DNG specs and dual illuminant

Question

I am currently testing the DNG profile editor and the camera specific profiles and I really like the results so far. At first impression they seem to significantly improve controllability of images taken at sunset for example. (Specifically blue saturation of the eastern sky, Canon 40D).
After briefly scanning over the DNG specs i understand that the specs allow for a dual color matrix which accommodates the ACR/LR workflow.
However, isn't this somewhat restrictive?
The whole point of the DNG profiler is that it enables one to optimize the converter for the colorresponse of the camera under certain lighting conditions. A single colortemperature may be the result of wildly different lighting conditions which result in a significantly different colorresponse in the camera.
So, wouldn't it be more appropriate to be able to add an entire list of "named colormatrices" which can correspond to given lighting conditions? Like the usual sunny, shady, daylight, incandescent, fluorescent, etc...
ACR/LR can simply pick the 2 most appropriate matrices to do its interpolation thingy, but it would be really useful if in addition one could add "named sets" of matrices which allows ACR/LR to do their interpolation thingy with a specific set (possibly over more than 2 profiles).
That way one can create a specific "daylight response" set which combines a 2800K sunset profile with a 5500K daylight response. (and then possibly an additional 10K n.sky response or something). Or an "artificial lighting" set of say a 2800K bulb light response combined with a 5000K tube light response.
In ACR/LR the WB popup can be used to select one of these sets...
Anyways: my point really is the DNG specs themselves. If it is eventually to become an industry wide open standard, the colormatrix implementation should probably not be coupled so intimately with the whitebalancing paradigm of ACR/LR.
(if this has been previously discussed, my apologies, I haven't scanned the entire list of discussion threads).

Answer 1

> This bests corresponds with conventional photography
> wisdom of selecting a WB according to original shooting
> conditions as well as conventional colormanagement wisdom
> of a single colorresponse/illuminant for a single device.
> Adjustment of WB T can still be allowed within reason.
> To allow for a wider range of WB T adjustments WITHOUT
> having to select different colorresponses may be
> implemented by interpolation between colorresponses.
That's exactly what a dual-illuminant profile accomplishes.
Note that the DNG spec does not require that profiles be dual-illuminant. A profile may in fact be single-illuminant, and some cameras which support in-camera DNG create such profiles.
> Interpolating between 2 colorresponses should then consist
> of selecting 2 or more profiles based on the properties
> desired.
> One of those properties might be "convenience", i.e.
> interpolating between an artificial 2850K colorresponse
> and a D65 daylight response to cover most processing
> demands.
> Another property might be "accuracy". This could consist
> of interpolating between an entire set of daylight
> response profiles so that sunset images aren't processed
> with an artificial light profile.
Again, this can already be done with the current DNG 1.2 spec. You can define one profile to be used with natural daylight images and another profile to be used with artificial light images. These profiles can be either single-illuminant or dual-illuminant; it is the choice of the profile author (or profile-building software).
The choice of which profile to use for a given image falls outside the scope of the DNG spec.
Consider the following example. Suppose a camera shoots DNG raw files. If the camera knew that a given image was shot under natural light (e.g., it has a built-in spectroradiometer that measures the spectrum of the incoming light and determines that it's a flavor of daylight) then it could then generate a "daylight" DNG profile that is then embedded in the DNG, to be used for that specific image.
This is all possible with the DNG 1.2 spec, as it's currently written. However, the generation of the an appropriate profile is a higher-level functionality that falls outside the scope of the spec itself. In other words, the spec dictates the format of a profile and its behavior, but it does not dictate how that profile gets generated in the first place (i.e., based on which measurements, etc.).
> However, defining a profile as describing 2 colorresponses
> for 2 illuminants and "claiming" that interpolation
> between those responses is in any way relevant to scene
> referred colormatching for a single device or DNG file is,
> with all due respect, simply ridiculous.
Why?
> For "correct" rawdata interpretation i'm pretty sure i
> only need a single colorresponse and illuminant, namely
> the colorresponse and illuminant matching the exact
> conditions at the time of capture.
For a single image, yes.
Questions: (1) Do you actually have the color response information for your camera's sensor, and if so, where did you obtain it from? (2) Do you actually have the spectrum of the illuminant at the time of capture, as well as the reflectance spectrum of the materials being photographed?
If you have both, then you can generate an optimal profile for that capture. That was the point I made above: the on-the-fly profile generation is something that can be done if you have this info, and it's completely compatible with the existing DNG 1.2 spec. There is nothing in CR, LR, or the DNG spec that prevents this.
The real issue is that in most cases, the practical answer to both of my above questions is no. (Even if you have #1, #2 is almost certainly no.)
> dual matrix tags are superfluous because 2 profiles
> containing a single matrix carry the same amount of
> information
Again, as noted above, the DNG spec allows for single-illuminant profiles, so if you wish to go that route, you are free to do so.
> dual matrix tags are an incorrect representation of the
> colorresponse paradigm because there is no singular
> relation FROM colortemperature TO colorresponse.
The calibration illuminant tag indicates the actual light source, not just the color temperature. For a given illuminant there is actually a single mapping between illuminant and color response.
> Note also that if you really aim for consistency of tone
> and color results across implementations then Adobe should
> also disclose their debayer colormixing code, it is not
> solely dependent on interpolation of colormatrixes...
No, the two issues are completely independent except for pixel-level effects which are largely irrelevant to the overall color and tone appearance.
Eric