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Drawing with Cairo in PyGTK

In this part of the PyGTK programming tutorial, we will 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, and 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 will demonstrate these two operations.

simpledrawing.py
#!/usr/bin/python

# ZetCode PyGTK tutorial 
#
# This code example draws a circle
# using the cairo library
#
# author: jan bodnar
# website: zetcode.com 
# last edited: February 2009


import gtk
import math

class PyApp(gtk.Window):

    def __init__(self):
        super(PyApp, self).__init__()

        self.set_title("Simple drawing")
        self.resize(230, 150)
        self.set_position(gtk.WIN_POS_CENTER)

        self.connect("destroy", gtk.main_quit)

        darea = gtk.DrawingArea()
        darea.connect("expose-event", self.expose)
        self.add(darea)

        self.show_all()
    
    def expose(self, widget, event):

        cr = widget.window.cairo_create()

        cr.set_line_width(9)
        cr.set_source_rgb(0.7, 0.2, 0.0)
                
        w = self.allocation.width
        h = self.allocation.height

        cr.translate(w/2, h/2)
        cr.arc(0, 0, 50, 0, 2*math.pi)
        cr.stroke_preserve()
        
        cr.set_source_rgb(0.3, 0.4, 0.6)
        cr.fill()
    

PyApp()
gtk.main()

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

darea = gtk.DrawingArea()

We will be doing our drawing operations on the DrawingArea widget.

darea.connect("expose-event", self.expose)

We do all drawing in a methods that is a handler for the expose-event signal.

cr = widget.window.cairo_create()

We create the cairo context object from the gdk.Window of the drawing area. The context is an object that is used to draw on all Drawable objects.

cr.set_line_width(9)

We set the width of the line to 9 pixels.

cr.set_source_rgb(0.7, 0.2, 0.0)

We set the colour to dark red.

w = self.allocation.width
h = self.allocation.height

cr.translate(w/2, h/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, 50, 0, 2*math.pi)
cr.stroke_preserve()

We draw the outside shape of a circle. In red colour. The stroke_preserve() 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.set_source_rgb(0.3, 0.4, 0.6)
cr.fill()

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

Simple drawing
Figure: Simple drawing

Basic shapes

The next example draws some basic shapes onto the window.

basicshapes.py
#!/usr/bin/python

# ZetCode PyGTK tutorial 
#
# This code example draws basic shapes
# with the cairo library
#
# author: jan bodnar
# website: zetcode.com 
# last edited: February 2009

import gtk
import math

class PyApp(gtk.Window):

    def __init__(self):
        super(PyApp, self).__init__()
        
        self.set_title("Basic shapes")
        self.set_size_request(390, 240)
        self.set_position(gtk.WIN_POS_CENTER)

        self.connect("destroy", gtk.main_quit)

        darea = gtk.DrawingArea()
        darea.connect("expose-event", self.expose)
        self.add(darea)
        
        self.show_all()
    
    def expose(self, widget, event):

        cr = widget.window.cairo_create()
        cr.set_source_rgb(0.6, 0.6, 0.6)

        cr.rectangle(20, 20, 120, 80)
        cr.rectangle(180, 20, 80, 80)
        cr.fill()

        cr.arc(330, 60, 40, 0, 2*math.pi)
        cr.fill()
        
        cr.arc(90, 160, 40, math.pi/4, math.pi)
        cr.fill()

        cr.translate(220, 180)
        cr.scale(1, 0.7)
        cr.arc(0, 0, 50, 0, 2*math.pi)
        cr.fill()
    

PyApp()
gtk.main()

In this example, we will create a rectangle, a square, a circle, an arc, and an ellipse.

cr.rectangle(20, 20, 120, 80)
cr.rectangle(180, 20, 80, 80)
cr.fill()

These lines draw a rectangle and a square.

cr.arc(330, 60, 40, 0, 2*math.pi)
cr.fill()

Here the arc() method draws a full circle.

cr.scale(1, 0.7)
cr.arc(0, 0, 50, 0, 2*math.pi)
cr.fill()

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

Basic shapes
Figure: Basic shapes

Colors

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.

