Drawing with Cairo in GTK#

last modified October 18, 2023

In this part of the GTK# programming tutorial, we do some drawing with the Cairo library.

Cairo is a library for creating 2D vector graphics. We can use it to draw our own widgets, charts or various effects or animations.

Simple drawing

The stroke operation draws the outlines of shapes and the fill operation fills the insides of shapes. Next we demonstrate these two operations.

using Gtk;
using Cairo;
using System;
 
class SharpApp : Window {
 

    public SharpApp() : base("Simple drawing")
    {
        SetDefaultSize(230, 150);
        SetPosition(WindowPosition.Center);
        DeleteEvent += delegate { Application.Quit(); };;
        
        DrawingArea darea = new DrawingArea();
        darea.ExposeEvent += OnExpose;

        Add(darea);

        ShowAll();
    }

    void OnExpose(object sender, ExposeEventArgs args)
    {
        DrawingArea area = (DrawingArea) sender;
        Cairo.Context cr =  Gdk.CairoHelper.Create(area.GdkWindow);
        
        cr.LineWidth = 9;
        cr.SetSourceRGB(0.7, 0.2, 0.0);
                
        int width, height;
        width = Allocation.Width;
        height = Allocation.Height;

        cr.Translate(width/2, height/2);
        cr.Arc(0, 0, (width < height ? width : height) / 2 - 10, 0, 2*Math.PI);
        cr.StrokePreserve();
        
        cr.SetSourceRGB(0.3, 0.4, 0.6);
        cr.Fill();
         
        ((IDisposable) cr.Target).Dispose();                                      
        ((IDisposable) cr).Dispose();
    }

    public static void Main()
    {
        Application.Init();
        new SharpApp();
        Application.Run();
    }
}

In our example, we draw a circle and will it with a solid color.

gmcs -pkg:gtk-sharp-2.0 -r:/usr/lib/mono/2.0/Mono.Cairo.dll  simple.cs

Here is how we compile the example.

DrawingArea darea = new DrawingArea();

We be doing our drawing operations on the DrawingArea widget.

darea.ExposeEvent += OnExpose;

All drawing is done in a method that we plug into the ExposeEvent.

DrawingArea area = (DrawingArea) sender;
Cairo.Context cr =  Gdk.CairoHelper.Create(area.GdkWindow);

We create the Cairo.Context object from the GdkWindow of the drawing area. The context is an object that is used to draw on all Drawable objects.

cr.LineWidth = 9;

We set the width of the line to 9 pixels.

cr.SetSourceRGB(0.7, 0.2, 0.0);

We set the colour to dark red.

int width, height;
width = Allocation.Width;
height = Allocation.Height;

cr.Translate(width/2, height/2);

We get the width and height of the drawing area. We move the origin into the middle of the window.

cr.Arc(0, 0, (width < height ? width : height) / 2 - 10, 0, 2*Math.PI);
cr.StrokePreserve();

We draw the outside shape of a circle. The StrokePreserve strokes the current path according to the current line width, line join, line cap, and dash settings. Unlike the Stroke, it preserves the path within the cairo context.

cr.SetSourceRGB(0.3, 0.4, 0.6);
cr.Fill();

This fills the interior of the circle with some blue colour.

Basic shapes

The next example draws some basic shapes onto the window.

using Gtk;
using Cairo;
using System;
 
class SharpApp : Window {
 

    public SharpApp() : base("Basic shapes")
    {
        SetDefaultSize(390, 240);
        SetPosition(WindowPosition.Center);
        DeleteEvent += delegate { Application.Quit(); };
        
        DrawingArea darea = new DrawingArea();
        darea.ExposeEvent += OnExpose;

