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<HTML><HEAD><TITLE>Xlib Programming Manual: Chapter 6: Color Management Functions</TITLE></HEAD><BODY><H1 ALIGN=center>Chapter 6: Color Management Functions</H1>Each X window always has an associated colormap thatprovides a level of indirection between pixel values and colors displayedon the screen.Xlib provides functions that you can use to manipulate a colormap.The X protocol defines colors using values in the RGB color space.The RGB color space is device-dependent;rendering an RGB value on differing output devices typically resultsin different colors.Xlib also provides a means for clients to specify color usingdevice-independent color spaces for consistent results across devices.Xlib supports device-independent color spaces derivable from the CIE XYZcolor space.This includes the CIE XYZ, xyY, L*u*v*, and L*a*b* color spaces as well asthe TekHVC color space.<P>This chapter discusses how to:<UL><P><LI><A HREF="colormap.html">Create, copy, and destroy a colormap</A><P><LI><A HREF="strings/">Specify colors by name or value</A><P><LI><A HREF="cell.html">Allocate, modify, and free color cells</A><P><LI><A HREF="modifying-and-querying-cells.html">Read entries in a colormap</A><P><LI><A HREF="converting-between-color-spaces.html">Convert between color spaces</A><P><LI><A HREF="callback/">Control aspects of color conversion</A><P><LI><A HREF="gamut-querying/">Query the color gamut of a screen</A><P><LI><A HREF="management-extensions/">Add new color spaces</A></UL><P>All functions, types, and symbols in this chapter with the prefix ``Xcms''are defined in<B><TT><X11/Xcms.h>.</TT></B>The remaining functions and types are defined in<B><TT><X11/Xlib.h>.</TT></B><A NAME="colormap"></A><P>Functions in this chapter manipulate the representation of color on thescreen.For each possible value that a pixel can take in a window,there is a color cell in the colormap.For example, if a window is four bits deep, pixel values 0 through 15 are defined. A colormap is a collection of color cells.A color cell consists of a triple of red, green, and blue values.The hardware imposes limits on the number of significantbits in these values.As each pixel is read out of display memory, the pixelis looked up in a colormap.The RGB value of the cell determines what color is displayed on the screen.On a grayscale display with a black-and-white monitor, the values are combined to determine the brightness on the screen.<P>Typically, an application allocates color cells or sets of color cellsto obtain the desired colors.The client can allocate read-only cells.In which case, the pixel values for these colors can be shared among multiple applications, and the RGB value of the cell cannot be changed.If the client allocates read/write cells,they are exclusively owned by the client,and the color associated with the pixel value can be changed at will.Cells must be allocated (and, if read/write, initialized with an RGB value)by a client to obtain desired colors.The use of pixel value for anunallocated cell results in an undefined color.<P>Because colormaps are associated with windows, X supports displayswith multiple colormaps and, indeed, different types of colormaps.If there are insufficient colormap resources in the display,some windows will display in their true colors, and otherswill display with incorrect colors.A window manager usually controls which windows are displayedin their true colors if more than one colormap is required forthe color resources the applications are using.At any time, there is a set of installed colormaps for a screen.Windows using one of the installed colormaps display with true colors, andwindows using other colormaps generally display with incorrect colors.You can control the set of installed colormaps by using<B><A HREF="../window-and-session-manager/XInstallColormap.html">XInstallColormap()</A></B>and<B><A HREF="../window-and-session-manager/XUninstallColormap.html">XUninstallColormap()</A></B>.<P>Colormaps are local to a particular screen.Screens always have a default colormap,and programs typically allocate cells out of this colormap.Generally, you should not write applications that monopolize color resources.Although some hardware supports multiple colormaps installed at one time,many of the hardware displaysbuilt today support only a single installed colormap, so the primitivesare written to encourage sharing of colormap entries between applications.<P>The <B><A HREF="../display/display-macros.html#DefaultColormap">DefaultColormap()</A></B>macro returns the default colormap.The <B><A HREF="../display/display-macros.html#DefaultVisual">DefaultVisual()</A></B>macroreturns the default visual type for the specified screen.<!.IN "Color map">Possible visual types are <B><A HREF="../window/visual-types.html">StaticGray</A></B> , <B><A HREF="../window/visual-types.html">GrayScale</A></B> , <B><A HREF="../window/visual-types.html">StaticColor</A></B> ,<B><A HREF="../window/visual-types.html">PseudoColor</A></B> , <B><A HREF="../window/visual-types.html">TrueColor</A></B> , or <B><A HREF="../window/visual-types.html">DirectColor</A></B> (see "<A HREF="../window/visual-types.html">Visual Types</A>").<H5 ALIGN=right><I>Next: <A HREF="structures.html">Color Structures</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>⌨️ 快捷键说明
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