Elle Stone wrote:
By "the white and black points will be determined and adjusted after color management" do you mean setting a manual white balance by eye-droppering off a neutral target such as a neutral object in the image and/or a separately exposed white balance card? Or do you mean using levels to set the brightest bright and darkest dark - changing L with no regard for a and b values? I'm guessing you mean both.
Any of the above (although it crosses my mind that such methods aren't the best you can do if they are computed in RGB or L*a*b* space rather than using something like a Bradford matrix - but that's an application issue).
For a digital camera, white balancing (color balancing) already corrects chromaticity (right word?) errors in the highlights and shadows, because the color of the light illuminating the scene (assuming only one light source and ignoring the fact that objects in shade are really in a different light source) doesn't vary from the brightest to the darkest part of the scene. Yes?
Well of course it can, if the darkest object has some reflectance and it is not neutral (ie. not a flat reflectance spectrum), but it comes down to how one defines and uses a black point.
But for a printer, for example, the situation is different. The chromaticity of the darkest dark can and often is different from the chromaticity of the white patch (ink vs paper). (And for a monitor?) So for a printer, "color balancing" so to speak has to be done with respect to both the brightest and darkest part of the image, along with setting the L values.
I don't see it as much different, but this is because Argyll doesn't use the chromaticity of the black point in its perceptual gamut mapping (other software may do this though). Instead it assumes that the observers sense of neutral corresponds to the chromaticity of the media white point, since it is assumed that the image dominates their view and hence their state of adaptation. So the neutral axis is assumed to be those colors with the same chromaticity as the white point. In practice it then "bends" the neutral axis close to the black point, so as to achieve a neutral looking result over the majority of the luminance range with the darkest possible black. [Note that some sources talk about the observer being adapted to the chromaticity of the illuminant, but I dispute this is most situations. You can't see an illuminant unless it is blinding you, so in practice you see it reflected or diffused through other objects, and if it has any influence over your state of adaptation independent of the image being viewed, then you are in a "mixed adaptation" situation, which is outside the scope of a simple ICC profile. In practice when looking at images it doesn't often matter anyway, since cognitive mechanisms complete the adaptation to the neutral of the image.]
One issue with NOT using "-u" is that even the it8 target shot itself has to be re-white-balanced if it is re-rendered from the raw file and has the non "-u" profile applied. Otherwise the gs00 patch is rendered as neutral (R=G=B) and the rest of the image has a yellow cast (because on my target gs00 actually has a slight blue cast as indicated by the Lab values in the reference file).
Hmm. It's an assumption in my part that the lightest test patch is neutral. It's pity the white patch is not neutral in respect to the other neutral patches for your test chart then. I guess the white point chromaticity could be computed in some other fashion that attempts to average the chromaticity of all the neutral seeming patches, but for typical paper media, the paper is assumed to be the reference white, and usually people want the paper background "removed" when they scan documents in relative colorimetric mode.
Using the "-u" switch in 111beta to make the profile, then applying to the target during raw rendering, gs00 comes out properly slightly blue and gs06 comes out neutral, which is what I would expect to happen. Does this mean I got lucky in my choice of time of day, color of sunlight when exposing the target? The "-u" profile might not produce an image already color-balanced to the gs06 patch if the target had been shot in deep shade? I can see some experimentation is in order.
I think that's the nature of it. The test chart's XYZ reference values are relative to the illuminant used to measure them, so naturally if the measurement illuminant is normalized to D50, and D50 is used as the white reference (which it is with ICC PCS), then a neutral (ie. spectrally flat) test patch value will have a chromaticity of D50. So when you profile with that test chart, a spectrally flat test patch will map the camera RGB to an XYZ with a chromaticity of D50, so it will look neutral. Graeme Gill.