4 Replies Latest reply: May 1, 2012 10:27 AM by photodrawken RSS

    Colour curves

    photodrawken Community Member
    photodrawken Apr 28, 2012 10:14 PM

      I've just started experimenting with colour curves (the SmartCurves plug-in) and want to know if I'm understanding their concept correctly:  It seems that what colour curves allows one to do is adjust the R, G, and B values for specific image luminances.  In other words, more or less of a specific colour in the shadows and highlights areas.

       

      Here's an example:

       

      This is the before and after view of my adjustment of Jen's skin tones:

      Jennifer Aniston 08.png

       

      I got rid of most of the magenta colour cast.  The curves used and the resulting histogram is shown here:

      Jennifer Aniston 08 curves.png

       

      If I'm reading the curves diagram correctly, it means that I've reduced the red in the darker areas and boosted the red in the lighter areas.  It also shows that I've reduced the blue across all luminances, with more reduction in the middle of the scale.

       

      The histogram seems to show that the B & G values are equally strong at their peaks, both at the dark end and at the lighter end.  It also shows that the R values are consistently lower at their peaks both at the lower and higher luminance values.

       

      Am I getting this right?

       

      Ken

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        • 1. Re: Colour curves
          ronzie99 Community Member
          ronzie99 Apr 30, 2012 11:06 PM (in response to photodrawken)

          Your interpretation is correct as far as I can see.

           

          .

          • 2. Re: Colour curves
            photodrawken Community Member
            photodrawken Apr 30, 2012 11:43 PM (in response to ronzie99)

            Thanks for the feedback.

             

            Now that I have a handle on the what of Curves, the important question becomes why use Curves. 

             

            From what I've read, it all seems to come down to contrast and luminance -- Curves allows modifying the contrast in an image according to the luminance values.  IOW, one can apply more or less contrast to the shadows, midtones or highlights of an image.  That contrast can either be the overall B&W type of contrast (if working on the combined RGB channel) or contrast for each of the colour channels.  It's kind of a complex effect because the contrast is obtained by changing the output values (more or less Red, e.g.).

             

            So, after some more experimenting, the before and after of Jen looks like this:

            Jen before and after.png

             

            I tried to limit most of the adjustment of the Curves for the Red and the Blue to the brighter areas, since her face was more washed out by the flash than the rest of her; then I used a Green Curve to warm up the skin tones overall.  I also used an RGB Curve to give some more contrast, again mostly in the lighter tones (in this case the Curves adjustment acted like an enhanced Levels adjustment).

             

            It's a very interesting tool to use, for sure.  I think my workflow will be to first use the gross tools (like Adjust Color for Skin Tones, Remove Color Cast, Levels, etc.) and then switch to the Curves to make any subtle overall corrections or corrections that need to be precisely targetted.

             

            Ken

            • 3. Re: Colour curves
              ronzie99 Community Member
              ronzie99 May 1, 2012 12:32 AM (in response to photodrawken)

              Smart Curves is a powerful tool. You can define a curve as point to point, Beziette, independent added jagged point drag, whatever. You can therefore notch out a portion of a channel. The eye dropper tools to give you sampled curves as a guide are nice (if I have that correct). The curves do not have to have symmetrical continuous form as I've illustrated here:

               

              smart curve plug in beziette.jpg

               

              (Don't pay attention to the image <G>)

               

              The ability to save a curve for "batch recall" use to apply to a set of images in the same environment is a nice feature.

               

              For those interested here is the link for a number of filter plug-ins at no charge. Titles are English mostly but descriptions are in English or multiple languages for most once you open the filter/function link:

              http://free.pages.at/easyfilter/

              and smart curve is here with instructions in multiple languages:

              http://free.pages.at/easyfilter/curves.html

               

              http://free.pages.at/easyfilter/colorconverter.html I've noticed has been a requested color coordinate/space converter such as RGB to CMYK. It might help with direct scans of color negatives for one when that function is not in the scanner software.

               

              http://free.pages.at/easyfilter/filtermeister.html is a sheet for coders but also describes some of the filteers (in English).

               

              BTW, this link is from one of Barbara B's appendixes from the Missing Manual series for PSE.

               

              Thanks, Barbara.

              • 4. Re: Colour curves
                photodrawken Community Member
                photodrawken May 1, 2012 10:27 AM (in response to ronzie99)

                Yes, the SmartCurves page you mentioned has a link to this page:

                http://www.cambridgeincolour.com/tutorials/photoshop-curves.htm

                which I found very helpful -- good explanations and well-illustrated.

                 

                Ken