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<div class="appendix" title="Appendix F. Drawing With GDK">
<div class="titlepage"><div><div><h2 class="title">
<a name="chapter-gdk-drawing"></a>Appendix F. Drawing With GDK</h2></div></div></div>
<p>The GDK drawing API described here is now deprecated, in favour of Cairo. See the <a class="link" href="chapter-drawingarea.html" title="Chapter 15. The Drawing Area Widget">DrawingArea</a> chapter for information about the Cairo API.</p>
<p>
Gdk graphics contexts (<code class="classname">Gdk::GC</code>) are a
server-side resource. They contain information that describes how
drawing is to be done. This provides for fewer arguments to the
drawing methods, and less communication between the client and the
server. The following example shows how to set up a graphics context
with a foreground color of red for drawing.
</p>
<pre class="programlisting">Gdk::GC some_gc;
some_gc.create(get_window());
Gdk::Color some_color;
Gdk::Colormap some_colormap(Gdk::Colormap::get_system());
some_color.set_red(65535);
some_color.set_green(0);
some_color.set_blue(0);
some_colormap.alloc(some_color);
some_gc.set_foreground(some_color);</pre>
<p>
The first two lines create the graphics context and assign it to the
appropriate widget. The <code class="methodname">get_window()</code> method is
a part of the <code class="classname">Gtk::Widget</code> class, so if you put
this code into a derived widget's implementation then you can call it
just as it is, otherwise you'd use
<code class="methodname">some_widget.get_window()</code>.
</p>
<p>
The next two lines create the <code class="classname">Gdk::Color</code> and
<code class="classname">Gdk::Colormap</code>. After setting the color values
you then need to allocate the color. The system figures out what to
do in this case. The colormap contains information about how colors
can be displayed on your screen, and is able to allocate the
requested color. For example, on a display of only 256 colors the
exact color requested may not be available, so the closest color to
the one requested will be used instead. The final line sets the color
as the foreground color.
</p>
<p>
There are a number of attributes that can be set for a graphics
context. There's the foreground and background color. When drawing
lines, you can set the thickness of the line with
<code class="methodname">set_line_width()</code>. Whether a solid or dashed line
is drawn can be set with <code class="methodname">set_line_style()</code>. The
size and proportions of the dashes are set with
<code class="methodname">set_dashes</code>. How two lines join together, whether
round or pointed or beveled off, is set with
<code class="methodname">set_join_style()</code>. Other things that can be set
within a graphics context include font style, stippling and tiling
for the filling of solid polygons.
</p>
<p>
Graphics contexts are central to drawing with Gdk, because nearly all Gdk
drawing functions and many Pango functions take a
<code class="classname">Gdk::GC</code> object as an argument. So although Cairo
has largely superceded many Gdk drawing functions, you're still likely to
run into <code class="classname">Gdk::GC</code> objects quite often, so it's
important to know what they are and how they're used.
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