Is LAB useful for color correction?

For color correction, it might seem sensible to use a perceptually uniform color space. CIE L*a*b* is one such color space that is designed to be perceptually uniform.

In the context of LAB, perceptual uniformity means that the ability to tell two shades apart is uniform. This is usually measured in just noticeable differences (JNDs). Different shades are presented to the human test observers. Two shades are 1 JND apart when the observers can differentiate between the shades a certain percentage of the time (e.g. when they can tell the shades apart 50% of the time and get it wrong the rest of the time). LAB is designed so that distances in LAB space will correspond to JNDs.

However, LAB is not perceptually uniform when using other meanings of that phrase. Perceived hue and saturation (as interpreted by the human visual system) are not uniform in LAB space. The most noticeable situation where this occurs is when blue turns purple when lowering saturation[1]. The diagram below shows a patch of blue being de-saturated with different algorithms in Photoshop. The image you are looking at is likely inaccurate if you are using an uncalibrated monitor, the white point is not D65, etc. etc. etc. But what you should see is that LAB color space is the worst of the bunch- blue turns purple and it is more purple than all the other algorithms.

In my opinion, LAB color space is not ideal for color correction. Every other saturation algorithm shown here does a better job at maintaining constant hue! This makes sense, as LAB color space was designed for uniform JNDs and was not designed to be useful for color correction.

An explanation of the 5 different saturation algorithms:

• The LAB result was generated using Photoshop. The a and b channels were reduced 50% via Levels.
• Photoshop Hue/Saturation is the Hue/Saturation adjustment in Photoshop with a setting of -50.
• Colormancer 0.25X and 0.5X are saturation adjustments via the Colormancer color correction tools.
• Rec. 709 chroma gain takes the original R’G'B’ values, converts them to Rec. 709 Y’CbCr color space, and multiplies the resulting chroma values by 0.5X. This type of algorithm is found in some high definition editing systems and color correction systems. The result appears dark because this algorithm does not follow the principle of constant luminance[2].

[1] For more information, see the section on The “Blue Turns Purple” Problem on Bruce Lindbloom’s website: http://www.brucelindbloom.com/MunsellCalcHelp.html

[2] Charles Poynton has a technical article on constant luminance here: http://www.poynton.com/notes/video/Constant_luminance.html