Color notes - references, links, and unsorted stuff

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

You will hear "technically, magenta doesn't exist".

This is fairly wrong, or at least a semantic argument in that it depends on a singular definition of color.

Yet magenta is still almost-uniquely interesting, in a way that this statement is indeed pointing at.

For context: When reading about spectra (e.g. at e.g. SPDs mentioned here), and metameters, you realize that there may be distinct SPDs that our eyes happen to simplify into the same color sensation.

There is a possible concept of a "spectral color", those represented by a single wavelength.

This is the small subset of all colors for which yes, there may be a set of metameters, but you can cull away parts of the SPD and reduce it to a single wavelength.

For these spectral colors, this can usefully act as a minimal representation. Which is useful because we can then pretend all that metameter business doesn't exist, and it can simplify specifying color a bunch.

These spectral colors happen to correlate well with the rainbow colors. Violet is on one edge of the spectrum.

Magenta isn't one of those spectral colors.

But the thing is, almost most of the colors we see everyday aren't a spectral color either.

Most colors we see, most colors we would say exist, are blends. And most cannot be reduced without altering our color sensation.

That said, magenta has an unusual property among those blends, because it is a blend with a break in the middle.

The most minimal among its metameters would be one peak in red (around 400nm) and one in violet (around 750nm).

(This property is not unique, but magenta is by far the most visible example to our eyes)

"What's the resolution of the human eye?"

Unsorted

http://en.wikipedia.org/wiki/Deep_Color

http://en.wikipedia.org/wiki/XvYCC

Grayscale conversion from RGB

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

The easiest conversion to grayscale is a simple average, (r+g+b)/3, but this is not very perception-accurate.

There are various RGB standards that mention grayscale conversion as weighing of the channels. In some cases this is based on perception, in some it correction for RGB primaries used in the standard (probably based on phosphors?).

These include:

ITU-601-1 and a few others use nearly identical numbers, 0.298954*R + 0.586434*G + 0.114612*B (apparently originating from CIE-XYZ 1931(verify))
EBU/ITU 3213 (PAL(verify))): 0.222*R + 0.707*G + 0.071*B
BT/ITU-709 (NTSC(verify))): 0.213*R + 0.715*G + 0.072*B
...more

Also, "RGB" is not actually a color space -- the best choice depends mostly on the flavour of RGB colorspace you have.

It can also depend on specific wishes. Consider how photographers sometimes use specific colored lenses (or nearly-equivalent photoshop filters) for specific effects, such as lessening the visibility of freckles, making skies more contrasted, and such.

Undetailed so far

Mostly colour spaces, colour models, and transforms. Again, these are not necessarily accurate.

HSV, HSB, HSL/HLS, HSI - hue, saturation, and brightness/level/luminisity/intensity. * IHS,
NTSC YIQ, NTSC CMY
YUV, YIQ, YCbCr
CMY, CMYK
HMMD
StW, I1I2I3

Retinal Cone
Munsell
Karhonen-Loeve

And things I've read about them:

NTSC RGB seems to refer to the defined conversion from YIQ to RGB inside the TV.
SMPTE RGB apparently does the same but matches modern phosphors better.
IHS conversions seems inconsistently defined between books.
I1I2I3 is defined from an (unspecified?) RGB as I1=(R+G+B)/3, I2=(R-B)/2, I3=(2G-R-B)/4, so seems to be a luminance-and-opposite type of system.