When I have a profile applied, created based on the measured camera SSFs, and then adjust the white balance slider in RawTherapee using the pick function on a neutral gray target: Does the color temperature shown by the slider roughly matches the color temperature at the real scene?
Under the assumption of course of a similar characteristic light curve used for the profile as it is in the scene, i.e. a smooth continuous single peak curve without strange multiple peaks. But with the profile color temperature for example being stdA and the real scene more around sunlight.
Iām asking as I look at various test images and notice the exact same color temperature as result when picking the same spot independent from the profile generated for stdA, D50 or D65. The base of all those SSF-based profiles are the same SSFs as it is the same camera used.
Note, that the SSF-based profile is a single-illuminant DCP profile where you can define the illuminant freely during creation of the DCP file as the SSFs are independent from any illuminant.
The context of my question is a research project I do for identifying the sceneās color temperature which is not known, only the general characteristic is known and the cameraās SSFs are also known.
The results will be public, but I donāt like to talk much about the details here for now.
That would explain the same result as I compare within the same DNG image, especially as my DNG is converted from the pure camera data without any further information than the bare pixels and the conversion tool most likely adds some default attributes.
It can be tough sometimes teasing out the other processing to isolate the effect of a single variable. Even in my hack raw processor where I have explicit control over each operator and their order, Iāve got to pay attention to display rendition which has two functions, color and tone conversion. I recently exposed browser behavior where, if thereās an embedded profile in an image, Chrome appears to convert it to sRGB for display, even without color management set up in the computerā¦
Yes, through similar effects when using other image viewer Iāve been through already back then when I used ICC profiles instead of now DCP profiles. Thatās why I now use only DCP and RawTherapee to view my results and then create a plain standard sRGB JPG or PNG from them if needed.
Are you using a dual-illuminant DCP? Hardest part of making one these days is finding a tungsten bulb for the StdA target shotā¦ I havenāt tried one yet, but itās supposed to handle color better if youāre shooting at less-than daylight temps.
Product description says itās 2700K. The nearest standard illuminant is StdA, at 2856K, but in dcamprof you can specify the temperature directly, e.g.,
From the mainstream standpoint where we process all our pictures with a D65 or D50 set of primaries, any effort to interpolate between two primary sets that cover the illuminant range has to be better, no? Even if the numbers are bit offā¦ ??
I am not certain, but I always thought the idea is that you want to interpolate between two black body emitters with different temperatures. Mixing two different types of light sources seems like a bad idea in terms of calibration ( only change one variable at a time!).
Edit: @ggbutcher I think there is some spectral processing going on while making the DCP. Perhaps with the right flags, it could work to mix and match.
Wouldnāt daylight and a tungsten bulb be just that? If not, I messed up a profile I made for @hanatosā¦
With spectral data and dcamprof, you can make a dual illuminant profile by making two spectral profiles, one D65 the other whatever you choose for the low end, and then combine the two with dcamprof make-profile. No target shotsā¦
I was commenting on the idea of combining shots of a color chart under daylight and halogen, and combining those for a dual-illuminant DCP. Iām not sure thatās a great idea.
Whatās wrong with halogen? Itās a tungsten radiator surrounded with halogen gas. Does that affect the black-body reference? If so, all my spectral work is shotā¦ 'cept, I get great dE against the common training spectraā¦
Eh, my mind is not awake - itās 5 am and I couldnāt sleep anymore. Of course a halogen lamp has a tungsten filament and should give a similar spectrum as an incandescent bulb. What the influence of the halogens is, I am actually not sure of and a quick Google search didnāt provide me with any reliable comparison spectra between halogen and incandescent.
For my experiment I generate the profile mathematically from the measured spectral sensitivity function of the camera. Thatās the camera response function for each RGB channel separately measured every 10nm over the spectrum. No actual test target photo is taken for this process. For generating a profile from that, dcamprof only offers the single illuminant mode, not the dual. Or at least I couldnāt figure out yet how to create a dual profile in that mode.
About tungsten vs halogen-tungsten I found this: Fig.3 shows the differences in the spectrum in the visible light range. The shape of the complete curves appears to be the same as a blackbody, but the part used by the camera is a bit different in the green. And they mention how the first halogen bulbs used iodine which added a purple color, but todayās donāt have that anymore.
Nice to see it relatively confirmed!
Iāll do some further experiments under known light sources with removed meta data from the raw images to verify.
All that is of course under the assumption of this single peak smooth spectrum as otherwise you would need a multi channel spectrometer.
I havenāt tried one yet, but itās supposed to handle color better if youāre shooting at less-than daylight temps.
Halogen but 2850ā¦