Chromaticity versus chrominance
June 23, 2008
Color spaces can generally be classified as chromaticity or chrominance based. Both are different styles of defining color. Chromaticity is “color” defined independent of luminance (or the equivalent of luminance in a particular color space). Suppose you used a neutral density filter on a camera or changed the intensity of the lighting. The chromaticity values of the image would stay the same. In a chrominance based color space, the chrominance values of the image will change if the intensity of light is varied as in the example. Chrominance can be thought of as a paint/pigment that adds “colorfulness” to an image. If mixing a gallon of paint, a certain amount of such a pigment would be needed. If mixing twice as much paint, twice as much of the pigment would be needed. And for three times the paint, three times the pigment would be needed, etc. etc. In a chromaticity based system, you would define color as the proportion to mix all the paints/pigments together.
If you are concerned with compression/bandwidth efficiency and/or uniform distribution of JNDs, then chrominance based color spaces tend to be more useful. For color correction however, chrominance based color spaces can cause problems if changing the luminance channel (or equivalent) without making similar changes to chrominance.
The first image below shows the original image. The second image below shows the L* channel in LAB space being halved in Photoshop. The third image shows the Luminosity channel being halved via a Levels adjustment Layer and the Luminosity blending mode (again, in Photoshop). The fourth image shows the RGB channels being halved - halving the luminance channel in a chromaticity based color space would yield similar/analogous results.
Original.
L* channel reduced 50% in LAB space (Photoshop).
Luminosity channel reduced 50% (Photoshop).
RGB channels reduced 50%. Halving the luminance channel in a chromaticity based color space will produce results similar to this.
The chrominance based color space adjustments cause the resulting image to have oversaturated colors. To go back to the paint mixing analogy, there is now too much “colorfulness” paint. This can be avoided by doing adjustments in a chromaticity based color space. It is possible to convert any chrominance based color space into a chromaticity based one by dividing chrominance by luminance (or its equivalent) to yield chromaticity. Most programs do not do this unfortunately. In some situations like the one here, it is possible to get this by reduce the chrominance components by the same amount. However, this generally will not work if curve-based manipulations (e.g. s shaped curves) are applied to the luminance channel.
If doing color correction work with chrominance based color spaces or transformations, this is something to watch out for! If the adjustments are very minor then the error will be low and likely at an acceptable level. But push the image too much and you run the risk of unintended saturation shifts.
References:
Kerr, Douglas. “Chromaticity and Chrominance in Color Definition” http://doug.kerr.home.att.net/pumpkin/
Filed under: color correction, color science |
Great post and nice blog! I’ll be checking back often! -swc
Thanks for the info !
Is there any good description about the mysterious nature of “vibrancy” available?
Regarding the “correct” contrast adjustments: I have the impression that in many cases (perhaps even the majority?), people actually are actually expecting the saturation shifts and predominantly the saturation enhancements that go along with the regular Photoshop or PhotoLine contrast adjustments.
Perhaps it is caused by the PS-way being the standard way and everybody being accustomed to it, but perhaps it is somewhat physiological?
(Colormancer clearly allowes for more control and produces less noise.)
I don’t believe that there is a standard definition for vibrancy. I take it to be a gamma/midpoint-like control on saturation, which seems to be what Adobe Camera RAW does.
Regarding contrast via RGB curves… it’s a valid way of doing color correction in my opinion. The contrast increase goes hand in hand with increases and decreases in saturation. Film inherently has s-shaped curves built it and it contributes strongly to film’s look.
Colormancer is different but not necessarily better- it may be preferable since changes in contrast don’t have obvious impacts on saturation.
While RGB curves do desaturate highlights, this is fine if you don’t push them to the extremes. The reason why it may look good is because the desaturation in the highlights looks similar to the effect of specular reflections. The desaturation is a similar look to a soft light source causing specular reflections on that object. Our visual system probably filters that out. That’s my guess anyways!
“Vibrancy” is saturation with an equalization mask (i.e., inversed saturation) in order to keep already saturated colors under control.