Digital BW, The Print

Ah!

> 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.

Ah!  (again).  I think I see the confusion.  You can construct an ellipse
from three points (actually an arc), if the three points are X,Y
coordinates.  These three data values are not two dimensional.

To do the conversion from RGB to grayscale, you are not reconstructing a
curve with the three data points, you are reconstructing only a SINGLE point
in the XY space from the three data points.  Frequency is one vector and
intensity is the other.  That's it.  No curve, no ellipse etc.  Just one XY
point from three RGB values.  You do this for EVERY different RGB value that
is valid for the film type you are "simulating", and you have reconstructed
the grayscale response curve for that film.

Austin