<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Drawing Curved Lines</title> <link rel="stylesheet" href="style.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.73.2"> <link rel="start" href="index.html" title="Programming with gtkmm"> <link rel="up" href="ch15.html" title="Chapter 15. The Drawing Area Widget"> <link rel="prev" href="ch15s02.html" title="Drawing Straight Lines"> <link rel="next" href="ch15s04.html" title="Drawing Arcs and Circles"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr><th colspan="3" align="center">Drawing Curved Lines</th></tr> <tr> <td width="20%" align="left"> <a accesskey="p" href="ch15s02.html"><img src="../icons/prev.png" alt="Prev"></a> </td> <th width="60%" align="center">Chapter 15. The Drawing Area Widget</th> <td width="20%" align="right"> <a accesskey="n" href="ch15s04.html"><img src="../icons/next.png" alt="Next"></a> </td> </tr> </table> <hr> </div> <div class="sect1" lang="en"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="id2585435"></a>Drawing Curved Lines</h2></div></div></div> <p> In addition to drawing straight lines Cairo allows you to easily draw curved lines (technically a cubic Bézier spline) using the <code class="function">Cairo::Context::curve_to()</code> and <code class="function">Cairo::Context::rel_curve_to()</code> functions. These functions take coordinates for a destination point as well as coordinates for two 'control' points. This is best explained using an example, so let's dive in. </p> <div class="sect2" lang="en"> <div class="titlepage"><div><div><h3 class="title"> <a name="id2585466"></a>Example</h3></div></div></div> <p> This simple application draws a curve with Cairo and displays the control points for each end of the curve. </p> <div class="figure"> <a name="figure-drawingarea-curve"></a><p class="title"><b>Figure 15.3. Drawing Area - Lines</b></p> <div class="figure-contents"><div class="screenshot"><div><img src="../figures/drawingarea_curve.png" alt="Drawing Area - Lines"></div></div></div> </div> <br class="figure-break"><p><a class="ulink" href="../../../examples/book/drawingarea/curve" target="_top">Source Code</a></p> <p>File: <code class="filename">myarea.h</code> </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: virtual bool on_expose_event(GdkEventExpose* event); }; #endif // GTKMM_EXAMPLE_MYAREA_H </pre> <p>File: <code class="filename">main.cc</code> </p> <pre class="programlisting"> #include "myarea.h" #include <gtkmm/main.h> #include <gtkmm/window.h> int main(int argc, char** argv) { Gtk::Main kit(argc, argv); Gtk::Window win; win.set_title("DrawingArea"); MyArea area; win.add(area); area.show(); Gtk::Main::run(win); return 0; } </pre> <p>File: <code class="filename">myarea.cc</code> </p> <pre class="programlisting"> #include "myarea.h" #include <cairomm/context.h> MyArea::MyArea() { } MyArea::~MyArea() { } bool MyArea::on_expose_event(GdkEventExpose* event) { // This is where we draw on the window Glib::RefPtr<Gdk::Window> window = get_window(); if(window) { Gtk::Allocation allocation = get_allocation(); const int width = allocation.get_width(); const int height = allocation.get_height(); double x0=0.1, y0=0.5, // start point x1=0.4, y1=0.9, // control point #1 x2=0.6, y2=0.1, // control point #2 x3=0.9, y3=0.5; // end point Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context(); // clip to the area indicated by the expose event so that we only redraw // the portion of the window that needs to be redrawn cr->rectangle(event->area.x, event->area.y, event->area.width, event->area.height); cr->clip(); // scale to unit square (0 to 1 with and height) cr->scale(width, height); cr->set_line_width(0.05); // draw curve cr->move_to(x0, y0); cr->curve_to(x1, y1, x2, y2, x3, y3); cr->stroke(); // show control points cr->set_source_rgba(1, 0.2, 0.2, 0.6); cr->move_to(x0, y0); cr->line_to (x1, y1); cr->move_to(x2, y2); cr->line_to (x3, y3); cr->stroke(); } return true; } </pre> <p> The only difference between this example and the straight line example is in the <code class="function">on_expose_event()</code> function, but there are a few new concepts and functions introduced here, so let's examine them briefly. </p> <p> Note that we clip to the area that needs re-exposing just as we did in the last example. After clipping, however, we make a call to <code class="function">Cairo::Context::scale()</code>, passing in the width and height of the drawing area. This scales the user-space coordinate system such that the the width and height of the widget are both equal to 1.0 'units'. There's no particular reason to scale the coordinate system in this case, but sometimes it can make drawing operations easier. </p> <p> The call to <code class="function">Cairo::Context::curve_to()</code> should be fairly self-explanatory. The first pair of coordinates define the control point for the beginning of the curve. The second set of coordinates define the control point for the end of the curve, and the last set of coordinates define the destination point. To make the concept of control points a bit easier to visualize, a line has been draw from each control point to the end-point on the curve that it is associated with. Note that these control point lines are both translucent. This is achieved with a variant of <code class="function">set_source_rgb()</code> called <code class="function">set_source_rgba()</code>. This function takes a fourth argument specifying the alpha value of the color (valid values are between 0 and 1). </p> </div> </div> <div class="navfooter"> <hr> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"> <a accesskey="p" href="ch15s02.html"><img src="../icons/prev.png" alt="Prev"></a> </td> <td width="20%" align="center"><a accesskey="u" href="ch15.html"><img src="../icons/up.png" alt="Up"></a></td> <td width="40%" align="right"> <a accesskey="n" href="ch15s04.html"><img src="../icons/next.png" alt="Next"></a> </td> </tr> <tr> <td width="40%" align="left" valign="top">Drawing Straight Lines </td> <td width="20%" align="center"><a accesskey="h" href="index.html"><img src="../icons/home.png" alt="Home"></a></td> <td width="40%" align="right" valign="top"> Drawing Arcs and Circles</td> </tr> </table> </div> </body> </html>