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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html> <head> <title> Drawables and Pixmaps </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="Visuals and Colormaps" href= "sec-gdkvisual.html"> <link rel="NEXT" title="Events" href="sec-gdkevent.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-gdkvisual.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="sec-gdkevent.html"><font color="#0000ff" size= "2"><b>Next >>></b></font></a> </td> </tr> </table> </div> <div class="SECT1"> <h1 class="SECT1"> <a name="SEC-GDKDRAWABLE">Drawables and Pixmaps</a> </h1> <p> A <i class="FIRSTTERM">pixmap</i> is an off-screen buffer you can draw graphics into. After drawing into a pixmap, you can copy it to a window, causing it to appear on the screen (when the window is visible). (You can also draw into a window directly, of course. Using a pixmap as a buffer allows you to rapidly update the screen without repeating a series of primitive drawing operations.) Pixmaps are also good to store image data loaded from disk, such as icons and logos. You can then copy the image to a window. In GDK, the pixmap type is called <span class= "STRUCTNAME">GdkPixmap</span>. A pixmap with a single bit representing each pixel is called a <span class= "STRUCTNAME">bitmap</span>; GDK's bitmap type is <span class="STRUCTNAME">GdkBitmap</span>. "Bitmap" is not really a separate type; from X's point of view, it is simply a pixmap with a depth of 1. Like windows, pixmaps are server-side resources. </p> <p> In X terminology, a <i class="FIRSTTERM">drawable</i> is anything you can draw graphics on. GDK has a corresponding type, called <span class="STRUCTNAME">GdkDrawable</span>. Drawables include windows, pixmaps, and bitmaps. Here is how the types are defined in GDK: </p> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="PROGRAMLISTING"> typedef struct _GdkWindow GdkWindow;typedef struct _GdkWindow GdkPixmap;typedef struct _GdkWindow GdkBitmap;typedef struct _GdkWindow GdkDrawable; </pre> </td> </tr> </table> <p> On the client side, pixmaps and bitmaps are just <span class="STRUCTNAME">GdkWindow</span>s with type <span class= "STRUCTNAME">GDK_WINDOW_PIXMAP</span>. <span class= "STRUCTNAME">GdkDrawable</span> is used in function declarations when either a window or a pixmap is an acceptable argument. Functions that draw graphics take either type; functions that move windows around or set window manager hints accept only windows. Only windows can receive events. <span class="STRUCTNAME"> GDK_INPUT_ONLY</span> windows are a special case; they are not drawables and you can't draw on them. </p> <p> Three of the four logical combinations of "window features" and drawability actually exist: </p> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="PROGRAMLISTING"> Drawable Not DrawableWindow Features Normal Window Input Only WindowNo Window Features Pixmap/Bitmap --- </pre> </td> </tr> </table> <p> Unfortunately, all three of these logically distinct cases appear the same from a type-checking point of view. So be careful not to use the wrong one. Also keep in mind that a normal window is not drawable until it actually appears on the screen; you should wait until you receive an expose event before you draw. Expose events are covered in <a href="sec-gdkevent.html#SEC-EXPOSEEVENTS">the section called <i>Expose Events</i></a>. </p> <p> Like <span class="STRUCTNAME">GdkWindow</span>, a <span class="STRUCTNAME">GdkPixmap</span> is merely a client-side handle for an object located on the X server. Because of this, some things are entirely infeasible from a performance point of view; notably, if you are doing anything which requires significant manipulation of individual pixels, drawables will be far too slow. On the other hand, copying a pixmap to a window is not as slow as you might think, because both objects are on the same machine. </p> <p> Creating a pixmap is much easier than creating a window, because most of the window attributes are not relevant to pixmaps. The function is <tt class="FUNCTION"> gdk_pixmap_new()</tt> (<a href= "sec-gdkdrawable.html#FL-PIXNEW">Figure 4</a>). It accepts an initial size, and a bit depth. If a depth of <span class="STRUCTNAME">-1</span> is given, the depth is copied from its <span class="STRUCTNAME">GdkWindow</span> argument. You can't choose an arbitrary number for the depth---the server will not support all depths, and the pixmap's depth must match the depth of any windows you plan to copy it to. To destroy a pixmap, call <tt class= "FUNCTION">gdk_pixmap_unref()</tt>. </p> <p> The <span class="STRUCTNAME">GdkWindow</span> argument to <tt class="FUNCTION">gdk_pixmap_new()</tt> may not seem strictly necessary. However, the function wraps <tt class= "FUNCTION">XCreatePixmap()</tt>, which takes an X window as an argument. It uses this argument to determine which <i class="FIRSTTERM">screen</i> to create the window on; some X servers have multiple displays. Screens are an Xlib concept totally concealed by GDK; GDK supports only one screen at a time. Thus the <span class="STRUCTNAME"> window</span> argument to <tt class="FUNCTION"> gdk_pixmap_new()</tt> seems mysterious from a GDK point of view. </p> <div class="FIGURE"> <a name="FL-PIXNEW"></a> <div class="FUNCSYNOPSIS"> <a name="FL-PIXNEW.SYNOPSIS"></a> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="FUNCSYNOPSISINFO">#include <gdk/gdk.h></pre> </td> </tr> </table> <p> <code><code class="FUNCDEF">GdkPixmap* <tt class= "FUNCTION">gdk_pixmap_new</tt></code>(GdkWindow* <tt class="PARAMETER"><i>window</i></tt>, gint <tt class= "PARAMETER"><i>width</i></tt>, gint <tt class= "PARAMETER"><i>height</i></tt>, gint <tt class= "PARAMETER"><i>depth</i></tt>);</code> </p> <p> <code><code class="FUNCDEF">void <tt class="FUNCTION"> gdk_pixmap_unref</tt></code>(GdkPixmap* <tt class= "PARAMETER"><i>pixmap</i></tt>);</code> </p> </div> <p> <b>Figure 4. <span class="STRUCTNAME">GdkPixmap</span> Constructor</b> </p> </div> </div> <div class="NAVFOOTER"> <br> <br> <table width="100%" border="0" bgcolor="#ffffff" cellpadding= "1" cellspacing="0"> <tr> <td width="25%" bgcolor="#ffffff" align="left"> <a href="sec-gdkvisual.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="sec-gdkevent.html"><font color="#0000ff" size= "2"><b>Next >>></b></font></a> </td> </tr> <tr> <td colspan="2" align="left"> <font color="#000000" size="2"><b>Visuals and Colormaps</b></font> </td> <td colspan="2" align="right"> <font color="#000000" size="2"><b>Events</b></font> </td> </tr> </table> </div> </body></html>⌨️ 快捷键说明
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