[cairo] Subtractive API, part 0

Chris Murphy lists at colorremedies.com
Sun Jan 31 11:21:42 PST 2010

On Jan 30, 2010, at 10:12 PM, Jon Cruz wrote:

> On Jan 31, 2010, at 9:44 AM, ecir hana wrote:
>> CMS is basically two things, one simple and one really difficult.
>> There actually is one and only, true color space - LAB.
> Not so much. Lab is a colorspace that is not as used nowadays. L*a*b* is really the more common one, and is often called "CIELAB". We also have XYZ . But... as you pointed out there is another complication. Without a white point being specified, "Lab" values do not define absolute colors.
> Not trying to get too pedantic, but with the things we're discussing the difference between "Lab" and "L*a*b*" are significant.

The color space is "CIE 1976 (L*a*b*)". It gets referred to as "Lab" and "CIELAB" and "L*a*b*" and all three can mean the exact same thing in the proper context.

"Lab" as in "Lab Color" found in Photoshop and a number of other applications (including ICC profiles) is possibly better described as a particular (and common) encoding of "CIE 1976 (L*a*b*)" The color space CIE 1976 (L*a*b*) itself  has a rather peculiar shape which is neither like a cylinder or cube or cone. Whereas the way it gets encoded (either 8bpc or 16bpc) it's treated as a cube. In fact that set of encodable values leaves out some colors that do exist in CIE 1976 (L*a*b*), and it also includes some encodable values that are not valid CIE 1976 (L*a*b*) colors - and hence aren't values representing a real color.

Chris Murphy
Color Remedies (TM)
New York, NY
Co-author "Real World Color Management, 2nd Ed"

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