An Introduction to Cairo with Python

Cairo is a library for drawing vector graphics. Vector graphics are interesting because they don’t lose clarity when resized or transformed.

Pycairo is a set of bindings for cairo. It provides the cairo module which can be used to call cairo commands from Python.

Understanding How to use Cairo

The best way to understand how to use cairo is to imagine that you are an artist using a paintbrush to draw out a shape on canvas.

To begin, you can choose a few characteristics of your brush. You can choose the thickness of your brush and the colour you want to paint with. You can also choose the shape of your brush tip - You can choose either a circle or a square.

Once you have chosen your brush, you are ready to start painting. You have to be quite precise when describing what you want to appear.

Firstly, decide where you want to place your brush on the canvas. You do this by supplying an x & y coordinate. Next you define how you want your brush stroke to look - an arc, a straight line etc. Finally you define the point where you want your stoke to end, again by supplying an x & y coordinate. Triangles and squares are very easy to do!

More complex graphics are generated using variations of the above theme with a few additions such as Fills (colouring in), transformations (zooming in, moving) etc. Using the Python interface to cairo

Nearly all the work revolves around using the cairo.Context (or cairo_t in the cairo C API). This is the object that you send your drawing commands to. There are a few options available to initialize this object in different ways.

Initializing the cairo.Context Object

One Very Important thing to realize is there is a difference between the coordinates you are describing your graphics on and the coordinates you will be displaying your graphic on.

(Ex - When giving a presentation you draw on your transparent acetate before hand, and then display it on your overhead projector - cairo calls the transparent acetate the user space coordinates and the projected image the device space coordinates)

On initializing the cairo context object, we tell it how to transform our description to how it should be displayed. To do this we supply a transformation matrix. Modifying the transformation matrix can lead to some very interesting results.
One of cairo’s most powerful features is that it can output graphics in many different formats (it can use multiple back ends). For printing, we can have cairo translate our graphics into Postscript to be sent off to the printer. For on screen display, we can have cairo translate our graphics into something glitz can understand for hardware accelerated rendering! It has many more important and useful target back ends. On initializing the cairo.Context, we set its target back end, supplying a few details (such as colour depth and size), as in the example below.

Example

#!/usr/bin/env python

import math
import cairo

WIDTH, HEIGHT = 256, 256

surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, WIDTH, HEIGHT)
ctx = cairo.Context(surface)

ctx.scale(WIDTH, HEIGHT)  # Normalizing the canvas

pat = cairo.LinearGradient(0.0, 0.0, 0.0, 1.0)
pat.add_color_stop_rgba(1, 0.7, 0, 0, 0.5)  # First stop, 50% opacity
pat.add_color_stop_rgba(0, 0.9, 0.7, 0.2, 1)  # Last stop, 100% opacity

ctx.rectangle(0, 0, 1, 1)  # Rectangle(x0, y0, x1, y1)
ctx.set_source(pat)
ctx.fill()

ctx.translate(0.1, 0.1)  # Changing the current transformation matrix

ctx.move_to(0, 0)
# Arc(cx, cy, radius, start_angle, stop_angle)
ctx.arc(0.2, 0.1, 0.1, -math.pi / 2, 0)
ctx.line_to(0.5, 0.1)  # Line to (x,y)
# Curve(x1, y1, x2, y2, x3, y3)
ctx.curve_to(0.5, 0.2, 0.5, 0.4, 0.2, 0.8)
ctx.close_path()

ctx.set_source_rgb(0.3, 0.2, 0.5)  # Solid color
ctx.set_line_width(0.02)
ctx.stroke()

surface.write_to_png("example.png")  # Output to PNG