Mike Russell wrote:
The ability to alter Lightness without changing
color is indeed a major advantage of Lab
Please do this test:
1) Take an underexposed raw shot of a color input target (e.g. the Kodak Q60). Use a strong underxposure, say -4EV, so that you will see the errors clearly.
2) Convert the raw shot into a RGB color-space (but not to the sadRGB) and do not let the raw converter to correct the underexposure.
3) Open the converted image to Photoshop.
4) make a duplicate of that and convert the duplicate to Lab.
5) Scale up the RGB mode image using the right input levels slider of
the "RGB channel"–> the appearance of the image will pretty closely match the appearance of the target itself.
6) Scale up the Lab mode image similarly using the right input levels slider of the "Lightness channel" –> the image shows _very_ larger luminance, hue and saturation errors.
Please do the above experiment, you will learn that it is _not_ possible to change the Lightness without changing "color" in the Lab mode.
In this context the commonplace expression "color" is way too loose term. What we refer to with (reflected or self luminous) "color" is actually composed by three properties: Luminance, hue, and saturation. The luminance of a dark surface is low and the luminance of a white surface (e.g. illuminated by the sun) is high. The hue of the strawberry is red and the hue of the grass is green. Pink is less saturated red than what the saturation of the red of a strawberry is.
A change (or scaling) on the "L"channel in the Lab space _does_ affect, very strongly indeed, to both the hue and the saturation, it is a widely spread misunderstanding that it would not. I have already presented a demonstration that shows that when the hue and/or saturation is changed in the Lab space then the luminance will change also. The contrary is _equally_ true, a change on the "L" channel will also change the hue and/or saturation of the color. Naturally the numerical values on the "a" and "b" channels do not change due to a change on the "L" channel but the displayed hue and/or saturation will, a great deal. Note that the "a" and "b" channels are not hue nor saturation -channels, they are just arbitrary numerical scales.
The related operation in a RGB space is a change (or scaling) on the "RGB channel". The result depends on the properties of the particular RGB space. In case the RGB space is linear (has gamma 1.0 transfer function) or an accurate mathematical gamma function then the hue and/or saturation are not affected at all due to that operation and the lumimance scaling happens properly. In case of the sadRGB and the Lab spaces, hue and/or saturation errors are induced and the scaling of the luminance happens incorrectly.
In RGB, bumping the contrast of an image
generally increases the color saturation.
To what operation do you refer to by "bumping the contrast"? With digital images the "contrast" is somewhat a difficult property to be adjusted without creating some kind of errors, no matter in what color space you perform this such operation. In the real life it is much more easy to increase contrast, you just add more light to the scene. In digital imaging light can not be added freely since there is the upper limit (255,255,255), so other kind of rendering has to be performed in order to "bump up the contrast".
I suspect the above could initiate some discussion about the property called contrast, so: Take absolutely all the light away from a scene, the result is that absolutely no contrast is available from the scene. Lit up a candle, now there is just a little of contrast available from the scene. Lit up a sun, plenty of contrast is available.
In practical terms, this makes RGB an ideal color space for dealing with
Correctly chosen RGB space is ideal for all image manipulation.
underexposed images. Since such images are generally lacking in color as well as brightness,
Underexposed images are just that, underexposed. They do not lack "color", nor saturation nor hue. They do not lack even the luminance. The digital coding of underexposed shots simply is scaled down due to the underexposure.
The same is true of washed out images – making them darker by moving the black end of the RGB curve increases contrast, and adds saturation.
Such adjustment moves the blackpoint of the coding upwards, this will _clip_ any data that there might be in that clipped portion of the dark end. In addition, if you do this in non-linear working-space then the "image-gamma" (a.k.a. "file-gamma") is affected resulting errors to all three color properties. Blackpoint adjustment should be performed only when the blackpoint is incorrectly coded.
If the saturation is already about right, RGB may add or remove saturation inappropriately. […] In addition to saturation, hue angle can also change as a side effect of any RGB correction. […] This is the main reason you may find yourself accidentally getting very saturated red or orange skin
tones when correcting in RGB.
Editing work in a non-linear RGB working-space will create such (and other) errors, editing work in the Lab space also does so. Editing work in the linear RGB space either completely avoids or at least minimizes all such errors.
Timo Autiokari
http://www.aim-dtp.net