Introduction to PyCairo

This is PyCairo tutorial. This tutorial will teach you the basics and some advanced topics of the Cairo 2D library with the Python language. In most examples, we will use the Python GTK backend to produce our output. Images used in this tutorial can be downloaded here.

Computer graphics

There are two different computer graphics: vector and raster graphics. Raster graphics represents images as a collection of pixels. Vector graphics is the use of geometric primitives such as points, lines, curves, or polygons to represent images. These primitives are created using mathematical equations.

Both types of computer graphics have advantages and disadvantages. The advantages of vector graphics over raster are:

Cairo

Cairo is a library for creating 2D vector graphics. It is written in the C programming language. Bindings for other computer languages exist, including Python, Perl, C++, C#, or Java. Cairo is a multiplatform library; it works on Linux, BSDs, Windows, and OSX.

Cairo supports various backends. Backends are output devices for displaying the created graphics.

This means that we can use the library to draw on Windows, Linux, Windows, OSX and we can use the library to create PNG images, PDF files, PostScript files, and SVG files.

We can compare the Cairo library to the GDI+ library on Windows OS and the Quartz 2D on Mac OS. Cairo is an open source software library. From version 2.8, Cairo is part of the GTK system.

Definitions

Here we provide some useful definitions. To do some drawing in PyCairo, we must first create a drawing context. The drawing context holds all of the graphics state parameters that describe how drawing is to be done. This includes information such as line width, color, the surface to draw to, and many other things. It allows the actual drawing functions to take fewer arguments to simplify the interface.

A path is a collection of points used to create primitive shapes such as lines, arcs, and curves. There are two kinds of paths: open and closed paths. In a closed path, starting and ending points meet. In an open path, starting and ending point do not meet. In PyCairo, we start with an empty path. First, we define a path and then we make them visible by stroking and/or filling them. After each stroke() or fill() method call, the path is emptied. We have to define a new path. If we want to keep the existing path for later drawing, we can use the stroke_preserve() and fill_preserve() methods. A path is made of subpaths.

A source is the paint we use in drawing. We can compare the source to a pen or ink that we use to draw the outlines and fill the shapes. There are four kinds of basic sources: colors, gradients, patterns, and images.

A surface is a destination that we are drawing to. We can render documents using the PDF or PostScript surfaces, directly draw to a platform via the Xlib and Win32 surfaces.

Before the source is applied to the surface, it is filtered first. The mask is used as a filter. It determines where the source is applied and where not. Opaque parts of the mask allow to copy the source. Transparent parts do not let to copy the source to the surface.

A pattern represents a source when drawing onto a surface. In PyCairo, a pattern is something that you can read from and that is used as the source or mask of a drawing operation. Patterns can be solid, surface-based, or gradients.

Sources

To create this tutorial, we have used the following materials. The Apple Cocoa drawing guide, the PyCairo reference, and the Cairo documentation.