3 Replies Latest reply on Nov 19, 2007 8:34 AM by (Jeff_Schewe)

    ACR and LAB Color

    Level 1
      Hi, all -

      Earlier this year, I was reading Dan Margulis' book on LAB color, and one day when I was making some adjustments in ACR, it dawned on me that the white balance slider is the same (blue => yellow) as the "b" channel in LAB and that the tint slider (green => magenta) is the same as the "a" channel. That got me thinking (too much time on my hands, I guess!)... Is there any correlation between how those sliders work and the LAB colorspace (in other words, are those adjustments in ACR being made using the LAB colorspace, even if the image is in RBG)? Then, I thought some more... and realized that the exposure, recovery, blacks, fill light, brightness, and clarity sliders all seem as though they'd affect only the "L" channel in LAB, which made me wonder even more.

      There was no mention of ACR in the book I was reading, and I have not read any of the Real World Camera Raw books (waiting for the latest to come out so I can ask Santa to bring it). Is this just a wild coincidence, or are the ACR adjustments based on the LAB colorspace or color theory?

      Thanks!
        • 1. Re: ACR and LAB Color
          Level 1
          >the white balance slider is the same (blue => yellow) as the "b" channel in LAB and that the tint slider (green => magenta) is the same as the "a" channel.

          Actually no...it's NOT the same. Lab does not use the Kelvin scale to describe the ab channels. Kelvin is based upon a blackbody radiation and is actually blue/amber and the magenta/green is perpendicular to that axis, which is NOT the same as the ab channels in Lab.

          Camera Raw uses the Pro Photo RGB chromaticities in linear gamma for its internal color space and while a large set of functions are luminance only, it does that and the HSL corrections based on HSL which also is not the same as Lab.
          • 2. Re: ACR and LAB Color
            Level 1
            Jeff, thank you for your reply. So, the bottom line is that the apparent correlation is coincidence only, right?
            • 3. Re: ACR and LAB Color
              Level 1
              >the apparent correlation is coincidence only

              Well, there is no direct correlation between Lab and Kelvin scale and Camera Raw uses HSL type controls, not Lab. There's a BIG difference between Hue, Saturation and Luminance and Lab. So, Camera Raw has NOTHING to do with Lab, correct.