Your information is not true from what I can find about metamerism. For
more information about metamerism, see
http://www.colourware.co.uk/cpfaq/q5-2.htm
or
http://en.wikipedia.org/wiki/Metamerism_(color)
Metamerism is related to reflective, or illuminative isomorphisms and
not to spectral isomorphisms.
The continous spectrum is not what is the main issue, its what the
absorptive mapping of the film is to that spectrum. If you know that
mapping, then you should be able to apply that mapping to the RGB values
of the RAW sensor sites, at least some aproximation to it. The RAW
helps you here, because if you know the mapping in terms of RGB, then
you can do this before you do the Bayer interpolation, kind of like the
way you can do white balance with the RGB data.
rg
John Francis wrote:
That doesn't help. You're still reducing a continuous spectrum
to a single sample value. Whether that's still just at the
single sensor site in the RAW file, or has been interpolated
to a component in an RGB value, makes no difference.
Consider that 'yellow light or red/green light' case again.
There's nothing you can do with the sensor values to differentiate
between a monochromatic light source or a broad-spectrum source,
and yet without that information you can't predict how any real
film will respond.
Fortunately the sensitivities of the various layers in colour
films (and the different sensors in the digital sensor array)
are all fairly close to the spectral sensitivities of the human
eye (gee - I wonder why that should be the case? :-), so ignoring
this issue and just dealing with overall hue and saturation will
get you something fairly close.
You could even argue that not reproducing the way that certain
shades of orange tended to show up as purple on some films is
a plus - you're recreating the ideal version of the film as it
should have been, not how it actually behaved.
On Tue, Jan 24, 2006 at 02:10:34PM -0600, Gonz wrote:
It sounds like the ideal place to do some type of film characteristic
mapping is through the RAW sensor data, before it has been combined
through the Bayer interpolation. I.e. modify the Bayer mechanism to
mimic a film type.
rg
John Francis wrote:
But that's not really enough to reproduce a film characteristic.
As we all know, colour film (and the human eye, and digital
sensors, and digital colour spaces) are tri-stimulus systems;
any particular colour is reduced to three measured values.
The real world, though, is not so discrete - it's possible
for the same value triple to be produced by different inputs.
For a very simple example, consider a bright yellow light.
This may be a monochromatic light source, emitting light
at one very specific frequency, or it maybe a combination
of red and green light sources, or any number of different
options. But by the time it is reduced to a value triple,
there's no way to differentiate between any of the original
light spectra that map to the same triple of sample values.
(This is technically known as metamerism).
But (and it's a very important but) the mapping to triples
depends on the frequency response of the sensor - it's not
an absolute. To continue the analogy of a pure yellow light
versus a combination of red and green, two different colour
films may very well behave differently; the perceived colour
of the red/green combination may match the monochromatic
yellow light on one film, and yet appear to be a different
colour on the other film. Photographers have long known
this, and have chosen different films for different purposes.
Once two real-world lighting conditions have been mapped to
the same recorded tri-stimulus values, though, there's nothing
you can do split them apart again. Given an image recorded
on the first film, and showing that yellow light source,
there's just no way for you to map it to the right colour
to mimic the behaviour of the second type of film.
On Tue, Jan 24, 2006 at 06:52:20AM -0500, Paul Stenquist wrote:
With enough time and patience, you can copy any tint/saturation
level/contrast level through digital manipulation.
Paul
On Jan 24, 2006, at 2:00 AM, Scott Loveless wrote:
I just spent some time looking through some portraits taken with
Kodachrome 25 from the late 70s or early 80s. That's right,
portraits. The color is amazing. Anyone have a recommendation how I
might achieve this look today?
--
Scott Loveless
http://www.twosixteen.com
--
"You have to hold the button down" -Arnold Newman
--
Someone handed me a picture and said, "This is a picture of me when I
was younger." Every picture of you is when you were younger. "...Here's
a picture of me when I'm older." Where'd you get that camera man?
- Mitch Hedberg
