A question regarding Lab color mode in Photoshop

[Jim Kasson]

Lab in Photoshop predates RGB working space by many years and was at the time, the only device independent color space one could use (Lab IS useful for that).

I dunno. How about CIEL*u*v*? Both Lab and Luv were standardized in 1976, well before Ps. I'd like it if Ps at least had a Luv color picker and Luv readout, if not being a three-quarter-fledged working space like Lab. I agree with Andrew that the sun is setting on Lab's usefulness as a working space. 16-bit color helped it along (it was asking for posterization to use it in 8-bit color), but ProPhoto RGB has gotten rid of one set of reasons to use it.

Luv has the property that, at any L*, the range of the visible colors is easy to figure out and plot. The display even looks familiar to many, since it's pretty close to u'v' chromaticity, with its horseshoe for the spectral locus. Having that spectral locus means that Luv has a measure that Lab lacks: saturation, defined as the ratio of the distance of a color to the white point at the luminance of the color to the distance from the white point to the spectral locus at the hue angle of the color. So, in Luv, we know if any set of values is visible, and how far away it is from not being visible.

Luv is not completely perceptually uniform, and neither is Lab; both are off by about the same amount, but in different places.

I know why Ps picked Lab in the first place; a big part of the target market was the color prepress business, and Lab was a standard there. Luv was used by the people who worked with displays. But today, lots of images are prepared for displays.

I'm not holding my breath, but I'd sure like to see Luv in Ps. Having it there would solve much of the OP's concern.

What would *really* be nice is a cleaned up Luv, kind of like what Bruce Lindbloom did with Lab, so that constant Munsell hues are on constant Luv hue angles.

Jim

[Robert Ardill]

The question is, what can't you do while in a well behaved RGB working space or better, when rendering the raw that can only be achieved in Lab?

I guess I'm thinking about Lab in Photoshop rather than raw processing in Lightroom. 

Perhaps another way of putting your question is: "What can you not do in a well behaved Lab working space that you can only achieve in RGB?".  That is, assuming that all of the adjustments available in RGB in Photoshop were also available in Lab in Photoshop.

The answer for me is that adjusting colors by manipulating the R, G and B values is hopeless, whereas it is not at all hopeless doing so by manipulating the L, a, b values.  For example, white-balancing a ColorChecker perfectly in Lab is really easy (to get all the gray values balanced, not just the middle gray).  Doing that in RGB is certainly possible, but I don't think it's easy.

At the end of the day, the kind of weird colors I started this topic with are not a problem really, because mostly in photography (most of us) are going to stay in a pretty conservative color gamut.  Then it's a question of preference, surely: what color space you feel most comfortable in (for certain tasks, perhaps white-balancing, as an example).

And actually I don't see why there has to be a (significant) loss of information in going from an RGB working-space to Lab and back again.  There is of course the potential for rounding and truncation errors but these should be minimal in 16 bit.  I would certainly have no hesitation in going to Lab for fear of information loss.

Robert

[digitaldog]

You CAN adjust Lab values while in an RGB space. Just set the info palette for Lab.

[Robert Ardill]

You CAN adjust Lab values while in an RGB space. Just set the info palette for Lab.

Well ....... you can SEE the Lab values, but you can't change the individual L, a or b channels!  To change the Lab values you have to manipulate the R, G and B channels.

Robert