        Add(darea);
        ShowAll();
    }

    void OnExpose(object sender, ExposeEventArgs args)
    {
        DrawingArea area = (DrawingArea) sender;
        Cairo.Context cc =  Gdk.CairoHelper.Create(area.GdkWindow);
                
        cc.SetSourceRGB(0.2, 0.23, 0.9);
        cc.LineWidth = 1;

        cc.Rectangle(20, 20, 120, 80);
        cc.Rectangle(180, 20, 80, 80);
        cc.StrokePreserve();
        cc.SetSourceRGB(1, 1, 1);
        cc.Fill();

        cc.SetSourceRGB(0.2, 0.23, 0.9);
        cc.Arc(330, 60, 40, 0, 2*Math.PI);
        cc.StrokePreserve();
        cc.SetSourceRGB(1, 1, 1);
        cc.Fill();

        cc.SetSourceRGB(0.2, 0.23, 0.9);
        cc.Arc(90, 160, 40, Math.PI/4, Math.PI);
        cc.ClosePath();
        cc.StrokePreserve();
        cc.SetSourceRGB(1, 1, 1);
        cc.Fill();

        cc.SetSourceRGB(0.2, 0.23, 0.9);
        cc.Translate(220, 180);
        cc.Scale(1, 0.7);        
        cc.Arc(0, 0, 50, 0, 2*Math.PI);
        cc.StrokePreserve();
        cc.SetSourceRGB(1, 1, 1);
        cc.Fill();          

        ((IDisposable) cc.Target).Dispose ();                                      
        ((IDisposable) cc).Dispose ();
    }

    public static void Main()
    {
        Application.Init();
        new SharpApp();
        Application.Run();
    }
}

In this example, we create a rectangle, a square, a circle, an arc, and an ellipse. We draw outlines in blue colour, insides in white.

cc.Rectangle(20, 20, 120, 80);
cc.Rectangle(180, 20, 80, 80);
cc.StrokePreserve();
cc.SetSourceRGB(1, 1, 1);
cc.Fill();

These lines draw a rectangle and a square.

cc.Arc(330, 60, 40, 0, 2*Math.PI);

Here the Arc method draws a full circle.

cc.Scale(1, 0.7);        
cc.Arc(0, 0, 50, 0, 2*Math.PI);

If we want to draw an oval, we do some scaling first. Here the Scale method shrinks the y axis.

Colours

A colour is an object representing a combination of Red, Green, and Blue (RGB) intensity values. Cairo valid RGB values are in the range 0 to 1.

using Gtk;
using Cairo;
using System;
 
class SharpApp : Window {
 

    public SharpApp() : base("Colors")
    {
        SetDefaultSize(360, 100);
        SetPosition(WindowPosition.Center);
        DeleteEvent += delegate { Application.Quit(); };
        
        DrawingArea darea = new DrawingArea();
        darea.ExposeEvent += OnExpose;

        Add(darea);

        ShowAll();
    }

    void OnExpose(object sender, ExposeEventArgs args)
    {
        DrawingArea area = (DrawingArea) sender;
        Cairo.Context cr =  Gdk.CairoHelper.Create(area.GdkWindow);
                
        cr.SetSourceRGB(0.2, 0.23, 0.9);
        cr.Rectangle(10, 15, 90, 60);
        cr.Fill();
         
        cr.SetSourceRGB(0.9, 0.1, 0.1);
        cr.Rectangle(130, 15, 90, 60);
        cr.Fill();

        cr.SetSourceRGB(0.4, 0.9, 0.4);
        cr.Rectangle(250, 15, 90, 60);
        cr.Fill();

        ((IDisposable) cr.Target).Dispose();                                      
        ((IDisposable) cr).Dispose();
    }

    public static void Main()
    {
        Application.Init();
        new SharpApp();
        Application.Run();
    }
}

We draw three rectangles in three different colours.

cr.SetSourceRGB(0.2, 0.23, 0.9);

The SetSourceRGB method sets a colour for the cairo context. The three parameters of the method are the colour intensity values.

cr.Rectangle(10, 15, 90, 60);
cr.Fill();

We create a rectangle shape and fill it with the previously specified colour.

