Manual of Lua-OOCairo

Manual of Lua-OOCairo

lua-oocairo - Lua API for the Cairo vector graphics library

Introduction

This is a Lua binding for the Cairo graphics library, with an object oriented style of API. There is at least one other Cairo binding available in Lua, but it has a more C-like API and is much less complete.

Simple Example

The code below is a complete example showing how to load this module and do some basic drawing with it. Other features are demonstrated by the files in the examples directory included with the module.

First load the module. The module doesn't create a global variable, so you have to assign it to something. 

local Cairo = require "oocairo"

Create a bitmap image surface for drawing on, and create a context object which is used to do the drawing: 

local surface = Cairo.image_surface_create("rgb24", 200, 200)
local cr = Cairo.context_create(surface)

Fill whole image with white. The paint method will copy the whole of the source to the surface. In this case the source is a solid colour, but it can be other things like an image or gradient. The default source is solid fully opaque black. Since the default colour of the image surface is also black, nothing would happen if we didn't set the source to something else before painting. 

cr:set_source_rgb(1, 1, 1)
cr:paint()

Define an arbitrary shape, then fill it in purple. The move_to and line_to methods, among others, define the shape of the path which is used for drawing operations like filling. 

cr:move_to(0, 0)
cr:line_to(190, 100)
cr:line_to(100, 185)
cr:line_to(200, 200)
cr:line_to(30, 130)
cr:close_path()
cr:set_source_rgb(0.8, 0.4, 1)
cr:fill()

Another shape, this time both filled and with a line drawn along it's edge. The path is normally discarded after a drawing operation, so here fill_preserve is used to keep it so that it can also be stoked. Note that the stroked line won't go all the way around the filled area, because the path wasn't closed with close_path. The black outline created by the stroke method will be three pixels wide, and will be centered on the path, half of it inside the filled area. 

cr:move_to(180, 30)
cr:line_to(100, 20)
cr:line_to(80, 120)
cr:set_source_rgb(0.5, 0.7, 0.3)
cr:fill_preserve()

cr:set_line_width(3)
cr:set_source_rgb(0, 0, 0)
cr:stroke()

Finally, write the output to a file in PNG format. 

surface:write_to_png("simple-example.png")

Surface constructor functions

The following functions can be accessed from the module table (the table returned from require). One of them must be used to create a surface on which to draw on before Cairo can be used for anything interesting. These all return a surface object, for which the methods are documented at lua-oocairo-surface(3). Most drawing is done through a context object, which should be constructed with a call to the context_create function (also in the module table), passing the surface object as its only argument. The methods for context objects are documented at lua-oocairo-context(3).

image_surface_create (format, width, height)

Return a surface object for drawing an image in memory. The width and height values must be numbers and are measured in pixels. The value of format can be any of the following strings:

argb32

Each pixel is 32 bits, encoding both colour and alpha (transparency) values.

rgb24

Each pixel is 32 bits with only 24 bits used, encoding a colour value. All pixels are implicitly 100% opaque.

a8

Each pixel is a single byte encoding the alpha (transparency) value, with no colour information.

a1

Each pixel is a single bit encoding whether a pixel is transparent or opaque, with no colour information.

The image created will be black by default, and fully transparent if the pixels have an alpha component.

image_surface_create_from_data (data, format, width, height, stride)

Creates a new image surface with the size width by height pixels, using format just as the image_surface_create function does, but the image is initialized using pixel data from the string data. The stride value should be whatever is returned from the format_stride_for_width function for the given width and pixel format. The data should be encoded in the format described for the surf:get_data() method in lua-oocairo-surface(3).

Note that since Lua strings are immutable, a copy of the data is made and used as the live buffer. There is currently no support for sharing an image surface buffer between Cairo and something else.

This binds the native Cairo function cairo_image_surface_create_for_data, but has a slightly different name because the data you provide is only used at construction time, not kept around for drawing.

image_surface_create_from_png (file/filename)

Creates a new image surface containing the image in a PNG file. The file is read from the filename or file handle specified. See below for details about what kind of file handles can be used.

pdf_surface_create (file/filename, width, height)

Create a surface which writes drawing instructions out to a PDF file, at the given filename or through a file handle. The width and height values must be numbers and are measured in points.

ps_surface_create (file/filename, width, height)

Create a surface which writes drawing instructions out to an Adobe™ file, at the given filename or through a file handle. The width and height values must be numbers and are measured in points.

surface_create_similar (surface, content, width, height)

Create a new surface of the same type as surface, but with the width and height specified. The content value indicates whether the surface should include colour and/or transparency information, and must be one of the following strings:

color

Surface will have only colour content.

alpha

Surface will have only transparency content.

color-alpha

Surface will have colour and transparency content.

svg_surface_create (file/filename, width, height)

Create a surface which writes drawing instructions out to an SVG file, at the given filename or through a file handle. The width and height values must be numbers and are measured in points.

Other top-level functions

The following additional methods are available. The are rarely needed except for context_create and the gradient constructors.

