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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sec-cairo-drawing-lines"></a>Drawing Straight Lines</h2></div></div></div>
<p>
Now that we understand the basics of the Cairo graphics library, we're
almost ready to start drawing. We'll start with the simplest of
drawing elements: the straight line. But first you need to know a
little bit about Cairo's coordinate system. The origin of the Cairo
coordinate system is located in the upper-left corner of the window
with positive x values to the right and positive y values going down.
</p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="icons/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>Since the Cairo graphics library was written with support for
multiple output targets (the X window system, PNG images, OpenGL,
etc), there is a distinction between user-space and device-space
coordinates. The mapping between these two coordinate systems
defaults to one-to-one so that integer values map roughly to pixels
on the screen, but this setting can be adjusted if desired.
Sometimes it may be useful to scale the coordinates so that the
full width and height of a window both range from 0 to 1 (the 'unit
square') or some other mapping that works for your application.
This can be done with the
<code class="methodname">Cairo::Context::scale()</code> function.</p></td></tr>
</table></div>
<p>
</p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cairo-example-lines"></a>Example</h3></div></div></div>
<p>
In this example, we'll construct a small but fully functional <span class="application">gtkmm</span>
program and draw some lines into the window. The lines are drawn by
creating a path and then stroking it. A path is created using the
functions <code class="methodname">Cairo::Context::move_to()</code> and
<code class="methodname">Cairo::Context::line_to()</code>. The function
<code class="methodname">move_to()</code> is similar to the act of lifting your
pen off of the paper and placing it somewhere else -- no line is drawn
between the point you were at and the point you moved to. To draw a
line between two points, use the <code class="methodname">line_to()</code>
function.
</p>
<p>
After you've finished creating your path, you still haven't
drawn anything visible yet. To make the path visible, you must use the
function <code class="methodname">stroke()</code> which will stroke the current
path with the line width and style specified in your
<code class="classname">Cairo::Context</code> object. After stroking, the
current path will be cleared so that you can start on your next path.
</p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="icons/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>Many Cairo drawing functions have a <code class="methodname">_preserve()</code>
variant. Normally drawing functions such as
<code class="methodname">clip()</code>, <code class="methodname">fill()</code>, or
<code class="methodname">stroke()</code> will clear the current path. If you
use the <code class="methodname">_preserve()</code> variant, the current path
will be retained so that you can use the same path with the next
drawing function.</p></td></tr>
</table></div>
<div class="figure">
<a name="figure-drawingarea-lines"></a><p class="title"><b>Figure 17.1. Drawing Area - Lines</b></p>
<div class="figure-contents"><div class="screenshot"><div><img src="figures/drawingarea_lines.png" alt="Drawing Area - Lines"></div></div></div>
</div>
<br class="figure-break"><p><a class="ulink" href="http://git.gnome.org/browse/gtkmm-documentation/tree/examples/book/drawingarea/simple?h=gtkmm-3-24" target="_top">Source Code</a></p>
<p>File: <code class="filename">myarea.h</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#ifndef GTKMM_EXAMPLE_MYAREA_H
#define GTKMM_EXAMPLE_MYAREA_H
#include <gtkmm/drawingarea.h>
class MyArea : public Gtk::DrawingArea
{
public:
MyArea();
virtual ~MyArea();
protected:
//Override default signal handler:
bool on_draw(const Cairo::RefPtr<Cairo::Context>& cr) override;
};
#endif // GTKMM_EXAMPLE_MYAREA_H
</pre>
<p>File: <code class="filename">main.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "myarea.h"
#include <gtkmm/application.h>
#include <gtkmm/window.h>
int main(int argc, char** argv)
{
auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example");
Gtk::Window win;
win.set_title("DrawingArea");
MyArea area;
win.add(area);
area.show();
return app->run(win);
}
</pre>
<p>File: <code class="filename">myarea.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "myarea.h"
#include <cairomm/context.h>
MyArea::MyArea()
{
}
MyArea::~MyArea()
{
}
bool MyArea::on_draw(const Cairo::RefPtr<Cairo::Context>& cr)
{
Gtk::Allocation allocation = get_allocation();
const int width = allocation.get_width();
const int height = allocation.get_height();
// coordinates for the center of the window
int xc, yc;
xc = width / 2;
yc = height / 2;
cr->set_line_width(10.0);
// draw red lines out from the center of the window
cr->set_source_rgb(0.8, 0.0, 0.0);
cr->move_to(0, 0);
cr->line_to(xc, yc);
cr->line_to(0, height);
cr->move_to(xc, yc);
cr->line_to(width, yc);
cr->stroke();
return true;
}
</pre>
<p>
This program contains a single class, <code class="classname">MyArea</code>,
which is a subclass of <code class="classname">Gtk::DrawingArea</code> and
contains an <code class="methodname">on_draw()</code> member function.
