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is_widgetserver - Function

	Package	display
	Module	widgets

Is the argument a widgetserver tool?

Synopsis
is_widgetserver(tool)

Description
This global function can be used to determine if the supplied argument is a widgetserver tool. If so it returns True, otherwise it returns False.

Arguments

tool		The variable that you wish to test
		Allowed:	Any glish variable
		Default:	no default

Returns
T or F, indicating if the argument is a widgetserver tool or not.

Example

      include 'widgetserver.g'
      if (!is_widgetserver(dws)) {	
        fail 'Default widgetserver not available!';	
      }

This example shows how you can use this function to determine if the supplied variable is a widgetserver.

A note on colors

At present, the Aipsview and Viewer tools will only work on X window screens which can provide a ``PseudoColor visual.'' Most present-day Sun workstations provide an 8-bit PseudoColor visual by default; most present-day SGI workstations provide a 10-bit PseudoColor visual by default; and most Linux workstations and X-terminals offer a 16-bit TrueColor visual by default, but commonly offer instead a 15-bit TrueColor visual or a 24-bit TrueColor visual. Some TrueColor X servers (notably the Sun servers, and a few commercial servers running under Linux on selected hardware) are actually able to provide PseudoColor visuals within selected windows.

Without a PseudoColor visual, the Aipsview and Viewer tools will fail. This is because additional work is needed for these tools to support TrueColor (and DirectColor) visuals correctly.

However, it is important to note that for standard false color images, PseudoColor visuals are generally better than TrueColor visuals. This is because PseudoColor visuals have ``hardware colormaps,'' into which selected colors can be placed for use. A false color image is normally displayed by first mapping a set of colors (eg. a rainbow or greyscale) into part of the hardware colormap, and then drawing the pixels to the screen, specifying for each pixel with which entry in the hardware colormap it should color itself. Once drawn, the colors can be changed in the hardware colormap, and the display is updated instantly. For example, the rainbow colors could be replaced with topographic colors. Or as is more common, the colors within the hardware colormap can be shifted lengthwise and compressed or stretched to give more range to the signal or noise regimes of an image: this is the standard ``colormap fiddling'' technique.

TrueColor visuals, on the other hand, do not have a hardware colormap, and so for each pixel the exact color must be specified by red, green and blue components at the time of drawing. Once drawn, the only way to modify the coloring of the image is to completely redraw the image. Thus, the ability to interactive fiddle the colormap of an image is lost.

Furthermore, while a screen having an 8-bit PsuedoColor visual can display at maximum 256 colors on the screen at once, and a 15-bit TrueColor visual can display upto 32768 colors, in the second case, the individual color components (red, green and blue) can be specified at only 5-bit resolution. For the former, 8-bit resolution is available for the color components. Thus, smooth color shadings are easier to obtain on an 8 or 10-bit PseudoColor visual than on the typical 15/16-bit TrueColor visuals used on Linux machines.