computer imaging is in no way unusual in this regard.

> Ideally, scan with a gamma correction that maximizes the linear dynamic
> range of the data recorded, which is much less than that suggested by
> the density.  Clip and baseline clip to throwout the data which lies
> outside the linear dynamic range.  Using brightness and contrast
> adjustments supported by histogram analysis to restore the data to a 0
> to max range of representation (ie. 0 to 255 for Photoshop etc).
> Process as desired.  In an ideal world, you have the option of wysiwyg.

But the gamma of the display device is different from that of the input
device and the computer software manipulating it (which is probably the
only linear part of the equation). That's how this whole thing started;
out of the recognition that the monitor does *not* present a wysiwyg
situation.

Anyway, I'll try to find out what the equivelent f/stop range of
computer imaging systems generally are and pass it along to the list if
anyone out there cares... BobH, don't you get your stuff transfered to
CD-ROM?

Eric G.


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