Color Glossary D-Z

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Version 1.0, 03/18/2003 - Last Update: 02/24/2009

The following glossary lists and explains color and visual perception terms which are relevant for graphic and Web design as well as usability. The information was taken from several sources and adapted to the needs of this glossary. See References for more information.

In this version, some ISO definitions have been added.

Dither, Dithering

A process of placing different-colored pixels next to one another to create the illusion of additional colors. The eye sees the two adjacent colors, and the mind blends them into a third color. (From Looking Good in Color)

Figure: Original continuous tone photo for the demonstration of dithering (200% enlargement)

Figure: Dither pattern (left) and diffusing dithering (irregular pattern, right) for creating intermediate colors (200% enlargement)

Figure-Ground Phenomenon

The figure-ground phenomenon, also called figure-ground separation, refers to the human ability to separate figures or "foreground" information from a surrounding background or noise information. This phenomenon is often demonstrated with ambiguous images, where you can see only one of two possible interpretations of a scene at a time.

Figure: Different figures demonstrate the figure-ground or foreground-background phenomenon

Although there can be a number of perceptual attributes listed that distinguish figures from ground, we are still far away from fully understanding the stimulus factors and psychological processes that distinguish figures from ground. One of these is, for example, the smaller the stimulus the more likely it is seen as figure.

Figure-ground phenomena also lead to the perception of subjective contours.

Foreground-Background

See Figure-Ground Phenomenon

Gamut

The gamut is the set of possible colors within a color system. No one system can reproduce all possible colors in the spectrum. Thus, it is not possible to create every color in the spectrum with either additive or subtractive colors. Both systems can reproduce only a subset of all visible colors. While those subsets generally overlap, there are colors which can be reproduced with additive color and not with subtractive color and vise-versa. (©1998 ACM)

Figure: Relative gamuts of additive (monitor display) and subtractive (printed) color; the larger area on the graph represents all visible colors. Click on the graph to see a more detailed version of the chart. (From projectcool Website)

The colors that can be created by each system are called a "gamut". The ACM diagram to the left, shows the relative gamuts of additive (monitor display) and subtractive (printed) color. The larger area on the graph represents all visible color. Click on the graph to see a more detailed version of the chart. (From projectcool Website)

Gestalt Laws

The Gestalt psychologists approached lightness perception, and perception generally, in a different manner from the Hering or the Helmholtz schools. They emphasized the importance of perceptual organization, much of it based on mechanisms that might be characterized as mid-level. The key concepts include grouping, belongingness, good continuation, proximity. (From Adelson, MIT)

Figure: Grouping by color (or gray level, left ) and form (right)

See also the guest editorial in Perception 11/1997 (vol. 26)"Colour in a larger perspective: the rebirth of Gestalt psychology"

GIF

See Image File Formats.

Halftone

The process of converting a continuous-tone image into a series of one-color dots of varying sizes. The larger and closer together the dots, the darker the image. The finer and aparter the dots, the lighter the image. (From lgc)

Figure: Continuous tone image (upper left) as it is possible on computer screens and a pure black and white version of the image created by a threshold process (upper right) in comparison to different halftone processes for printing: patterned dither (middle left),diffusion dither (mezzotint, middle right), dots at an angle of 45 degree (screened halftone, lower left) and the same image with a coarser dot pattern (lower right). Click the lower right image to see an even larger version and to see the computer simulation of the dots in detail.

HSL, HSV, HSB

See Color Space -> HSL/HSV/HSB (see also RGB, CMYK, Lab)

Hue

Hue, along with saturation and brightness make up the three distinct attributes of color (as defined in the HSL color space). The terms "red" and "blue" are primarily describing hue. Hue is related to wavelength for spectral colors (see Light). It is convenient to arrange the saturated hues around a Newton Color Circle. Starting from red and proceeding clockwise around the circle below to blue proceeds from long to shorter wavelengths. However, not all hues can be represented by spectral colors since there is no single wavelength of light which has the magenta hue - it may be produced by an equal mixture of red and blue.

