Sense & Simplicity of ACR Calibration
"Could it be that the hue-accuracy of this memory color is sacrificed to get dead-on Red..."
Peter might be on to something... according to this: http://photoshopnews.com/2005/05/01/photos...e-real-history/
, we can maybe say the opposite, that the pure reds have maybe been sacrified for the (ears) skintones in ACR?
… was the title of a post which I placed some long time ago in the Adobe forums. Purpose was to share a quite simple, manually operated procedure for ACR Calibration. Some points may still be of interest:Guiding principles:1.) In the course of ACR Calibration, it is hardly possible to compute a best-fit matrix space by just nailing the three primary patches i.e. red, green and blue.
Background is as follows: When the spectral response of a real-world sensor deviates from an ideal linear-combination of CIE XYZ weighting function, colors and RGB data released from Bayer interpolation deviate from their ideal position (within a matrix space as a subset of CIE XYZ) in a quite idiosyncratic way. There are no obvious systematic shifts in terms of hue and saturation. For example, get the red patch right, and the hue of skin tones becomes worse… (see above).
The latter is one problem with the Fors Script which just analyzes the three primary patches. This may make sense if your camera is considerably different from the one profiled by Thomas Knoll (device-to-device deviations). However, if your camera is somewhat close to the camera which was used by Adobe, the Fors Script will just violate Mr. Knoll’s profiling efforts. At least that’s obvious for me from quite a bit numerical analysis.
Anyway, it’s of course possible to compute a best-fit matrix by minimizing the color error for many colors. But then, we are having memory colors which are more important for us than others - and that’s an individual thing. So any procedure for ACR Calibration which doesn’t call for an input by the user about preferred memory colors is prone to fail in practice.2.) There is no need to linearize Camera Raw i.e. to match the grayscale first, for the purpose of subsequent ACR Calibration.
This is not only unnecessary, at the end it leads to a high-sat look due to the RGB tone curve (and it’s side effect on color saturation) which in practice is applied on the top – mainly via Brightness and Contrast settings.
That’s the second problem with the Fors script and again there is evidence that the in-built Thomas profiles already absorb this more or less unavoidable boost of saturation from such sigmoidal RGB curve which is needed for a pleasing tonality.
Anyway, the Calibrate Tab sliders actually do what the labels suggest. They adjust the red, green and blue Hue & Saturation for these primary colors with decreasing influence on secondary colors, respectively. While this is done behind the scenes on a CIE xy basis, there are enough similarities with the HSB color model to trace respective changes via HSB readings.
In other words, given that ACR calibration is 2D per primary color, why bother with a 3D fit.
---- Proposed procedure:
/> Reset the Main Adjust Tab and in particular all tonal controls to ACR 2.x defaults i.e. Shadows (Blacks) 5, Brightness 50 and Contrast 25. Everything else zero and Curve Tab linear.
/> Nail the second gray i.e. patch #20 of Gretag’s ColorChecker to RGB= 190 by means of click-whitebalance and the Exposure slider (All numbers referring to ProPhoto RGB).
/> Address the color patches in pairs of one primary color and one corresponding memory color. My choice is: Red #15 + Light skin #2, Green #14 + Yellow-green #11 and Blue #13 + Blue sky #3.
Adjust the Red Hue slider of the Calibrate Tab to get the HSB-hue somewhat right for both, the Red patch #15 and Light skin patch #2. In all probability this will require to make a compromise. It’s a balance. Continue with the Green Hue slider of the Calibrate Tab to get the HSB-hue somewhat right for both, the Green patch #14 and the Yellow-green patch #11. Same with the Blue Hue slider and the respective patches Blue #13 and Blue sky #3.
Same principle applies to the adjustment of saturation, at least as far as the Red Saturation is concerned. With the Green Saturation my preference is to stay a bit under-saturated in order to prevent fresh green grass from getting too vivid. With the Blue Saturation my preference is to have it a bit over-saturated in order to get a deep blue sky. However, that’s not really essential. You can also strive for a somewhat accurate calibration of saturation while making such tweaks later on via the HSL Tab.
Finally, re-exercise through all sliders in order to eliminate interactions. My experience is that finally only more or less small adjustments are needed. A kind of minimum invasion which tells me that this approach is quite close to the logic of Thomas Knoll’s profiling software.
As for the target values for Gretag’s ColorChecker, the following numbers given in terms of HSB hue & saturation were derived by averaging the simulated charts as offered by ColorRemedies as well by BabelColor. Perhaps you have better ones, if not, the following ones should not be so bad:
red patch # 15: H= 10°, S= 62%
# 2 light skin: H= 30°, S= 28%
green patch #14: H= 101°, S= 45%
# 11 yellow green: H= 75°, S= 56%
blue patch # 13: H= 248°, S= 60%
# 3 blue sky: H= 230°, S= 29%
Again, the numbers are referring to HSB readings in ProPhoto RGB. In the past it was necessary to convert from ACR to Photoshop in order to access HSB readings. So in the course of above described procedure it was necessary to do repeated RAW conversions while proceeding step by step. However, while I have not yet arrived with latest version of Camera Raw I can not say if HSB readings are now accessible in ACR. Would be nice.
That’s it - really not as complicate as it may sound. It’s however nice to see from above quoted article that Mr Knoll also had to go through a learning curve.