Transparent rectangles

Transparency is the quality of being able to see through a material. The easiest way to understand transparency is to imagine a piece of glass or water. Technically, the rays of light can go through the glass and this way we can see objects behind the glass.

In computer graphics, we can achieve transparency effects using alpha compositing. Alpha compositing is the process of combining an image with a background to create the appearance of partial transparency. The composition process uses an alpha channel. (wikipedia.org, answers.com)

using Gtk;
using Cairo;
using System;
 

class SharpApp : Window {
 

    public SharpApp() : base("Transparent rectangles")
    {
        SetDefaultSize(590, 90);
        SetPosition(WindowPosition.Center);
        DeleteEvent += delegate { Application.Quit(); } ;
        
        DrawingArea darea = new DrawingArea();
        darea.ExposeEvent += OnExpose;

        Add(darea);

        ShowAll();
    }

    void OnExpose(object sender, ExposeEventArgs args)
    {
        DrawingArea area = (DrawingArea) sender;
        Cairo.Context cr =  Gdk.CairoHelper.Create(area.GdkWindow);

        for ( int i = 1; i <= 10; i++) {
            cr.SetSourceRGBA(0, 0, 1, i*0.1);
            cr.Rectangle(50*i, 20, 40, 40);
            cr.Fill();  
        }

        ((IDisposable) cr.Target).Dispose();                                      
        ((IDisposable) cr).Dispose();
    }

    public static void Main()
    {
        Application.Init();
        new SharpApp();
        Application.Run();
    }
}

In the example we draw ten rectangles with different levels of transparency.

cr.SetSourceRGBA(0, 0, 1, i*0.1);

The last parameter of the SetSourceRGBA method is the alpha transparency.

Soulmate

In the next example, we draw some text on the window.

using Gtk;
using Cairo;
using System;
 
class SharpApp : Window {
 

    public SharpApp() : base("Soulmate")
    {
        SetDefaultSize(420, 250);
        SetPosition(WindowPosition.Center);
        DeleteEvent += delegate { Application.Quit(); };
        
        DrawingArea darea = new DrawingArea();
        darea.ExposeEvent += OnExpose;

        Add(darea);

        ShowAll();
    }

    void OnExpose(object sender, ExposeEventArgs args)
    {
        DrawingArea area = (DrawingArea) sender;
        Cairo.Context cr =  Gdk.CairoHelper.Create(area.GdkWindow);
        
        cr.SetSourceRGB(0.1, 0.1, 0.1);
         
        cr.SelectFontFace("Purisa", FontSlant.Normal, FontWeight.Bold);
        cr.SetFontSize(13);
       
        cr.MoveTo(20, 30);
        cr.ShowText("Most relationships seem so transitory");
        cr.MoveTo(20, 60);
        cr.ShowText("They're all good but not the permanent one");
        cr.MoveTo(20, 120);
        cr.ShowText("Who doesn't long for someone to hold");
        cr.MoveTo(20, 150);
        cr.ShowText("Who knows how to love without being told");
        cr.MoveTo(20, 180);
        cr.ShowText("Somebody tell me why I'm on my own");
        cr.MoveTo(20, 210);
        cr.ShowText("If there's a soulmate for everyone");

        ((IDisposable) cr.Target).Dispose();                                      
        ((IDisposable) cr).Dispose();
    }

    public static void Main()
    {
        Application.Init();
        new SharpApp();
        Application.Run();
    }
}

We display part of the lyrics from the Natasha Bedingfields Soulmate song.

cr.SelectFontFace("Purisa", FontSlant.Normal, FontWeight.Bold);

Here we specify the font that we use—Purisa bold.

cr.SetFontSize(13);

We specify the size of the font.

cr.MoveTo(20, 30);

We move to the point, where we draw the text.

cr.ShowText("Most relationships seem so transitory");

The ShowText method draws text onto the window.

In this chapter of the GTK# programming library, we were drawing with Cairo library.