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Re: Color in "digital cinema" projection



> As you point out, wider gamut (film) primaries must be used ...

There is no such a thing as a set of film primaries.

Grassmann's Third Law: 

   Sources of the same color produce identical effects in a mixture 
   regardless of their spectral composition.

This law applies to additive mixture. 

Take two SPDs, add them, and compute the tristimulus values [X, Y, Z] of the
result. (If you like,
compute the [x, y, Y] of that. Or, use [R, G, B] tristimulus.) Set that triple
(color) aside for a
moment.  Now, compute the [X, Y, Z] of each SPD individually. Add the
tristimulus values, pairwise, to
get [X, Y, Z] of the mixture. Grassmann says that in additive mixture, these
two calculations produce
the same result. You can predict the color of any mixture from the colors of
the primaries. You do not
need to do the calculation spectrally - three components suffice.

This does NOT apply to subtractive mixture (as in film). The reason is that in
so-called subtractive
mixture, the SPDs mix directly (through wavelength-by-wavelength
multiplication). The tristimulus
values do not simply add up. You cannot predict the color of a mixture by the
colors of the pure dyes.
You cannot (linearly) compute the color of the mixture using just three
components.

For example, as the concentration of the magenta dye changes, some unwanted
absorption affects the
transmission in the shortwave and longwave bands. If you were thinking of the
modulation of the
magenta dye as having the effect of a "green" primary, then you will realize
that as "minus green"
causes the shortwave and longwave to change, your effective green primary
drifts around the
chromaticity chart. Some film experts refer to this as film having "unstable
primaries."

Please stop talking about film primaries.  There's no such thing.

[Soapbox mode off]

---
thanks to Gary Adams for support in 1999
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