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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html> <head> <title> Graphics Contexts </title> <meta name="GENERATOR" content= "Modular DocBook HTML Stylesheet Version 1.45"> <link rel="HOME" title="GTK+ / Gnome Application Development" href="ggad.html"> <link rel="UP" title="GDK Basics" href="cha-gdk.html"> <link rel="PREVIOUS" title="Fonts" href="sec-gdkfont.html"> <link rel="NEXT" title="Drawing" href="z132.html"> </head> <body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink= "#840084" alink="#0000FF"> <div class="NAVHEADER"> <table width="100%" border="0" bgcolor="#ffffff" cellpadding= "1" cellspacing="0"> <tr> <th colspan="4" align="center"> <font color="#000000" size="2">GTK+ / Gnome Application Development</font> </th> </tr> <tr> <td width="25%" bgcolor="#ffffff" align="left"> <a href="sec-gdkfont.html"><font color="#0000ff" size= "2"><b><<< Previous</b></font></a> </td> <td width="25%" colspan="2" bgcolor="#ffffff" align= "center"> <font color="#0000ff" size="2"><b><a href="ggad.html"> <font color="#0000ff" size="2"><b> Home</b></font></a></b></font> </td> <td width="25%" bgcolor="#ffffff" align="right"> <a href="z132.html"><font color="#0000ff" size="2"><b> Next >>></b></font></a> </td> </tr> </table> </div> <div class="SECT1"> <h1 class="SECT1"> <a name="SEC-GC">Graphics Contexts</a> </h1> <p> A <i class="FIRSTTERM">graphics context</i>, or GC, is simply a set of parameters to be used when drawing (such as color, clip mask, font, and so on). It is a server-side resource, just as pixmaps and windows are. GCs reduce the number of arguments to the GDK drawing functions, and also reduce the number of parameters passed from client to server with each drawing request. </p> <p> A graphics context can be created with a <span class= "STRUCTNAME">GdkGCValues</span> struct, analagous to <span class="STRUCTNAME">GdkWindowAttr</span>; the struct contains all the interesting features of a graphics context, and you pass <tt class="FUNCTION"> gdk_gc_new_with_values()</tt> flags indicating which fields are valid. The other fields retain their default value. You can also create an all-default GC with <tt class= "FUNCTION">gdk_gc_new()</tt> (this is usually easier). Functions are provided to change GC settings after the GC is created as well---but remember that each change requires a message to the X server. These functions are summarized in <a href="sec-gc.html#FL-GDKGC">Figure 16</a>. The attributes of a GC, and the flags used as the final argument to <tt class="FUNCTION"> gdk_gc_new_with_values()</tt>, are summarized in <a href= "sec-gc.html#TAB-GDKGC">Table 6</a>. </p> <p> All GCs are not interchangeable; they are tied to a particular depth and visual. The GC's depth and visual must match the depth and visual of the drawable you are drawing to. A GC's depth and visual are taken from the <span class= "STRUCTNAME">GdkWindow*</span> argument to <tt class= "FUNCTION">gdk_gc_new()</tt>, so the easiest way to handle this issue is to create the GC with the window you plan to draw on. </p> <p> <span class="STRUCTNAME">GdkGCValues</span> is a nice summary of a GC's attributes: </p> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="PROGRAMLISTING"> typedef struct _GdkGCValues GdkGCValues;struct _GdkGCValues{ GdkColor foreground; GdkColor background; GdkFont *font; GdkFunction function; GdkFill fill; GdkPixmap *tile; GdkPixmap *stipple; GdkPixmap *clip_mask; GdkSubwindowMode subwindow_mode; gint ts_x_origin; gint ts_y_origin; gint clip_x_origin; gint clip_y_origin; gint graphics_exposures; gint line_width; GdkLineStyle line_style; GdkCapStyle cap_style; GdkJoinStyle join_style;}; </pre> </td> </tr> </table> <p> The <span class="STRUCTNAME">foreground</span> color is the "pen color" used to draw lines, circles, and other shapes. The purpose of the <span class="STRUCTNAME"> background</span> color depends on the particular drawing operation. These colors must be allocated in the current colormap with <tt class="FUNCTION">gdk_color_alloc()</tt>. </p> <p> The <span class="STRUCTNAME">font</span> field is unused: in Xlib, it specifies the font to use when drawing text. In GDK, it used to have the same purpose; but now the GDK routines for drawing text all require a <span class= "STRUCTNAME">GdkFont*</span> argument instead. An Xlib graphics context can only store plain fonts, but a <span class="STRUCTNAME">GdkFont</span> can also represent a fontset (used to render some foreign languages). GDK should probably store a font field in its <span class= "STRUCTNAME">GdkGC</span> instead of requiring a font argument to the text-drawing functions, but it doesn't. </p> <p> The <span class="STRUCTNAME">function</span> field specifies how each pixel being drawn is combined with the pixel that already exists in the drawable. There are many possible values, but only two are ever used: </p> <ul> <li> <p> <span class="STRUCTNAME">GDK_COPY</span> is the default. It ignores the existing pixel (just writes the new pixel