There are many different mixtures of wavelengths which can produce the same perceived hue. The achromatic line from black to gray to white through the center of the circle represents light which has no hue.

Figure: The color wheel (left) shows spectral and non spectral hues; the color circle (right) also shows different levels of saturation

Hue Induction

Hue Induction through Simultaneous Contrast

The target color seems to be tinged with the complementary color of the surround (most noticeable with a gray target).

Note: Hues in the red to blue range are most stable, i.e. are least affected by chromatic induction.

Figure: Hue induction makes the gray look different.

Figure: Depending of the color of the surrounding square the central neutral (gray) square gets a slight tint in the direction of the complementary color.

Hue Induction through Color Assimilation

When the areas of color in a pattern subtend very small angles to the observer (less than about a third of a degree), an effect opposite to simultaneous contrast occurs: Colors appear to become more like their neighbor instead of less like them.

Figure: The Bezold-Brücke effect (hue induction) makes the red (left) and green (right) assimilate their neighboring colors; see also an animated example

Consequences of hue induction:

• same colors appear to be different
• different colors appear to be the same

Fore more examples see Simultaneous Contrast -> Simultaneous Color Contrast.

Effect of Surrounding Color

Colorfulness of a target is also affected by the surround. In general colors look most colorful against another color in lower lightness, particularly gray.

Image File Formats

BMP

Native bitmap file format (BMP = bitmap) for Microsoft Windows and OS/2 (slightly different). Files can be in 8-bit or 24-bit format and may utilize a lossless LZW compression.

Used throughout Microsoft Windows and OS/2 for bitmap files.

GIF

GIF = Graphic Interchange Format, 8-bit file format (or lesser bit depths, that is, 256 or less colors) developed specifically for the Internet by Compuserve. Offers automatic lossless compression (LZW, but best suited to horizontal patterns) and one transparency color.

Used for Web graphics which require only 256 or less colors, like graphics, text, screen dumps etc.

JPEG, JPG

24-bit file format with lossy compression. Offers variable degrees of compression: Higher compression rates result in smaller file sizes but also increase compression artifacts. Basically the compression is based on the reduction of the number of colors. When the image is compressed, colors may change slightly.

Used for Web graphics which require more then 256 colors, like photos, graphics with smooth gradations etc.

PICT

Native bitmap and object-oriented file format for the Apple Macintosh (comparable to WMF in Windows).

Used on the Apple Macintosh for bitmapped, vector-oriented and mixed files.

PNG

PNG = Portable Network Graphics, developed as a licence-free successor to the GIF file format. Offers 8 bit and 24 bit color depths, lossless compression and an 8-bit transparency mask for smooth transitions between foreground and background images.

Some older browsers do not support PNG well.

TIFF, TIF

TIFF = Tagged Image File Format, platform independent bitmap file format (there are slight differences between Motorola-based systems (Apple Macintosh) and Intel-based systems (Microsoft Windows). Offers different bit depths, lossless LZW compression, alpha channels and more.

Used for exchanging images between different computer platforms.

WMF

Native bitmap and object-oriented file format for Microsoft Windows (comparable to PICT on the Apple Macintosh).

Used on Microsoft Windows for bitmapped, vector-oriented and mixed files (format of the clipboard).

Intensity

Light waves vary in intensity and wavelength (frequency). Intensity is proportional to the square of the amplitude of a light wave.

See also Light.

Jaggies

A term for "jagged" or stair-step effect found in enlarged bitmapped images. Anti-aliasing and higher resolution minimizes jaggies. (From lgc)

See also Aliasing and Anti-Aliasing.

JPEG, JPG

See Image File Formats.

Lab

See Color Space -> Lab (see also RGB, CMYK, HSL/HSV). See further CIE which introduced the Lab color system.

Lateral Inhibition

The combined effect of the summation of adjacent excitatory and inhibitory signals causes us to perceive an increased contrast at luminance boundaries (edges), but little or no contrast in uniform fields.

Figure: The Mach-band effect is caused by lateral inhibition

Figure: Simultaneous contrast (black and white) is also caused by lateral inhibition; it lets the same gray in the middle of the lager squares appear differently.

