Master RGB , RGB Luminosity, and Lightness Curves

February 27, 2008

On Dan Margulis' "colortheory: Applied Color Theory in Photoshop" list, there was a lengthily thread titled "accepted wisdom" - RGB Luminosity. This thread ran during January and February 2008. If you are interested in joining the list, it can be found at: http://tech.groups.yahoo.com/group/colortheory/

The discussion contained some very important information that took me awhile to completely absorb and understand how to leverage in color correction of images in Photoshop. As the colortheory list is text based, my hope is that the following images show the techniques being discussed by the various contributors to the thread. If I have misinterpreted anything, please let me know and I'll adjust this page accordingly. How to create the RGB Luminosity "channel" and the details of how to apply the curve have been illustrated with screenshots where appropriate.

When I photographed this painting, I knew I wanted to make contrast adjustments to put more shape into the flower. How that curve is applied can produce noticeably different results. Below is a screenshot of where the two "Color Sampler" points were placed. Points #1 and #2 were placed on the curve via Cmd/Ctrl clicking on the Color Samplers. Point #1 was moved up by 10 points (or clicks of the "up" arrow key), while point #2 was moved down by 15 points (or clicks of the "down" arrow key).

The purpose of this page is not to state that one method is better than the other, but rather to illustrate the various ways of applying a curve as the result is often different. I would suggest saving these images and opening them in Photoshop where you can compare them side by side. All images are in sRGB.

The image with Color Sampler points circled in green for easier recognition:

Version 1
 
Curve applied to the RGB Master curve.

Version 2

Curve applied as an Adjustment Layer clipped to a RGB Luminosity "Channel" layer.

The layer structure looks like this:

"Layer 1" is the RGB Luminosity. It was created from a duplicate of the file. In this duplicate, above the Background layer, a new layer containing a neutral solid fill (Black, Grey or White) set to "Color" blending mode, was added. These two layers were flattened and the result was dragged on top of the original RGB image. This new greyscale layer then had its blending mode set to "Luminosity". When the Curves adjustment layer was added, the checkbox to "Use Previous Layer to Create Clipping Mask" was ticked. Blending mode for the Curves layer was left at "Normal".

Extracting the RGB Luminosity appears as:

 

Version 3

The same curve again, but instead of clipping it to the RGB Luminosity layer, the curve is applied as an Adjustment Layer set to "Luminosity" blending mode. No RGB Luminosity "Channel" layer is present in this version.

Version 4

Curve applied to the L (Lightness) curve in LAB Color

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