
Editimage(Display Utils)     Khoros      Editimage(Display Utils)

     Colorspace Models

     When an image is displayed, a colormap, or look up table
     (LUT), is used to map actual image pixel values to values
     that are meaningful to the display hardware.  Most color
     displays require that values for all three primary (RGB)
     colors be specified.  Therefore the colormap maps the single
     image pixel value to a set of three values, one for red, one
     for green, and one for blue.

     Pixel values with respect to different color coordinate sys-
     tems are available with the Print Pixel Values, Interactive
     LUT, Pseudo Color, and Windowing/Thresholding features of
     editimage. On these subforms, accessible via the DISPLAY
     UTILITIES pulldown menu,  the desired color space can be
     selected by clicking one of the eight buttons labeled RGB,
     CMY, HSV, HSV, HLS, YIQ, XYZ, UVW, or GREY.

     If the conversion from one colorspace to another transforms
     a straight line in one color model into a straight line in
     the other colorspace, then the result of a linear interpola-
     tion in both colorspaces will be the same. This is the case
     for the RGB, CMY, YIQ, XYZ and UVW colorspace models.  On
     the other hand, a straight line in the RGB colorspace does
     not in general transform into a straight line in either HSV
     or HLS colorspaces.  To aid in interpreting the pixel
     values, brief explanations of the eight color coordinate
     systems are given below.  Before explaining the different
     colorspaces, a bit of history is necessary.

     The Commission Internationale de L'Eclairage (CIE) defined
     the properties of the CIE 1931 standard initially by two
     different but equivalent, linearly related, sets of color
     matching functions. The functions are referred to as RGB and
     XYZ. In 1960 the CIE adopted a coordinate system called the
     Uniform Chromaticity Scale (UCS).  The UVW color space was
     the first attempt to represent approximately uniform chroma-
     ticity spacing under the UCS system. The UVW values are the
     tristimulus values corresponding to the UCS system.  The UVW
     color space is linearly related to the XYZ color space.

     The HSV and HLS colorspaces are perceptual colorspaces.
     These colorspaces are used for better human interpretations
     of color objects in a scene.

     Conversions between the different colorspaces are were taken
     from Digital Image Processing by Pratt, and Computer Graph-
     ics, by Foley, vanDam, Reiner, Hughes. we have made adjust-
     ments to some of the conversions to give more accurate
     results.

     RGB: Red, Green, and Blue Color Coordinate System

          The red, green and blue (RGB) colorspace is mainly used
          in color CRT monitors. This colorspace employs a Carte-
          sian coordinate system, often referred to as an RGB
          cube. The RGB primaries are additive primaries; that
          is, individual contributions of red, green and blue are
          added together to produce some color. The main diagonal
          of the RGB cube, with equal amounts of red, green and
          blue is represents grey values. The range of RGB pixel
          values is between 0 and 255.

     CMY: Cyan, Magenta, and Yellow Color Coordinate System

          The cyan, magenta and yellow (CMY) colorspace is mainly
          used in hardcopy devices.  The CMY colorspace is the
          complement of the RGB colorspace (1 - RGB).  The CMY
          primaries are the subtractive primaries; that is, indi-
          vidual contributions of cyan, magenta and yellow are
          subtracted from each other to produce some color. This
          colorspace employs a Cartesian coordinate system, the
          same coordinate system as the RGB colorspace. The main
          diagonal of the CMY cube, with equal amounts of cyan,
          magenta and yellow, represents grey values. The diago-
          nal represented in CMY is the opposite of that in RGB
          (they are complements).  The range of CMY pixel values
          is between 0 and 255.

     YIQ: Color Coordinate System for Commercial Color Television
     Broadcasting

          The YIQ colorspace is used in U.S. commercial color
          television broadcasting, which is closely related to
          raster graphics. YIQ is actually a recoding of RGB for
          transmission efficiency and for downward compatibility
          with black and white television. The Y component is
          luminance, and is defined the same as the CIE Y com-
          ponent. The I and Q components together define the
          chromaticity. The definition of term luminance is very
          complicated. It basically denotes luminous flux, or the
          luminance of an object with no regard to the surround-
          ings. The chromaticity specifies the hue and satura-
          tion.  The YIQ colorspace uses a 3D Cartesian coordi-
          nated system, with the visible subset being a convex
          polyhedron that maps into the RGB cube.  This
          colorspace would be used to adjust colors so that lumi-
          nance is given more weight than hue and saturation for
          visualization or transmission.  The range of YIQ pixel
          values is between 0 to 1.

