4. Operating
Barco – TransForm A - X Terminal – DOC-3265-2 – user's manual – Revision 09 – September-2006
______________________________________________________
4-4
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 en-
try 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 & Associates, Inc.
•
Introduction to the X Window System
Oliver Jones, Prentice Hall, Inc.
