XLoadct - Color Control

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.

CAUTION: Using any of the features below will preempt selecting colors (black, white, red, green, yellow, and blue) for graphics. You should first go to File/Set Colors on the Main Display and click on Reset to set all of the choices to "None".

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.