However, lines of equal saturation are oblique (drawn dashed on the graph) and all go from black through each color patch, meaning that their directions are particular to each color. Lines of equal chroma are vertical (following the patches grid), meaning that chroma has the same direction for all colors in the gamut (see below). This means that chroma evolves at constant lightness, saturation evolves at constant brilliance/brightness, and vice versa: They are orthogonal spaces that can be converted from one to another by a simple rotation of the base. (Lightness + Chroma) or (Brilliance/Brightness + Saturation) are two different ways to encode the same reality. The following charts show luminance, lightness, chroma, brilliance/brightness and saturation varying from a “0” base color and how the resulting colors degrade: While the previous definitions are useful to give a meaning to the words, they don’t tell us what we should be looking at. High purity, at the other extreme, means the light is laser-like, composed of a single wavelength.Ĭolors can be described in many different color spaces, but no matter the color space, each color needs at least 3 components: some metric of luminance or brightness, and 2 metrics of chromaticity (hue and chroma, or opponent color coordinates). Zero purity means the light is achromatic. 7 purity The distance of the pixel chromaticity from the white point in the xy chromaticity plane. 6 saturation The colorfulness of an area judged in proportion to its brightness. brilliance The brightness of an area judged relative to the brightness of its surroundings. 5 Warning: chroma is not short for chrominance, which is the color part of a video signal (the Cb and Cr channels in YCbCr, for example). chroma The colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears gray, white or highly transmitting. Lightness is expressed by the L channel in CIE Lab and Luv 1976 and the J channel in JzAzBz. 4 Lightness is the perceptual, non-linear homologue of luminance (roughly equal to the cubic root of luminance Y). 3 lightness The brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting. brightness An attribute of visual perception according to which an area appears to emit, transmit or reflect more or less light. 2 Luminance is a property of scene-referred frameworks, and is expressed by the Y channel of the CIE XYZ 1931 space. luminance The density of luminous intensity with respect to a projected area in a specified direction at a specified point on a real or imaginary surface. 1 Hue is a shared property between the perceptual and scene-linear frameworks. The following dimensions of color are used by darktable: hue An attribute of visual perception in which an area appears to be similar to one of the colors red, yellow, green, or blue, or to a combination of adjacent pairs of these colors considered in a closed ring. These two frameworks provide us with metrics and dimensions to analyze color and allow us to change some of its properties while preserving others. A perceptual, psychological, framework that stacks the brain’s corrections on top of the retina signal, using color spaces such as CIE Lab 1976, CIE Luv 1976, CIE CAM 2016 and JzAzBz (2017).A scene-linear, physiological, framework, that mostly focuses on the response of the retina cone cells, using color spaces such as CIE XYZ 1931 or CIE LMS 2006,.There are two frameworks within which color properties may be analyzed and described: In color science, each of these terms has a precise meaning. □definitionsĬolor properties like “saturation”, “brightness” or “lightness” have passed into common usage but are largely misused and often used to mean different things. This section defines the perceptual properties of color, both conceptually and quantitatively, in order to characterize and quantify the creative and corrective adjustments made to color in darktable.
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