Doug Kerr
Well-known member
Many descriptions of the Granger Rainbow pattern state (or intimate) that "luminance" runs from 0-1 over the entire height of the pattern going from bottom to top (for the orientation of the sample we saw) and "saturation" runs from 0-1 over the entire height of the pattern going from top to bottom.
This is not so. "Luminance" is constant (at 100%) in the entire upper half, and declines to zero as we go from the "equator" to the bottom.
"Saturation" is constant (at 100%) in the entire lower half, and declines to zero as we go from the "equator" to the top.
In fact, what I have called "luminance" is actually the coordinate "B" ("brightness") of the Photoshop form of the "HSB" color space. What I have called "saturation" is actually the coordinate "S" ("saturation") of the Photoshop form of the HSB color space,
In the Photoshop form of the HSB color space, the coordinate B is defined as (if we ignore, for clarity, the need to convert from a 0-255 scale to a 1-100% scale):
The coordinate S is defined as (with the same proviso about scales):
The HSB color space is sometimes called the HSL (L="lightness") color space.
Note that not all forms of these color spaces use the same definitions of B (or L) and S.
I have not discussed the definitions of H ("hue") in terms of R, G, and B, which are complicated, owing to the angular, sectorized nature of the result, and again are not consistent over all forms of these color spaces.
These color spaces are primarily intended for use in "color choosers" in graphic applications, since they have a good relationship to the intuitive human views of color, in a "tidy" but imprecise form, ignoring the subtleties of how luminance (for example) really works.
They are of course not attractive for use in a (you should pardon the expression) "scientific" context.
Best regards,
Doug
This is not so. "Luminance" is constant (at 100%) in the entire upper half, and declines to zero as we go from the "equator" to the bottom.
"Saturation" is constant (at 100%) in the entire lower half, and declines to zero as we go from the "equator" to the top.
In fact, what I have called "luminance" is actually the coordinate "B" ("brightness") of the Photoshop form of the "HSB" color space. What I have called "saturation" is actually the coordinate "S" ("saturation") of the Photoshop form of the HSB color space,
In the Photoshop form of the HSB color space, the coordinate B is defined as (if we ignore, for clarity, the need to convert from a 0-255 scale to a 1-100% scale):
B= max[C]/2
where "max[C]" represents the maximum of R, G, and B.The coordinate S is defined as (with the same proviso about scales):
S= (max[C]-min[C])/max[C]
where "max[C]" represents the maximum of R, G, and B, and "min[C]" represents the minimum of R, G, and B.The HSB color space is sometimes called the HSL (L="lightness") color space.
Note that not all forms of these color spaces use the same definitions of B (or L) and S.
I have not discussed the definitions of H ("hue") in terms of R, G, and B, which are complicated, owing to the angular, sectorized nature of the result, and again are not consistent over all forms of these color spaces.
These color spaces are primarily intended for use in "color choosers" in graphic applications, since they have a good relationship to the intuitive human views of color, in a "tidy" but imprecise form, ignoring the subtleties of how luminance (for example) really works.
They are of course not attractive for use in a (you should pardon the expression) "scientific" context.
Best regards,
Doug