This function is called whenever the image in the drawing area needs to
be redrawn. It is passed a <code class="classname">Cairo::RefPtr</code>
pointer to a <code class="classname">Cairo::Context</code> that we use
for the drawing.
The actual drawing code sets the color we want to use for drawing by
using <code class="methodname">set_source_rgb()</code> which takes arguments
defining the Red, Green, and Blue components of the desired color
(valid values are between 0 and 1). After setting the color, we
created a new path using the functions <code class="methodname">move_to()</code>
and <code class="methodname">line_to()</code>, and then stroked this path with
<code class="methodname">stroke()</code>.
</p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Tip: Drawing with relative coordinates">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="icons/tip.png"></td>
<th align="left">Drawing with relative coordinates</th>
</tr>
<tr><td align="left" valign="top"><p>In the example above we drew everything using absolute coordinates. You can also draw using
relative coordinates. For a straight line, this is done with the
function <code class="methodname">Cairo::Context::rel_line_to()</code>.</p></td></tr>
</table></div>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cairo-line-styles"></a>Line styles</h3></div></div></div>
<p>
In addition to drawing basic straight lines, there are a number of
things that you can customize about a line. You've already seen
examples of setting a line's color and width, but there are others
as well.
</p>
<p>
If you've drawn a series of lines that form a path, you may
want them to join together in a certain way. Cairo offers
three different ways to join lines together: Miter, Bevel, and
Round. These are show below:
</p>
<div class="figure">
<a name="figure-cairo-joins"></a><p class="title"><b>Figure 17.2. Different join types in Cairo</b></p>
<div class="figure-contents"><div class="screenshot"><div><img src="figures/cairo_joins.png" alt="Different join types in Cairo"></div></div></div>
</div>
<br class="figure-break"><p>
The line join style is set using the function
<code class="methodname">Cairo::Context::set_line_join()</code>.
</p>
<p>
Line ends can have different styles as well. The default style
is for the line to start and stop exactly at the destination
points of the line. This is called a Butt cap. The other
options are Round (uses a round ending, with the center of the
circle at the end point) or Square (uses a squared ending, with
the center of the square at the end point). This setting is set
using the function
<code class="methodname">Cairo::Context::set_line_cap()</code>.
</p>
<p>
There are other things you can customize as well, including
creating dashed lines and other things. For more information, see
the Cairo API documentation.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="sec-cairo-thin-lines"></a>Drawing thin lines</h3></div></div></div>
<p>
If you try to draw one pixel wide lines, you may notice that the line
sometimes comes up blurred and wider than it ought to be.
This happens because Cairo will try to draw from the selected position,
to both sides (half to each), so if you're positioned right on the
intersection of the pixels, and want a one pixel wide line, Cairo will try
to use half of each adjacent pixel, which isn't possible (a pixel is the
smallest unit possible). This happens when the width of the line is an
odd number of pixels (not just one pixel).
</p>
<p>
The trick is to position in the middle of the pixel where you want the
line to be drawn, and thus guaranteeing you get the desired results.
See <a class="ulink" href="http://cairographics.org/FAQ/#sharp_lines" target="_top">Cairo FAQ</a>.