     XYZ: An Artificial Primary Color Coordinate System

          The XYZ color coordinate system s a redefinition of the
          1931 CIE spectral primary system. The objective of XYZ
          is to provide a color coordinate system in which all
          tristimulus values required to match the spectral
          colors are positive. The Y component is the luminance.
          The primaries of this system are physically unattain-
          able, but the system is used because it is convenient
          for colormetric calculations. This colorspace helps
          determine where the just noticeable changes in color
          for the human vision system occur. The way this works
          is ellipsoids are drawn on the chromaticity diagram
          which represent just noticeable difference regions.
          These regions are characterized such that perceived
          differences between colors inside the ellipsoid are
          indistinguishable.  Because of the nature of the XYZ
          colorspace, the RGB cube will actually fit inside the
          space defined by the XYZ colorspace.  It is possible to
          have invalid XYZ pixel values in the RGB space.  The
          range of XYZ pixel values is between 0 and 1.

     UVW: Uniform Chromaticity Scale

          The UVW colorspace was developed from the XYZ
          colorspace, and is very similar to the XYZ colorspace.
          The UVW colorspace is an attempt to transform the
          elliptical contours for the XYZ system into circles of
          approximately equal size in the chromaticity plane.
          This uniformity is desirable because a unit change in
          the chromaticity diagram should correspond to an
          equivalently noticeable color shift to an observer.
          Because of the nature of the UVW colorspace, the RGB
          cube will actually fit inside the space defined by the
          UVW colorspace.  It is possible to have invalid UVW
          pixel values in the RGB space.  The range of UVW pixel
          values is between 0 and 1.

     HSV: Hue, Saturation, and Value Color System

          The HSV system is a perceptual color space that allows
          scenes to appear based on the intuitive appeal of the
          user's tint, shade and tone.  The HSV colorspace is
          defined as a hexicone or six sided pyramid with its
          wide end pointed upwards. The top of the hexicone
          corresponds to V = 1, which contains relatively bright
          colors. The Hue, or color is measured by the angle
          around which the vertical axis of the hexicone. Red is
          a 0 degrees, green is at 120 degrees and blue at 240
          degrees. The value of S is a ratio ranging from 0 to
          the center line (V axis) to 1 on the triangular sides
          of the hexicone. The saturation is measured relative to
          the color gamut represented by the model, which is a
          subset of the entire CIE diagram.  saturation to the
          purity of the color. The hexicone is one unit high. The
          point of the hexicone apex is black (V = 0). At V = 0,
          values of H and S are irrelevant. The range of pixel
          values for the HSV model range between 0 and 1

          A normal Reference point for HSV is S = 1 and V = 1.
          Adding white pigment corresponds to decreasing S from 1
          to 0 without changing V. Shades are created by keeping
          S = 1 and decreasing V from 1 to 0. Tones are created
          by decreasing both S and V. H corresponds to changing
          the color or pure pigment.

          Hue: Taking a horizontal slice through the hexicone
          yields a hexagon whose boundaries represent the various
          hues.  Therefore, hue values can be represented as
          angles about the axis of the hexagon, ranging between
          zero and one as one moves counterclockwise.  Zero
          corresponds to red, .33 to green, and .66 to blue, with
          intermediate values representing combinations of these
          hues.  Since hue is defined in terms of angles, red is
          represented both by values that are very near zero and
          one.

          Saturation: The saturation value, S, is a ratio which
          varies from zero on the center line of the hexicone, to
          one at the outer edges of the cone.  Points which lie
          along the center line (S=0) are grey level values, and,
          along this line the angle which represents hue is unde-
          fined. In terms of RGB or CMY, a color will be totally
          saturated as long as one of the primaries is absent.
          Saturation approaches zero as the three primaries
          approach the same value.

          Value: Value is a measure of the intensity of a color.
          It is the diagonal line in the RGB cube from black to
          white.  The top of the hexicone, where V=1, corresponds
          to maximum color values, and the bottom, where V=0,
          corresponds to black. In gray level images, this is the
          only HSV component that provides useful information.

     HLS: Hue, Lightness and Saturation Color System

          The HLS system is actually a deformation of the HSV
          colorspace. This colorspace is defined in a double-
          hexicone. Basically it is the HSV hexicone mirrored
          with itself. The Hue and Saturation are the same as
          defined in the HSV model above. The lightness value is
          black at 0 and white at 1. In the HLS case when L = 0
          or L = 1, the Hue and Saturation are undefined. Only
          when L = 0.5 any Hue can achieve full saturation.  The
          question: why do we need HLS when we have HSV?  The HLS
          system gives us a balance (double-hexicone) model to
          work with. This allows the user to change the perceived
          brightness of the color at some particular saturation
          level.  The range of pixel values for the HSV model
          range between 0 and 1.

          Copyright 1991, University of New Mexico. All rights
          reserved.

Khoros                    Release: 1.0                          1

