![]() The various RGB color spaces that we use in the digital darkroom are simply useful subsets of all the colors contained in the XYZ color space. In fact, though seldom used directly in image editing, the XYZ color space is the basis of everything that relates to color in a color-managed image editing application. It amazes me that despite the somewhat limited experimental foundations of the XYZ color space (since supplemented with additional experiments), XYZ is nonetheless extremely useful. I don't know the composition of the humans that participates in the color matching experiments that led to the creation of the "average human" 1931 CIEXYZ color space, but if I had to guess, I would guess that they were healthy young adult British males. But as far as the 1931 CIEXYZ color space that we use in the digital darkroom is concerned, these "nonstandard color perception" colors aren't real and aren't imaginary, rather they simply weren't measured during the color matching experiments that led to the creation of the XYZ color space. And if a painting reflects it and a human tetrachromat sees it, it's real for the tetrachromat. To summarize, if a flower reflects it ("it" being that complex phenomenon we call light) and a bee sees it, of course it's real for the bee. These alternative color spaces would have their own sets of real and imaginary colors. But the resulting "tetrachromat-XYZ" color space (or "color-blind-XYZ" color space, or "bird-XYZ" color space) wouldn't be the same as the "average humans only" 1931 CIEXYZ color space. One could do (and I'm sure color scientists have done) color matching experiments with human tetrachromats, with color-blind humans, and perhaps even with birds, bees, dogs, and etc. Here's why:Īs mentioned in the first section of this article, light waves of different frequencies are out there in the world, but color happens in the eye and brain. However, for purposes of the digital darkroom, the colors that are seen by any being with non-standard color perception are neither real nor imaginary. For example, birds, bees, dogs, and humans with nonstandard color perception don't see the same colors in the same way as the average human. Not every being sees color exactly like the hypothetical average human. XYZ coordinates that are inside the locus of colors mapped by the color matching experiments are called real colors. XYZ coordinates that are outside the locus of colors mapped by the color matching experiments that led to the creation of the XYZ color space are called imaginary colors. However, not every set of coordinates in XYZ space corresponds to a color that the average human can see. Theoretically, the XYZ axes go off to infinity in both the positive and negative direction. X and Z carry additional information about how the cones in the human eye respond to light waves of varying frequencies. ![]() ![]() In the XYZ color space, Y corresponds to relative luminance Y also carries color information related to the eye's "M" (yellow-green) cone response. ![]() To visualize XYZ, think of a three-dimensional cartesian coordinate system (high school algebra) with axes labelled X, Y, and Z. In 1931 color scientists used the results of the Wright and Guild experiments to create the 1931 CIEXYZ color space ("XYZ" for short). In the late 1920s William David Wright and John Guild independently conducted a series of color matching experiments that mapped out all the colors the average human (meaning the average of the humans in the experiments) can see. XYZ Color mapping experiments: what the average human sees The naming of colors carries one out of the narrow realm of color perception, and into the larger realm of cultural and linguistic interpretation and classification of color, and thence into even larger philosophical, aesthetic, theological, and metaphysical considerations. So our perception of color is composed of both intensity information and chromaticity information. Light varies in wavelengths, which our eyes and brain interpret as varying colors, and also in intensity. Light enters the eyes, is processed by light receptors ( cones and rods), and sent via the optic nerves to the brain for further processing and interpretation. Rather color is part of how we sense the world around us. On the other hand, color isn't out there in the world in the same tangible way that light is. On the one hand, light comes from the sun or other radiant sources, and is refracted by mediums (water, the atmosphere, glass) and diffusely or specularly reflected by surfaces. Nominal ranges are often extended in practice. RGB numbers have the nominal range 0 to 1, as does Y from XYZ and xyY. Summary plus a short historical perspective on linear gamma image editingĪll RGB, XYZ, and yxY numbers in this tutorial are floating point numbers.xyY - enough colors, all the colors, and imaginary colors.RGB - locate black, white, red, green, and blue in XYZ space.
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