Light

Electromagnetic radiation emanating from light sources or reflected from non-radiating objects. Perceived by our eyes and processed in the brain.

• Physical dimensions: Frequency (wavelength), intensity (amplitude)
• Perceptual dimensions: Brightness - related to intensity; hue (tone, tint) - related to frequency (wavelength), lightness
• Psychological dimensions: warmness/coldness, moods induced by colors, color associations, cultural differences

The visible light spectrum lies between 380 nm (violet) and 760 NM (red).

Light travels at a constant velocity of 300.000 km/s; it takes 1,25 sec to the moon, and nearly 8 minutes to the sun.

Lightness

Lightness is the perceived reflectance of a surface. It represents the visual system's attempt to extract reflectance based on the luminances in the scene. (From Adelson, MIT)

ISO Definition: Brightness of an area judged relative to the brightness of a similary illuminated area that appears to be white or highly transmitting. (Source: ISO 9241-8 (1997-10-00) ISO/ TC 159)

See also Brightness, Intensity, Luminance.

Luminance

Luminance is the amount of visible light that comes to the eye from a surface.

Illuminance is the amount of light incident on a surface. Reflectance is the proportion of incident light that is reflected from a surface.

Luminance, illuminance, and reflectance, are physical quantities that can be measured by physical devices.

There are also two subjective variables:

• Lightness is the perceived reflectance of a surface. It represents the visual system's attempt to extract reflectance based on the luminances in the scene.
• Brightness is the perceived intensity of light coming from the image itself, rather than any property of the portrayed scene. Brightness is sometimes defined as perceived luminance.

ISO Definition: The physical measurement of the stimulus which produces the sensation of brightness, in terms of the luminous intensity in a given direction [epsilon] (usually towards the observer) per unit area of an emitting, transmitting or reflecting surface. It is the luminous intensity of the light emitted or reflected in a given direction from an element of the surface, divided by the area of the element projected in the same direction. Unit: candela per square metre (cd/m2)

Note: The luminance L, in candela per square metre, of a perfectly matt surface is given by the formula [see formula in the standard] where E is the illuminance, in lux; [rho] is the reflectance of the surface considered. (Source: ISO 8995 (1989-10-00) ISO/TC 159)

Moiré

Figure: A moiré pattern

Munsell System (Munsell Color Space)

The Munsell color system is based on the HSL color space and is used for calibrating color systems. It matches colors to a set of standard samples (typically color charts), with adjacent samples based upon equal perceived differences in color. It divides hue into 100 equal divisions around a color circle. This is similar in approach to the Newton Color Circle except that the circle is distorted by assigning a unit of radial distance to each perceptible difference in saturation (called units of chroma). Since there are more perceptible differences for some hues, the figure will bulge outward to 18 values for some hues compared to only 10 for another. Perpendicular to the plane formed by hue and saturation is the brightness scale divided into a scale of "value" from 0 (black) to 10 (white). A point in the color space so defined is specified by hue, value, and chroma in the form H V/C. (Adapted from Hyperphysics)

Figure: The Munsell color system (from Hyperphysics)

Optical Illusion

Optical illusions define illusions in perception. They can be attributed to physiological (due to the construction and functionality of the human eye) as well as psychological factors (due to misinterpretations while seizing and comprehending the perceived objects and situations). There are various types of optical illusions: Geometrical-optical illusions, illusions concerning motion, contrast, perspective etc.

See also "Goodie" Optical Illusions in the Resources section of the SAP Design Guild for detailed information.

Parallax

ISO Definition: Difference in the apparent relative positions of objects when viewed from different points (Source: ISO/FDIS 9241-9 (1999-10-07) ISO/TC 159)

Perception

ISO Definition: Psychophysiological process occurring in the central nervous system, the product of which is knowledge about the environment. Perception is a dynamic process and is not determined merely by the parameters of the signals which initiated it. As a consequence, it is possible that the information obtained may be incomplete, uncertain, or incorrect. Knowledge may be based on one or more of the following levels of perception: detection, identification, and interpretation. Detection is the perceptual process by which the operator becomes aware of the mere presence of a signal. Identification is the perceptual process by which the detected signal is distinguished from other signals. Interpretation is the combination of perceptual and cognitive processes by which the contents and significance of the identified signal are recognised. (Source: EN 894-2 (1997-02-00) CEN/ TC 122)

PICT

See Image File Formats.

