Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Peter 
Nelson" <peter@s...> wrote:
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, 
"Austin 
> Franklin" <darkroom@i...> wrote:
> > Hi Anthony,
> > 
> > > > I do understand EXACTLY what you are saying, AND
> > > > I simply disagree with it.
> > >
> > > So how would you reconstruct a continuous, irregular curve 
of 
> spectral
> > > energy distribution from just three data points?
> > >
> > > It's possible if the curve is a portion of, say, an ellipse.  But 
> the real
> > > world curves aren't, so you cannot reconstruct them from 
three
> > > data points.
> > > And because of this, you cannot use RGB values to 
reconstruct the
> > > information you would need to accurate simulate the 
spectral
> > > response of any
> > > arbitrary monochrome or RGB sensor or film.  The required 
> information just
> > > isn't there, period.
> > 
> > I understand that is what you are saying, and again, I simply 
> disagree.  I
> > don't know what it is that is missing in yours and or my 
> understanding, but
> > I simply don't see that it can't be done.  I have all the 
> information I
> > need, the frequency and the intensity.  Both the color and 
B&W films
> > response is deterministic to the frequency and intensity...so I 
> believe I
> > have the information necessary to map one to the other.  The 
> converse is, of
> > course, not true, you can't go from B&W to color.
> 
> I agree with Austin.   Tell us precisely what information is 
> missing.  Don't speak vaguely about the "curves" because we 
know what 
> the response curve is of the color dyes used in the film.   So we 
> know exactly how much to compensate the density by for any 
color.   
> Or put another way:  because we know the shape of the film's 
response 
> curves there is only one unique point on the spectrum that will 
> produce a given density in all three dyes.  So what's missing?

I suppose this is totally beaten to death, but...

The part I think you are missing is that given a particular
triplet of RGB densities you are NOT trying to go back to a
"unique point on the spectrum".   The light in a real scene that
exposed that point in the film is NOT one unique frequency.
Every single point is composed of the entire  visible spectrum.
You'll have an amplitude (probably different) for every single
frequency.   The three density numbers can't possibly tell
what all the amplitudes of all the frequencies are.  Sure it
tells you a fair amount about the general shape, and it's close 
enough for the human color perception.  But B&W film with a 
colored filter can be much more selective about what 
frequencies to be sensitve to and what ones not to be.  How
dark each piece of file gets is based on integrating over the
entire visible spectrum of the energy * filter * film sensitivity.
One RGB triplett just doesn't have enough information to
do that integration.

For the same RGB values you can and mostly will have 
different overall spectra and if you had taken the image 
with B&W and filter you could very likely have differentiation
in B&W that was lost if you went via RGB.

Roy

> 
> You say that other people here understand you, but I don't see 
them 
> jumping in to clarify what you're saying.