Digital BW, The Print

Austin writes:

> I've not got time to waste with this.  I KNOW that
> was I said is correct and based on fact.

In that case, you should be able to easily answer my question.  How would
you reconstruct the light and dark rendering of the yellow-sensitive film
from an RGB image?  Remember, RGB cannot distinguish between yellow light
and a blend of red and green light, so the light from the original scene
will seem equally bright everywhere.  In the yellow-sensitive film, however,
only the truly yellow-lit areas will seem bright.  There is _no way_ to
transform the RGB in a way that will produce the results obtained with the
yellow-sensitive film.  It simply cannot be done.

Feel free to prove me wrong, if you can.

> You really need to go look at the spectral
> response curves for color and B&W films, and
> you will see what I said is true.

I have.  None of them are regular enough to be reproduced from three data
points.  If all of them were points on regular ellipses, it could be done,
but they aren't, so it's not possible.

> I said that the spectral response of Portra is
> sufficient to encompass Tri-X ...

If you mean that it will respond to the same frequencies, that may be true
(I'd have to look).  But since it does not have the same response curves, an
RGB image from Portra cannot be converted into a B&W image from Tri-X.