Editing Color vision (section)

== Mathematics of color perception ==
{{Unreferenced section|date=May 2016 }}
A "physical color" is a combination of pure [[spectral color]]s (in the visible range).  In principle there exist infinitely many distinct spectral colors, and so the set of all physical colors may be thought of as an infinite-dimensional [[vector space]] (a [[Hilbert space]]).  This space is typically notated ''H''<sub>color</sub>. More technically, the space of physical colors may be considered to be the [[cone (topology)|topological cone]] over the [[simplex]] whose vertices are the spectral colors, with white at the [[centroid]] of the simplex, black at the apex of the cone, and the monochromatic color associated with any given vertex somewhere along the line from that vertex to the apex depending on its brightness.

An element ''C'' of ''H''<sub>color</sub> is a function from the range of visible wavelengths—considered as an interval of real numbers [''W''<sub>min</sub>,''W''<sub>max</sub>]—to the real numbers, assigning to each wavelength ''w'' in [''W''<sub>min</sub>,''W''<sub>max</sub>] its intensity ''C''(''w'').

A humanly perceived color may be modeled as three numbers: the extents to which each of the 3 types of cones is stimulated. Thus a humanly perceived color may be thought of as a point in 3-dimensional [[Euclidean space]]. We call this space '''R'''<sup>3</sup><sub>color</sub>.

Since each wavelength ''w'' stimulates each of the 3 types of cone cells to a known extent, these extents may be represented by 3 functions ''s''(''w''), ''m''(''w''), ''l''(''w'') corresponding to the response of the ''S'', ''M'', and ''L'' cone cells, respectively.

Finally, since a beam of light can be composed of many different wavelengths, to determine the extent to which a physical color ''C'' in ''H''<sub>color</sub> stimulates each cone cell, we must calculate the integral (with respect to ''w''), over the interval [''W''<sub>min</sub>,''W''<sub>max</sub>], of ''C''(''w'')·''s''(''w''), of ''C''(''w'')·''m''(''w''), and of ''C''(''w'')·''l''(''w''). The triple of resulting numbers associates with each physical color ''C'' (which is an element in ''H''<sub>color</sub>) a particular perceived color (which is a single point in '''R'''<sup>3</sup><sub>color</sub>). This association is easily seen to be linear. It may also easily be seen that many different elements in the "physical" space ''H''<sub>color</sub> can all result in the same single perceived color in '''R'''<sup>3</sup><sub>color</sub>, so a perceived color is not unique to one physical color.

Thus human color perception is determined by a specific, non-unique linear mapping from the infinite-dimensional Hilbert space ''H''<sub>color</sub> to the 3-dimensional Euclidean space '''R'''<sup>3</sup><sub>color</sub>.

Technically, the image of the (mathematical) cone over the simplex whose vertices are the spectral colors, by this linear mapping, is also a (mathematical) cone in '''R'''<sup>3</sup><sub>color</sub>.  Moving directly away from the vertex of this cone represents maintaining the same [[chromaticity]] while increasing its intensity. Taking a cross-section of this cone yields a 2D chromaticity space.  Both the 3D cone and its projection or cross-section are convex sets; that is, any mixture of spectral colors is also a color.

[[File:CIE chromaticity diagram 2012 version.png|thumb|upright=1.25|The CIE 1931 xy chromaticity diagram with a triangle showing the [[gamut]] of the [[Adobe RGB color space]]. The Planckian locus is shown with color temperatures labeled in [[kelvin]]s. The outer curved boundary is the [[spectral color|spectral locus]], with wavelengths shown in nanometers. Note that the colors in this file are specified in Adobe RGB. Areas outside the triangle cannot be accurately rendered because they are out of the gamut of Adobe RGB, therefore they have been interpreted. Note that the colors depicted depend on the color space of the device you use to view the image (number of colors on your monitor, etc.), and may not be a strictly accurate representation of the color at a particular position.]]

In practice, it would be quite difficult to physiologically measure an individual's three cone responses to various physical color stimuli. Instead, a [[psychophysics|psychophysical]] approach is taken.<ref>{{cite journal | vauthors = Jacobs DE, Gallo O, Cooper EA, Pulli K, Levoy M | title = Simulating the Visual Experience of Very Bright and Very Dark Scenes. | journal = ACM Trans. Graph. | date = May 2015 | volume = 34 | issue = 3 | pages = 15 | url = https://graphics.stanford.edu/papers/gazehdr/ | doi = 10.1145/2714573 | s2cid = 14960893 | doi-access = free }}</ref> Three specific benchmark test lights are typically used; let us call them ''S'', ''M'', and ''L''. To calibrate human perceptual space, scientists allowed human subjects to try to match any physical color by turning dials to create specific combinations of intensities (''I''<sub>''S''</sub>, ''I''<sub>''M''</sub>, ''I''<sub>''L''</sub>) for the ''S'', ''M'', and ''L'' lights, resp., until a match was found. This needed only to be done for physical colors that are spectral, since a linear combination of spectral colors will be matched by the same linear combination of their (''I''<sub>''S''</sub>, ''I''<sub>''M''</sub>, ''I''<sub>''L''</sub>) matches. Note that in practice, often at least one of ''S'', ''M'', ''L'' would have to be added with some intensity to the ''physical test color'', and that combination matched by a linear combination of the remaining 2 lights. Across different individuals (without color blindness), the matchings turned out to be nearly identical.

By considering all the resulting combinations of intensities (''I''<sub>''S''</sub>, ''I''<sub>''M''</sub>, ''I''<sub>''L''</sub>) as a subset of 3-space, a model for human perceptual color space is formed. (Note that when one of ''S'', ''M'', ''L'' had to be added to the test color, its intensity was counted as negative.) Again, this turns out to be a (mathematical) cone, not a quadric, but rather all rays through the origin in 3-space passing through a certain convex set. Again, this cone has the property that moving directly away from the origin corresponds to increasing the intensity of the ''S'', ''M'', ''L'' lights proportionately. Again, a cross-section of this cone is a planar shape that is (by definition) the space of "chromaticities" (informally: distinct colors); one particular such cross-section, corresponding to constant ''X''+''Y''+''Z'' of the [[CIE 1931 color space]], gives the CIE chromaticity diagram.

This system implies that for any hue or non-spectral color not on the boundary of the chromaticity diagram, there are infinitely many distinct physical spectra that are all perceived as that hue or color. So, in general, there is no such thing as ''the'' combination of spectral colors that we perceive as (say) a specific version of tan; instead, there are infinitely many possibilities that produce that exact color. The boundary colors that are pure spectral colors can be perceived only in response to light that is purely at the associated wavelength, while the boundary colors on the "line of purples" can each only be generated by a specific ratio of the pure violet and the pure red at the ends of the visible spectral colors.

The CIE chromaticity diagram is horseshoe-shaped, with its curved edge corresponding to all spectral colors (the ''spectral [[locus (mathematics)|locus]]''), and the remaining straight edge corresponding to the most saturated [[purple]]s, mixtures of [[red]] and [[Violet (color)|violet]].