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Visual class
Due to the different display capabilities of workstations reaching from a color depth of 1 bit up to 24 bits differ-
ent strategies for translating pixel values into on-screen colors have been developed in the X window system.
These strategies are called visual classes.
The X window system defines six visual classes, three of them are dynamic (PseudoColor, GrayScale,
DirectColor) and three are static (StaticColor, StaticGray, TrueColor):
•
PseudoColor
is a class having a color map in which a pixel value indexes the color map entry to produce an independent
RGB value; that is, the color map is viewed as an array of triples (RGB values). Shared and private color cells
can be allocated.
•
StaticColor
can be viewed as a case of PseudoColor in which the RGB values are predefined and read-
only; no private color cells are available.
•
GrayScale
can be viewed as a case of PseudoColor, in which the red, green, and blue values in any given color map
entry are equal and thus, produce shades of gray Shared and private color cells can be allocated.
•
StaticGray
can be viewed as a case of GrayScale in which the gray values are predefined and read-only;
no private color cells are available.
•
DirectColor
is a class of color map in which a pixel value is decomposed into three separate subfields for indexing. The
first subfield indexes an array to produce red intensity values. The second subfield indexes a second array to
produce blue intensity values. The third subfield indexes a third array to produce green intensity values.
Shared and private color cells can be allocated.
•
TrueColor
can be viewed as a case of DirectColor in which the subfields in the pixel value directly en-
code the corresponding RGB values. That is, the color map has predefined read-only RGB
values.
Further documentation
The color management of the X window system is explained in more detail in:
•
The Definitive Guide to the X Window System – Volume 1
Adrian Nye, O'Reilly & Assoc
O'Reilly & Associates, Inc.
•
Introduction to the X Window System
Oliver Jones, Prentice Hall, Inc.
Barco – Argus X Terminal – DOC-3265-2 – user's manual – Revision 04 – May-2004
PseudoColor in which the RGB values are predefined and read-
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4. Operating
4-4
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