colors.py
#!/usr/bin/python

# ZetCode PyGTK tutorial 
#
# This program shows how to work
# with colors in cairo
#
# author: jan bodnar
# website: zetcode.com 
# last edited: February 2009


import gtk

class PyApp(gtk.Window):

    def __init__(self):
        super(PyApp, self).__init__()
        
        self.set_title("Colors")
        self.resize(360, 100)
        self.set_position(gtk.WIN_POS_CENTER)

        self.connect("destroy", gtk.main_quit)

        darea = gtk.DrawingArea()
        darea.connect("expose-event", self.expose)
        self.add(darea)
        
        self.show_all()
    
    def expose(self, widget, event):

        cr = widget.window.cairo_create()

        cr.set_source_rgb(0.2, 0.23, 0.9)
        cr.rectangle(10, 15, 90, 60)
        cr.fill()
         
        cr.set_source_rgb(0.9, 0.1, 0.1)
        cr.rectangle(130, 15, 90, 60)
        cr.fill()

        cr.set_source_rgb(0.4, 0.9, 0.4)
        cr.rectangle(250, 15, 90, 60)
        cr.fill()

PyApp()
gtk.main()

We draw three rectangles in three different colours.

cr.set_source_rgb(0.2, 0.23, 0.9)

The set_source_rgb() 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.

Colors
Figure: Colors

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)

transparentrectangles.py
#!/usr/bin/python

# ZetCode PyGTK tutorial 
#
# This program shows transparent
# rectangles using cairo
#
# author: jan bodnar
# website: zetcode.com 
# last edited: February 2009


import gtk

class PyApp(gtk.Window):

    def __init__(self):
        super(PyApp, self).__init__()
        
        self.set_title("Transparent rectangles")
        self.resize(590, 90)
        self.set_position(gtk.WIN_POS_CENTER)

        self.connect("destroy", gtk.main_quit)

        darea = gtk.DrawingArea()
        darea.connect("expose-event", self.expose)
        self.add(darea)
        
        self.show_all()
    
    def expose(self, widget, event):

        cr = widget.window.cairo_create()

        for i in range(1, 11):
            cr.set_source_rgba(0, 0, 1, i*0.1)
            cr.rectangle(50*i, 20, 40, 40)
            cr.fill()


PyApp()
gtk.main()

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

cr.set_source_rgba(0, 0, 1, i*0.1)

The last parameter of the set_source_rgba() method is the alpha transparency.

Transparent rectangles
Figure: Transparent rectangles

Soulmate

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

soulmate.py
#!/usr/bin/python

# ZetCode PyGTK tutorial 
#
# This program draws text
# using cairo
#
# author: jan bodnar
# website: zetcode.com 
# last edited: February 2009


import gtk
import cairo

class PyApp(gtk.Window):

    def __init__(self):
        super(PyApp, self).__init__()
        
        self.set_title("Soulmate")
        self.set_size_request(370, 240)
        self.set_position(gtk.WIN_POS_CENTER)

        self.connect("destroy", gtk.main_quit)

        darea = gtk.DrawingArea()
        darea.connect("expose-event", self.expose)
        self.add(darea)
        
        self.show_all()
    
    def expose(self, widget, event):

        cr = widget.window.cairo_create()

        cr.set_source_rgb(0.1, 0.1, 0.1)
         
        cr.select_font_face("Purisa", cairo.FONT_SLANT_NORMAL, 
            cairo.FONT_WEIGHT_NORMAL)
        cr.set_font_size(13)
       
        cr.move_to(20, 30)
        cr.show_text("Most relationships seem so transitory")
        cr.move_to(20, 60)
        cr.show_text("They're all good but not the permanent one")
        cr.move_to(20, 120)
        cr.show_text("Who doesn't long for someone to hold")
        cr.move_to(20, 150)
        cr.show_text("Who knows how to love without being told")
        cr.move_to(20, 180)
        cr.show_text("Somebody tell me why I'm on my own")
        cr.move_to(20, 210)
        cr.show_text("If there's a soulmate for everyone")
        

PyApp()
gtk.main()

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

cr.select_font_face("Purisa", cairo.FONT_SLANT_NORMAL, 
    cairo.FONT_WEIGHT_NORMAL)

Here we specify the font that we use.

cr.set_font_size(13)

We specify the size of the font.

cr.move_to(20, 30)

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

cr.show_text("Most relationships seem so transitory")

The show_text() method draws text onto the window.

Soulmate
Figure: Soulmate

In this chapter of the PyGTK programming library, we were drawing with the Cairo graphics library.