</p>
<div class="figure">
<a name="figure-drawingarea-thin-lines"></a><p class="title"><b>Figure 17.3. Drawing Area - Thin Lines</b></p>
<div class="figure-contents"><div class="screenshot"><div><img src="figures/drawingarea_thin_lines.png" alt="Drawing Area - Thin Lines"></div></div></div>
</div>
<br class="figure-break"><p><a class="ulink" href="http://git.gnome.org/browse/gtkmm-documentation/tree/examples/book/drawingarea/thin_lines?h=gtkmm-3-24" target="_top">Source Code</a></p>
<p>File: <code class="filename">examplewindow.h</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#ifndef GTKMM_EXAMPLEWINDOW_H
#define GTKMM_EXAMPLEWINDOW_H
#include <gtkmm/window.h>
#include <gtkmm/grid.h>
#include <gtkmm/checkbutton.h>
#include "myarea.h"
class ExampleWindow : public Gtk::Window
{
public:
ExampleWindow();
virtual ~ExampleWindow();
protected:
//Signal handlers:
void on_button_toggled();
private:
Gtk::Grid m_Container;
MyArea m_Area_Lines;
Gtk::CheckButton m_Button_FixLines;
};
#endif //GTKMM_EXAMPLEWINDOW_H
</pre>
<p>File: <code class="filename">myarea.h</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#ifndef GTKMM_EXAMPLE_MYAREA_H
#define GTKMM_EXAMPLE_MYAREA_H
#include <gtkmm/drawingarea.h>
class MyArea : public Gtk::DrawingArea
{
public:
MyArea();
virtual ~MyArea();
void fix_lines(bool fix = true);
void force_redraw();
protected:
//Override default signal handler:
bool on_draw(const Cairo::RefPtr<Cairo::Context>& cr) override;
private:
double m_fix;
};
#endif // GTKMM_EXAMPLE_MYAREA_H
</pre>
<p>File: <code class="filename">main.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "examplewindow.h"
#include <gtkmm/application.h>
int main(int argc, char* argv[])
{
auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example");
ExampleWindow window;
//Shows the window and returns when it is closed.
return app->run(window);
}
</pre>
<p>File: <code class="filename">myarea.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "myarea.h"
MyArea::MyArea()
: m_fix (0)
{
set_size_request (200, 100);
}
MyArea::~MyArea()
{
}
bool MyArea::on_draw(const Cairo::RefPtr<Cairo::Context>& cr)
{
Gtk::Allocation allocation = get_allocation();
const int width = allocation.get_width();
const int height = allocation.get_height();
cr->set_line_width(1.0);
// draw one line, every two pixels
// without the 'fix', you won't notice any space between the lines,
// since each one will occupy two pixels (width)
for (int i = 0; i < width; i += 2)
{
cr->move_to(i + m_fix, 0);
cr->line_to(i + m_fix, height);
}
cr->stroke();
return true;
}
// Toogle between both values (0 or 0.5)
void MyArea::fix_lines(bool fix)
{
// to get the width right, we have to draw in the middle of the pixel
m_fix = fix ? 0.5 : 0.0;
force_redraw();
}
// force the redraw of the image
void MyArea::force_redraw()
{
auto win = get_window();
if (win)
{
Gdk::Rectangle r(0, 0, get_allocation().get_width(), get_allocation().get_height());
win->invalidate_rect(r, false);
}
}
</pre>
<p>File: <code class="filename">examplewindow.cc</code> (For use with gtkmm 3, not gtkmm 2)
</p>
<pre class="programlisting">
#include "examplewindow.h"
ExampleWindow::ExampleWindow()
: m_Button_FixLines("Fix lines")
{
set_title("Thin lines example");
m_Container.set_orientation(Gtk::ORIENTATION_HORIZONTAL);
m_Container.add(m_Area_Lines);
m_Container.add(m_Button_FixLines);
add(m_Container);
m_Button_FixLines.signal_toggled().connect(
sigc::mem_fun(*this, &ExampleWindow::on_button_toggled));
// Synchonize the drawing in m_Area_Lines with the state of the toggle button.
on_button_toggled();
show_all_children();
}
ExampleWindow::~ExampleWindow()
{
}
void ExampleWindow::on_button_toggled()
{
m_Area_Lines.fix_lines(m_Button_FixLines.get_active());
}
</pre>
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