Digital BW, The Print

Peter writes:

> Let's not get all mystical about it.

It is not mysticism, it is hard science, as I have explained ... several
times.

> A response curve is a vector
> of scalars.

Well, that vector has only three scalars in it after you take the picture:
red, green, and blue.  Try to reconstruct an arbitrary curve from that.

> It should be simple linear algebra to take two
> response curves and come up with a third curve
> that represents the conversion of one to the other.

Sure.  The problem is that you have nothing to which you can apply that
third curve.  You have only three values: red, blue, and green.  You need a
whole curve to which you can apply your conversion--but it's gone.

> The only case that would fail is if the
> target response curve had a wider latitude than
> the source one.

_Most_ cases will fail.  Unless the entire curve is a function of the three
RGB values, it will fail.