over it). </p> </li> <li> <p> <span class="STRUCTNAME">GDK_XOR</span> combines the old and new pixels in an invertable way. That is, if you perform exactly the same <span class="STRUCTNAME"> GDK_XOR</span> operation twice, the first draw is undone by the second. <span class="STRUCTNAME"> GDK_XOR</span> is often used for "rubberbanding," since it makes it easy to restore the original contents of the drawable once rubberbanding is complete. </p> </li> </ul> <p> The <span class="STRUCTNAME">fill</span> field determines how the <span class="STRUCTNAME">tile</span> and <span class="STRUCTNAME">stipple</span> fields are used. A <span class="STRUCTNAME">tile</span> is a pixmap with the same depth as the destination drawable; it is copied over and over into the destination drawable---the origin of the first tile is (<span class="STRUCTNAME">ts_x_origin</span>, <span class="STRUCTNAME">ts_y_origin</span>). A <span class="STRUCTNAME">stipple</span> is a bitmap (pixmap with depth 1); stipples are also tiled starting at (<span class= "STRUCTNAME">ts_x_origin</span>, <span class="STRUCTNAME"> ts_y_origin</span>). Possible <span class="STRUCTNAME"> fill</span> values are: </p> <ul> <li> <p> <span class="STRUCTNAME">GDK_SOLID</span> means to ignore the <span class="STRUCTNAME">tile</span> and <span class="STRUCTNAME">stipple</span>. Shapes are drawn in the foreground and background colors. </p> </li> <li> <p> <span class="STRUCTNAME">GDK_TILED</span> means that shapes are drawn with the tile, instead of the foreground and background colors. Imagine a tiled surface underneath your drawable; drawing in <span class="STRUCTNAME">GDK_TILED</span> mode will scratch away the contents of the drawable, revealing the tiled surface underneath. </p> </li> <li> <p> <span class="STRUCTNAME">GDK_STIPPLED</span> is like <span class="STRUCTNAME">GDK_SOLID</span> with a bitmask defined by the stipple. That is, bits not set in the stipple are not drawn. </p> </li> <li> <p> <span class="STRUCTNAME">GDK_OPAQUE_STIPPLED</span> draws bits set in the stipple with the foreground color, and bits not set in the stipple with the background color. </p> </li> </ul> <p> Some X servers do not implement the more obscure function and fill modes very efficiently. Don't be surprised if using them noticeably slows down drawing. </p> <p> The optional <span class="STRUCTNAME">clip_mask</span> is a bitmap; only bits set in this bitmap will be drawn. The mapping from the clip mask to the drawable is determined by <span class="STRUCTNAME">clip_x_origin</span> and <span class="STRUCTNAME">clip_y_origin</span>; these define the drawable coordinates corresponding to (0,0) in the clip mask. It is also possible to set a clip rectangle (the most common and useful form of clipping) or a clip region (a region is an arbitrary area on the screen, typically a polygon or list of rectangles). To set a clip rectangle, use <tt class="FUNCTION">gdk_gc_set_clip_rectangle()</tt>: </p> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="PROGRAMLISTING"> GdkRectangle clip_rect; clip_rect.x = 10; clip_rect.y = 20; clip_rect.width = 200; clip_rect.height = 100; gdk_gc_set_clip_rectangle(gc, &clip_rect); </pre> </td> </tr> </table> <p> To turn off clipping, set the clip rectangle, clip region, or clip mask to <span class="STRUCTNAME">NULL</span>. </p> <p> The <span class="STRUCTNAME">subwindow_mode</span> of a GC only matters if the drawable is a window. The default setting is <span class="STRUCTNAME"> GDK_CLIP_BY_CHILDREN</span>; this means that child windows are not affected by drawing on parent windows. This preserves the illusion that child windows are "on top" of parents, and child windows are opaque. <span class= "STRUCTNAME">GDK_INCLUDE_INFERIORS</span> will draw right over the top of any child windows, overwriting any graphics the child windows may contain; normally this mode is not used. If you do use <span class="STRUCTNAME"> GDK_INCLUDE_INFERIORS</span>, you will probably use <span class="STRUCTNAME">GDK_XOR</span> as your drawing function, since it allows you to restore the child windows' previous contents. </p> <p> <span class="STRUCTNAME">graphics_exposures</span> is a boolean value which defaults to <span class="STRUCTNAME"> TRUE</span>; it determines whether <tt class="FUNCTION"> gdk_window_copy_area()</tt> sometimes generates expose events. <a href="sec-gdkevent.html#SEC-EXPOSEEVENTS">the section called <i>Expose Events</i></a> explained this in more detail. </p> <p> The final four GC values determine how lines are drawn. These values are used for drawing lines, including the borders of unfilled polygons and arcs. The <span class= "STRUCTNAME">line_width</span> field specifies the width of a line, in pixels. A line width of 0 specifies a "thin line"; thin lines are one-pixel lines that can be drawn very quickly (usually with hardware acceleration), but the exact pixels drawn depend on the X server in use. For consistent results, use a width of 1 instead.⌨️ 快捷键说明
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