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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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Drawables and Pixmaps
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<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>
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<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 Drawable
Window Features Normal Window Input Only Window
No 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>
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<pre class="FUNCSYNOPSISINFO">
#include <gdk/gdk.h>
</pre>
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<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>
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