Pixel

ISO Definition: the smallest element that is capable of generating the full functionality of the display (Source: prEN ISO 13406-2 (1997-07-00) ISO/TC 159)

PNG

See Image File Formats.

Primary Colors

Primary colors theoretically cannot be mixed from any other colors. The RGB primary additive colors (red, green, blue) can be mixed in varying combinations to produce millions of additive colors. The subtractive colors red, yellow and blue or magenta, yellow, cyan can be mixed as printer's ink or artist's pigment to produce thousands of colors.

Primary colors are: red, green, blue (additive color mixture), red, yellow, blue (arts, subtractive color mixture for paints) or cyan, magenta, yellow (subtractive color mixture for print).

Note: Often the colors cyan, magenta, yellow are called blue, red and yellow (like the primary colors for arts). Note however, that the secondary colors which are created by mixing the primaries are different if the first set or the second set is used. Be careful and consult the respective illustrations in order to not get confused by this ambiguity.

Figure: Different triples of primary color - red, green, blue (additive color mixing), red. yellow - blue (arts, subtractive color mixing), and cyan, magenta, yellow (subtractive color mixing, print)

Process Colors

A term that is a synonym for CYMK (cyan, yellow, magenta, black), the four inks used by printers to produce full-color printing. Also referred to as four-color printing.

Figure: The four primaries of the CMYK color space cyan, magenta, yellow and black

Psychological Effects of Colors

Colors do not have only sensorial but also psychological effects and may affect people's moods:

• Cool: blue, turquoise, violet
• Warm: yellow, orange, red, brown
• Achromatic colors (white, gray, black) may take on either a warm or a cool character with just a hint of color.
• Green and magenta are marginal, depending on which colors surround them - they will tend to the opposite.

Warm hues act stimulating, while cool hues act relaxing. These effects are, for example, taken into account when decorating the interior of buildings.

Reflectance

Reflectance (of a surface) is the proportion of incident light the surface reflects.

ISO Definition: The ratio of the luminous flux reflected from a surface on it. The reflectance depends on the direction of the incident light, except for matt surfaces, and on its spectral distribution. (Source: ISO 8995 (1989-10- 00) ISO/TC 159)

RGB

See Color Space -> RGB.

Saturation

Saturation is related to how much white content is in the stimulus - it describes the purity of colors.

As used in HSL, HSV, or HSB it refers to the presence or lack of chroma or color pigment. A color that is 100 percent saturated contains no white (monochromatic colors). A color that has 0 percent saturation is white.

ISO Definition: chromaticness, or colorfulness, of an area judged in proportion to its brightness (Source: ISO 9241-8 (1997-10-00) ISO/TC 159)

Figure: A red changing from 0% saturation to 100% saturation in steps of 25% each

Secondary Colors

The three colors which are created by mixing the three primary colors. Additively mixing the primary colors red, green and blue results in the following secondary colors: cyan (blue-green), magenta (blue-red), yellow (green-red).

Depending on the primary colors, other secondary colors are created. For instance, the primaries red, yellow and blue, which are used in arts (pigments, thus subtractive color mixing), produce the secondaries green (blue-yellow), orange (red-yellow), and violet (red-blue).

Simultaneous Contrast

Simultaneous contrast is caused by lateral inhibition and can be observed for black-and-white as well as for colored images.

Simultaneous Contrast (Black and White)

Simultaneous contrast lets the central gray square look differently, depending on the surrounding gray level, although all the central squares are composed of the same gray.

Figure: Simultaneous contrast (black and white)

Simultaneous Color Contrast

Simultaneous color contrast effects that

• colors shift their hues into the direction of the complementary color (the opposite effect of Color Assimilation, where colors tend to adopt similar hues)
• colors look lighter or darker with respect to the background color (essentially the same effect as with black and white)

The change in appearance of a central area is caused by the presence of a neighboring area. The effect is strongest when the inducing field completely surrounds the target area.

