Color Tables in Direct Graphics

Although you can define your own color tables, IDL provides predefined color lookup tables. (LUTs). Each color table contained within this routine is specified through an index value, shown in Loading a Default Color Table.

Tip: If you are running IDL on a TrueColor display, set DEVICE, DECOMPOSED = 0 before your first color table related routine is used within an IDL session or program.

For a list of all the color tables and their definitions, see Loading Default Color Tables.

You can load a default color table in an iImage display, an Object Graphics Display or a Direct Graphics display as follows:

iImage — select the Edit Palette button on the image panel.
Object Graphics — use the LoactCT method of an IDLgrPalette object to define the color table (see IDLgrPalette::LoadCT for details). Associate the palette object with another object using the Palette property (for example, see the PALETTE property of the IDLgrImage object).
Direct Graphics — use the LOADCT Procedure or another color table related routine to set the color table. Also see Using Color in Direct Graphics.

Note: See Color Table Manipulation for a list of related routines.

Modifying and Converting Color Tables

IDL contains two graphical user interface (GUI) utilities for modifying a color table, XLOADCT and XPALETTE (. The MODIFYCT routine lets you create or modify and store a new color table. See the following topics for examples:

XLOADCT Procedure — allows you to preview and select among pre-defined color tables
XPALETTE Procedure — allows you to preview and adjust pre-defined color tables
MODIFYCT Procedure — shows how to add modified color tables to IDL’s list of pre-defined color tables.

These examples are based on the default RGB (red, green, and blue) color system. IDL also contains routines that allow you to use other color systems including hue, saturation, and value (HSV) and hue, lightness, and saturation (HLS).

HighlightingFeatures with a Color Table

For indexed images, custom color tables can be derived to highlight specific features. Color tables are usually designed to vary within certain ranges to show dramatic changes within an image. Some color tables are designed to highlight features with drastic color change in adjacent ranges (for example setting 0 through 20 to black and setting 21 through 40 to white).

Note: Color tables are associated with indexed images. RGB images already contain their own color information. If you want to derive a color table for an RGB image, you should convert it to an indexed image with the COLOR_QUAN routine. You should also set COLOR_QUAN’s CUBE keyword to 6 to insure the resulting indexed image is an intensity representation of the original RGB image. See COLOR_QUANfor more information.

See the following topics for examples:

IDLgrPalette provides an example that creates, defines and applies a palette object to an image
TVLCT Procedure creates, defines and applies a color table in a Direct Graphics display
H_EQ_CT applies histogram equalization to a color table to reveal previously indistinguishable feature

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