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<HTML><HEAD><TITLE>Xlib Programming Manual: Window Functions: Visual Types</TITLE></HEAD><BODY><H1 ALIGN=center>3.1 Visual Types</H1><!.IN "Visual Type" "" "@DEF@">On some display hardware, it may be possible to deal with color resources in more than one way.For example, you may be able to deal with a screen of either 12-bit depthwith arbitrary mapping of pixel to color (pseudo-color) or 24-bit depthwith 8 bits of the pixel dedicated to each of red, green, and blue.These different ways of dealing with the visual aspects of the screenare called visuals.For each screen of the display, there may be a list of valid visual typessupported at different depths of the screen.Because default windows and visual types are defined for each screen,most simple applications need not deal with this complexity.Xlib provides macros and functions that return the default root window, the default depth of the default root window, and the default visual type(see "<A HREF="../display/display-macros.html">Display Macros</A>" and "<A HREF="../utilities/visual.html">Determining the Appropriate Visual Type</A>").<A NAME="Visual"></A><P>Xlib uses an opaque <B>Visual</B><!.IN "Visual" "" "@DEF@">structure that contains information about the possible color mapping.The visual utility functions (see "<A HREF="../utilities/visual.html">Determining the Appropriate Visual Type</A>") use an<A HREF="../utilities/visual.html#XVisualInfo">XVisualInfo</A>structure to return this information to an application.The members of this structure pertinent to this discussion are class, red_mask,green_mask, blue_mask, bits_per_rgb, and colormap_size.The class member specifies one of the possible visual classes of the screenand can be <!.IN "Visual Classes" "StaticGray"><!.IN "Visual Classes" "StaticColor"><!.IN "Visual Classes" "TrueColor"><!.IN "Visual Classes" "StaticColor"><!.IN "Visual Classes" "GrayScale"><!.IN "Visual Classes" "PseudoColor"><B>StaticGray</B>,<B>StaticColor</B>,<B>TrueColor</B>,<B>GrayScale</B>,<B>PseudoColor</B>,or<B>DirectColor</B>.<P>The following concepts may serve to make the explanation ofvisual types clearer. The screen can be color or grayscale,can have a colormap that is writable or read-only,and can also have a colormap whose indices are decomposed into separate RGB pieces, provided one is not on a grayscale screen.This leads to the following diagram:<P><CENTER><TABLE BORDER><TR ALIGN=center><TD> <TD COLSPAN=2>Color <TD COLSPAN=2>Gray-scale<TR ALIGN=center><TD> <TD>R/O <TD>R/W <TD>R/O <TD>R/W<TR ALIGN=center><TD>Undecomposed Colormap <TD>Static Color <TD>Pseudo Color <TD>Static Gray <TD>Gray Scale<TR ALIGN=center><TD>Decomposed Colormap <TD>True Color <TD>Direct Color <TD COLSPAN=2></TABLE></CENTER><P>Conceptually, as each pixel is read out of video memory for display on the screen,it goes through a look-up stage by indexing into a colormap.Colormaps can be manipulated arbitrarily on some hardware, in limited ways on other hardware, and not at all on other hardware. The visual types affect the colormap and the RGB values in the following ways:<UL><P><LI> For <B>PseudoColor</B>, a pixel value indexes a colormap to produceindependent RGB values, and the RGB values can be changed dynamically.<P><LI> <B>GrayScale</B>is treated the same way as <B>PseudoColor</B>except that the primary that drives the screen is undefined. Thus, the client should always store thesame value for red, green, and blue in the colormaps. <P><LI> For <B>DirectColor</B>,a pixel value is decomposed into separate RGB subfields, and eachsubfield separately indexes the colormap for the corresponding value.The RGB values can be changed dynamically. <P><LI> <B>TrueColor</B>is treated the same way as <B>DirectColor</B>except that the colormap has predefined, read-only RGB values.These RGB values are server-dependent but provide linear or near-linearramps in each primary. <P><LI> <B>StaticColor</B>is treated the same way as <B>PseudoColor</B>except that the colormap has predefined, read-only, server-dependent RGB values.<P><LI> <B>StaticGray</B>is treated the same way as <B>StaticColor</B>except that the RGB values are equal for any single pixelvalue, thus resulting in shades of gray. <B>StaticGray</B>with a two-entrycolormap can be thought of as monochrome.</UL>The red_mask, green_mask, and blue_mask members are only defined for<B>DirectColor</B>and <B>TrueColor</B>. Each has one contiguous set of bits with nointersections.The bits_per_rgb member specifies the log base 2 of thenumber of distinct color values (individually) of red, green, and blue.Actual RGB values are unsigned 16-bit numbers.The colormap_size member defines the number of available colormap entriesin a newly created colormap. For <B>DirectColor</B>and <B>TrueColor</B>, this is the size of an individual pixel subfield.<P>To obtain the visual ID from a <A HREF="#Visual">Visual</A> ,use<B><A HREF="XVisualIDFromVisual.html">XVisualIDFromVisual()</A></B>.<H5 ALIGN=right><I>Next: <A HREF="attributes/">Window Attributes</A></I></H5><HR><ADDRESS><A HREF="http://tronche.com/">Christophe Tronche</A>, <A HREF="mailto:ch.tronche@computer.org">ch.tronche@computer.org</A></ADDRESS></BODY></HTML>?? 快捷鍵說明
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