Is LAB useful for color correction?

June 23, 2008

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.

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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 my Photoshop plug-in (see colormancer.com). They are added as a point of reference.
• 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

Filed under: color correction, color science | Comments (3)

Surround Effects (Part 2)

June 23, 2008

One area where surround effects make a big difference is when evaluating the black level on LCD monitors. Almost all LCD monitors cannot reproduce true black and have a raised black level. In a dark room, they will have a “glow in the dark” appearance to them. The image below shows the LCD on a Macbook Pro.

In a brightly lit environment (i.e. the surround is bright), the black level on a LCD appears to be better even though the inherent black level of the display is still the same. Unfortunately this effect cannot be reproduced here, but you can walk into any computer store and see that LCDs do not have a glow in the dark appearance in brightly lit environments. They simply don’t look like the image above. You can still see that the black level on the LCD is raised (e.g. when screen is powered on/off, forcing the backlight off). Nonetheless blacks appear to be richer if the display is in a bright environment.

This effect is something to watch out for when considering displays as the display can be made to look better by placing it in a bright surround. If you need to monitor on a LCD display, then it can be useful to avoid a dark surround as it will make the raised black level on the LCD look even worse.

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Surround Effects (Part 1)

June 22, 2008

One of the factors that affect the perception of an image is the content of the area surrounding it. In the example below, the center patches of the color have the same value (128 128 128 RGB) yet the left patch appears to be lighter than the one on the right.

Adding articulation increases the strength of the effect.

But what happens when we test real world images, not just patches of color? The example below show that surround effects seem to be weak or non-existent on real world images. Certainly the difference is nowhere as dramatic.

It seems that visual cues within the real world image overrides or overshadows the surround effects in this case. In my opinion, these examples show that surround effects *do* matter- but not so much for real world images. For an ideal monitoring environment, you should try to setup a proper surround anyways as it does matter in some situations and it is not very difficult or expensive to achieve.

References:

For more information on surround effects, see Edward H Adelson’s Lightness Perception and Lightness Illusions.

Filed under: color science, monitoring, visual illusions | Comments (1)