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<HTML><HEAD><TITLE>Xlib Programming Manual: Standard Colormaps</TITLE></HEAD><BODY><H1 ALIGN=center>14.3 Standard Colormaps</H1>Applications with color palettes, smooth-shaded drawings, or digitizedimages demand large numbers of colors. In addition, these applications often require an efficient mapping from color triples to pixel values that display the appropriate colors.<P>As an example, consider a three-dimensional display program that wants to draw a smoothly shaded sphere. At each pixel in the image of the sphere, the program computes the intensity and color of lightreflected back to the viewer. The result of each computation is a triple of RGBcoefficients in the range 0.0 to 1.0. To draw the sphere, the program needs a colormap that provides alarge range of uniformly distributed colors. The colormap should be arranged so that the program canconvert its RGB triples into pixel values very quickly,because drawing the entire sphere requires many suchconversions.<P>On many current workstations,the display is limited to 256 or fewer colors. Applications must allocate colors carefully, not only to make sure they cover the entire range they need but also to make use of as many of the available colors as possible.On a typical X display, many applications are active at once.Most workstations have only one hardware look-up table for colors,so only one application colormap can be installed at a given time.The application using the installed colormap is displayed correctly, and the other applications go technicolor and aredisplayed with false colors.<P>As another example, consider a user who is running an image processing program to display earth-resources data. The image processing program needs a colormap set up with 8 reds, 8 greens, and 4 blues, for a total of 256 colors.Because some colors are already in use in the default colormap, the image processing program allocates and installs a new colormap.<P>The user decides to alter some of the colors in the imageby invoking a color palette program to mix and choose colors.The color palette program also needs acolormap with eight reds, eight greens, and four blues, so just likethe image processing program, it must allocate andinstall a new colormap.<P>Because only one colormap can be installed at a time,the color palette may be displayed incorrectlywhenever the image processing program is active.Conversely, whenever the palette program is active, the image may be displayed incorrectly. The user can never match or compare colors in the palette and image.Contention for colormap resources can be reduced if applicationswith similar color needs share colormaps.<P>The image processing program and the color palette program could share the same colormap if there existed a convention that describedhow the colormap was set up. Whenever either program was active, both would be displayed correctly.<P>The standard colormap properties define a set of commonly usedcolormaps. Applications that share these colormaps and conventions display true colors more often and provide a better interface to the user.<P>Standard colormaps allow applications to share commonly used colorresources. This allows many applications to be displayed in true colorssimultaneously, even when each application needs an entirely filledcolormap.<P>Several standard colormaps are described in this section.Usually, a window manager creates these colormaps.Applications should use the standard colormaps if they already exist.<P>To allocate an<B>XStandardColormap</B>structure, use<B><A HREF="XAllocStandardColormap.html">XAllocStandardColormap()</A></B>.<A NAME="XStandardColormap"></A><P>The <B>XStandardColormap</B>structure contains:<P><PRE><CODE>/* Hints */#define <B>ReleaseByFreeingColormap</B> ( (XID) 1L)/* Values */<!.IN "XStandardColormap" "" "@DEF@">typedef struct { Colormap colormap; unsigned long red_max; unsigned long red_mult; unsigned long green_max; unsigned long green_mult; unsigned long blue_max; unsigned long blue_mult; unsigned long base_pixel; VisualID visualid; XID killid;} XStandardColormap;</PRE></CODE><P>The <B>colormap</B> member is the colormap created by the<B><A HREF="../../color/XCreateColormap.html">XCreateColormap()</A></B>function.The <B>red_max</B>, <B>green_max</B>, and <B>blue_max</B>members give the maximumred, green, and blue values, respectively. Each color coefficient ranges from zero to its max, inclusive. For example,a common colormap allocation is 3/3/2 (3 planes for red, 3planes for green, and 2 planes for blue). This colormap would have red_max = 7, green_max = 7, and blue_max = 3. An alternate allocation that uses only 216 colors is red_max = 5, green_max = 5, and blue_max = 5.<P>The <B>red_mult</B>, <B>green_mult</B>, and <B>blue_mult</B>members give thescale factors used to compose a full pixel value. (See the discussion of the <B>base_pixel</B> members for further information.)For a 3/3/2 allocation, red_mult might be 32,green_mult might be 4, and blue_mult might be 1. For a 6-colors-each allocation, red_mult might be 36, green_mult might be 6, and blue_mult might be 1.<P>The <B>base_pixel</B> member gives the base pixel value used tocompose a full pixel value. Usually, <B>the</B> base_pixel is obtained from a call to the <B><A HREF="../../color/XAllocColorPlanes.html">XAllocColorPlanes()</A></B>function. Given integer red, green, and blue coefficients in their appropriate ranges, one then can compute a corresponding pixel value byusing the following expression:<P><BLOCKQUOTE><I>(r * red_mult + g * green_mult + b * blue_mult + base_pixel) & 0xFFFFFFFF</I></BLOCKQUOTE><P>For <B><A HREF="../../window/visual-types.html">GrayScale</A></B>colormaps, only the <B>colormap</B>, <B>red_max</B>, <B>red_mult</B>, and <B>base_pixel</B> members are defined. The other members are ignored. To compute a <B>GrayScale</B>pixel value, use the following expression:<P><BLOCKQUOTE><I>(gray * red_mult + base_pixel) & 0xFFFFFFFF</I></BLOCKQUOTE><P>Negative multipliers can be represented by converting the 2'scomplement representation of the multiplier into an unsigned long andstoring the result in the appropriate _mult field.The step of masking by 0xFFFFFFFF effectively converts the resultingpositive multiplier into a negative one.The masking step will take place automatically on many machine architectures,depending on the size of the integer type used to do the computation,<P>The <B>visualid</B> member gives the ID number of the visual from which thecolormap was created.The <B>killid</B> member gives a resource ID that indicates whetherthe cells held by this standard colormap are to be released by freeing the colormap ID or by calling the<B><A HREF="../../window-and-session-manager/XKillClient.html">XKillClient()</A></B>function on the indicated resource.(Note that this method is necessary for allocating out of an existing colormap.)<P>The properties containing the <B><A HREF="./#XStandardColormap">XStandardColormap</A></B> information have the type RGB_COLOR_MAP.<P>The remainder of this section discusses standard colormap properties and atomsas well as how to manipulate standard colormaps.<H5 ALIGN=right><I>Next: <A HREF="properties-and-atoms.html">Standard Colormap Properties and Atoms</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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