I think you should do a little more research. Metamerism is a general term to describe the way that different colours appear to a sensor (usually the eye) when viewed under different lighting conditions. As such it applies equally well to the spectral-to- single-sample contractive mapping (and isomorphisms under that mapping) as it does to anything else.
Anyway, just ignore the big words and consider the example I gave. If a colour patch illuminated by two different lights maps to the same tristimulus value for a given sensor (such as, say, the RAW readings) then there's nothing you can do from then on to find out whether the illuminant was a pure monochromatic source or a broad-spectrum light source, so you can't decide how a different sensor, with rather different sensitivities, would respond to that subject. On Tue, Jan 24, 2006 at 08:24:17PM -0600, Gonz wrote: > 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 > > > >