Figure: Simultaneous contrast makes the central blue look different

Figure: Simultaneous contrast makes the central gray adopt different hues (and brightnesses, depending on the brightness of the surrounding color)

Figure: Color contrasts intensifies complementary colors (left); the same central colors look less intensive with other surrounding colors (confounded with brightness, therefore some colors still look intensive).

An Example

Color contrast and Color Assimilation or other effect of Hue Induction demonstrate that the perceived brightness/color of an object or region of a scene does not depend simply on its own brightness/color. There are basically two phenomena:

1. The same color may look differently
2. Different colors may lead to a similar color impression

Below are a few demonstrations of this, inspired by a book of Edward Tufte:

Figure: The same color looks differently depending on the surrounding color

Figure: Though the colors of the middle squares seem to look the same, they are different, as the next figure shows:

Figure: The colors of the central squares

Figure: This is how the same color (the left color) looks depending on the two backgrounds

See also Chromatic Induction, Hue Induction

Size Constancy

Size constancy denotes the stability of perceived size despite changes in objective distance and retinal image size.

Figure: Size constancy lets the person to the right appear much larger than the one to the left, even though both are of the same physical size (from Shepard)

Stereopsis

ISO Definition: Binocluar, visual perception of depth or three dimensional space. (Source: ISO 9241-8 (1997-10-00) ISO/TC 159)

Subjective Colors

Colors which are perceived from moving, e.g. rotating, black-and-white images are called subjective colors. One such device is Benham's disk. It was invented by a nineteenth-century toymaker who noticed colors in a black-and-white pattern he had mounted on a top. Toy spinning tops with Benham's disks are still available in toy stores. The effects are, however, rather subtle.

Figure: Benham's disk for inducing subjective colors

Figure: A variant of Benham's disk (from Sensation and Perception)

Subjective Contours

Subjective contours demonstrate that form can emerge through the use of depth cues in a two-dimensional array - the observer perceptually "creates" the contours defining a form. Coren (1972) suggested that subjective contours arise when we use depth cues within a configuration to help organize an otherwise meaningless array into a simpler, more meaningful figure. A typical such depth cue is interposition, that is, the occlusion of an object by another one. The occluded object is perceived as the more distant one (see figures below). The figure defined by the subjective contour acts remarkably like a "real" contour even to the extent that it can mask a real contour. (Adapted from Sensation and Perception)

Figure: Examples for subjective contours (left: a triangle is perceived; right: a circle is perceived)

Subtractive Color

The opposite of additive color, subtractive color represents inks or pigments that reflect the light of a certain color. Its primaries are cyan, magenta and yellow. The primary colors of magenta, cyan and yellow can be mixed together to make up millions of colors. See CMYK and Color Mixture -> Subtractive Color Mixture.

Subtractive Color Mixture

Figure: The primaries of subtractive color mixture for print are cyan, magenta, yellow, and black

See Color Mixture -> Subtractive Color Mixture.

TIFF, TIF

See Image File Formats.

Visual Impairments

Most visual impairments have physiological reasons. Some of them originate from the lens alone, some from its physical relation to the eye ball, others from the sensors, that is from malfunctions of rods or cones, or the neural pathways and processing.

The tables below provide an overview of the most common visual impairments. For more information on visual impairments, see Vision and Visual Disabilities - An Introduction in the Editions section of the SAP Design Guild.

Overview of Visual Impairments

The first table covers impairments that are related to malfunctions of sensors. The second table includes impairments caused by defects in the optical or neural system or by diseases.

Sensor-Related Impairments

Table 1: Overview of sensor-related visual impairments

Visual Impairments Caused by the Neural or Optical System or by Diseases

Table 2: Overview of further visual impairments

WMW

See Image File Formats.

References

Compiled and edited by Christine Wiegand and Gerd Waloszek (Product Design Center)

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Source:  Color Glossary