XLOADCT


XLOADCT is a widget based utility for the interactive
manipulation of color tables.  It may be used with any
IDL Window application, regardless of whether or not
the application uses widgets.

The color table maps the data values written to the
screen to different colors and intensities.  Its
operation is similar to that of a photographic wedge.
The slope and position of the wedge are manipulated to
best display a particular dataset.

The major controls are:

Stretch Bottom and Stretch Top Sliders:

  These sliders control the "contrast" of the color
  tables, and are expressed in percentages of full intensity.
  The color table wedge is "stretched" so that values equal
  to or less than "Stretch Bottom" are set to the lowest
  color table entry, which is usually black.  Values
  equal to or greater than "Stretch Top" are set to the
  last entry in the color table, usually white or some
  other bright color.  Values between the bottom and top
  are linearly scaled to the color table.
  
  Making "Stretch Bottom" more than "Stretch Top" inverts
  the color table.

Gamma Correction Slider:

  This slider can be used to compensate for the
  characteristics of your monitor. Values larger than 1.0
  have a steeper contrast curve at the top end of the
  ramp, while values less than 1.0 have a steeper
  contrast curve at the bottom end of the ramp.  A value
  of 1.0 results in a linear ramp.


Pre-defined Color Table Buttons:

  Pressing one of these buttons loads the selected
  pre-defined color table.  The settings of the other
  controls are not affected.

Options:

  Gang Sliders:  connects the "Stretch Bottom"
    and "Stretch Top".  Moving one slider moves the other.
    With the sliders ganged, movement of a slider
    causes the other slider to track keeping the width of
    the wedge constant, while moving it across the
    range of data values.  Normally, the sliders are
    independent.

    The width of the wedge is fixed when this button is
    depressed.

 Top: When set to CLIP, values larger than the Top are
    set to the largest color index.  If set to CHOP, values
    larger than "Stretch Top" are set to color index 0.

 Stretch: When set to the default of INDICES,
    manipulations affect the mapping between color indices
    and color table triples.  When set to INTENSITY, the
    mapping controls the intensity of each color table entry.
    This is a useful when displaying quantized images, where
    the pixel value is arbitrary and does not represent an
    intensity.  In this mode, the hue and saturation
    remain relatively constant for a given color index.



Transfer Function:

  Selecting this mode allows interactive editing of the
mapping of color table values to color indices by dragging
control points on a plot of the map.  Select and move a
control point by clicking and dragging on its box.  

  The first and last control points may only be moved in the
vertical direction.  Other control points may be moved
anywhere within the plot as long as they are to the right of
their predecessor and to the left of the next control point.

  You can add a control point, in the largest interval that
contains no control points with the "Add Control Point"
button.  The "Remove Control Point" button removes the
control central point in the smallest interval containing
three control points.   The "Reset Transfer Function"
restores a linear one-to-one transfer function.