PURPOSE:
    Interactively create color tables using the RGB, CMY,
    HSV, and HLS color systems using the mouse, three
    sliders, and a cell for each color index. Single colors
    can be defined or multiple color indices between two
    endpoints can be interpolated.

 PROCEDURE:
    XPALETTE has the following controls:

    Left: Three plots showing the current Red, Green, and
          Blue vectors.

    Center: A status region containing:
        1) The total number of colors.
        2) The current color. XPALETTE allows changing
           one color at a time. This color is known as
           the "current color" and is indicated in the
           color spectrum display with a special marker.
        3) The current mark index. The mark is used to
           remember a color index. It is established by
           pressing the "Set Mark Button" while the current
           color index is the desired mark index.
        4) The current color. The special marker used in
           color spectrum display prevents the user from
           seeing the color of the current index, but it
           is visible here.

    A panel of control buttons, which do the following:

        Done: Exits XPALETTE.

        Predefined: Starts XLOADCT to allow selection of one
            of the predefined color tables. Note that when
            you change the color map via XLOADCT, XPALETTE
            is not always able to keep its display accurate.
            This problem can be overcome by pressing the
            XPALETTE "Redraw" button after changing things
            via XLOADCT.

        Help: This text is displayed.

        Redraw: Completely redraws the display using the
            current state of the color map.

          Set Mark: Set the value of the mark index to the
            current index.

       Switch Mark: Exchange the mark and the current index.

      Copy Current: Every color lying between the current
            index and the mark index (inclusive) is given
            the current color.

       Interpolate: The colors lying between the current
            index and the mark index are interpolated
            linearly to lie between the colors of two
            endpoints.

    Three sliders (R, G, and B) that allow the user to
    modify the current color.

    Right: A display which shows the current color map as a
        series of squares. Color index 0 is at the upper
        left. The color index increases monotonically by
        rows going left to right and top to bottom.  The
        current color index is indicated by a special marker
        symbol. There are 4 ways to change the current
        color:
            1) Press any mouse button while the mouse
               pointer is over the color map display.
            2) Use the "By Index" slider to move to
               the desired color index.
            3) Use the "Row" Slider to move the marker
               vertically.
            4) Use the "Column" Slider to move the marker
               horizontally.

 SIDE EFFECTS:
    XPALETTE uses two colors from the current color table
    as drawing foreground and background colors. These
    are used for the RGB plots on the left, and the
    current index marker on the right. This means that if
    the user set these two colors to the same value, the
    XPALETTE display could become unreadable (like writing
    on black paper with black ink). XPALETTE minimizes
    this possibility by noting changes to the color map
    and always using the brightest available color for the
    foreground color and the darkest for the background.
    Thus, the only way to make XPALETTE's display
    unreadable is to set the entire color map to a single
    color, which is highly unlikely. The only side effect
    of this policy is that you may notice XPALETTE
    redrawing the entire display after you've modified the
    current color. This simply means that the change has
    made XPALETTE pick new drawing colors.

    The new color tables are saved in the COLORS common
    block and loaded to the display.