BYTE_ORDER

Not actually a function, this is a string indicating which order colour components are stored in pixels in raw image data on this platform. This will be argb on big-endian systems and bgra on little-endian ones. This is the order in which the bytes for each pixel are stored on rgb24 and argb32 image surfaces. For a1 it indicates the order the pixels are encoded as bits. It can be ignored when the image format is a8.

context_create (surface)

Return a new context object for drawing on surface. See lua-oocairo-context(3) for methods which can be called on this object.

context_create_gdk (drawable)

Return a new context object for drawing on the GdkDrawable object drawable, which must be from the Lua-Gnome library.

Fails unless Lua-Gnome has been loaded and the gtk module is available as a global variable.

See example programs gtk-image.lua and gtk-drawing.lua for examples of how to use this.

font_options_create ()

Returns a new Cairo font options object, with all its options set to default values. See lua-oocairo-fontopt(3) for information about what you can do with it.

format_stride_for_width (format, width)

Returns the number of bytes Cairo will used for storing a single row of pixels in the raw data of an image surface. This can be used as the stride value for the image_surface_create_from_data() function and the image surface method get_data(). The format value should be one of the pixel format strings such as rgb24, and the width should be a number.

matrix_create ()

Return a new copy of the identity matrix. All transformation matrices returned to Lua are in the format of a table of six numbers. See lua-oocairo-matrix(3) for methods which can be called on these.

pattern_create_for_surface (surface)

Return a new pattern object representing the image on the surface. See lua-oocairo-pattern(3) for methods which can be called on pattern objects.

pattern_create_linear (x1, y1, x2, y2)

Return a new pattern object representing a linear gradient, where the offset numbers go from zero at the point specified by the two numbers (x1, y1) to the point (x2, y2).

pattern_create_radial (x1, y1, r1, x2, y2, r2)

Return a new pattern object representing a radial gradient, where offsets move from the circle defined by the three numbers (x1, y1, r1), where r1 is the radius, to the circle (x2, y2, r2).

pattern_create_rgb (r, g, b)

Return a new pattern object representing a solid colour. The red, green, and blue values specified should all be numbers in the range zero to one inclusive.

pattern_create_rgba (r, g, b, a)

Same as pattern_create_rgb, but accepts an alpha value, so the solid colour can be semitransparent.

ps_get_levels ()

Return a table containing a list of strings indicating what levels of PostScript are supported by Cairo.

scaled_font_create (face, font_matrix, ctm, options)

Creates a new scaled font object, representing a scaled version of face. See lua-oocairo-scaledfont(3) for information about these objects.

The font_matrix and ctm values must be matrices (any table containing six numbers, see lua-oocairo-matrix(3) for details). The options argument can be nil for the default options, or a font options object as returned by the font_options_create function.

svg_get_versions ()

Return a table containing a list of strings indicating what versions of SVG are supported by Cairo.

toy_font_face_create (family, slant, weight)

Create and return a toy font face object (see lua-oocairo-fontface(3)). See the context method select_font_face for details of the arguments. This does the same, but returns the object instead of applying it to a context. Only available with Cairo 1.8 or better, otherwise this method won't exist.

user_font_face_create (callbacks)

Returns a font face object which uses the supplied callbacks for rendering and such. This is intended to be used for creating special fancy font effects or for providing access to obscure font formats which aren't supported by Cairo. The callbacks value must be a table, with string keys for the callbacks. The render_glyph callback is required, the others optional.

See lua-oocairo-userfont(3) for details of how to write the callbacks. Only available with Cairo 1.8 or better.

I/O through file handles

Functions in this module which are advertised as accepting a file/filename value can accept a filename (a simple string) or a Lua file handle. They will also work with any other object (a userdata value or table) which provides the appropriate input or output method.

For files which are to be used for output, a file handle object needs to provide a method called write, which will be called with a single string every time more output is available. Any return values from the method are ignored, but it can throw an exception if there's an error.

For files which are used for input, a file handle needs to provide a method called read, which will be called with a number indicating the number of bytes it should read. It should return a string containing exactly that number of bytes.

The memoryfile module available from LuaForge can be used to write output into a buffer in memory which can then be accessed as a Lua string. This can be useful as a way to get an image into another graphics library such as GD, where it can be written in other formats other than PNG.

Feature flags

When the module is compiled, it will only enable support for the features which were compiled into your copy of the Cairo library. For example, if you built Cairo without the PDF backend, then the module will not include the functions for creating a PDF surface.

There are flags in the module table which indicate which features are available. These are all wither true or false:

HAS_PDF_SURFACE

Support for creating a surface which writes drawing instructions out to an SVG file.

HAS_PNG_FUNCTIONS

Support for loading a PNG bitmap and creating an image surface from it, or for writing the contents of an image surface out to a PNG.

HAS_PS_SURFACE

Support for creating a surface which writes to a PostScript or EPS file.

HAS_SVG_SURFACE

Support for creating a surface which writes to a PDF file.

HAS_USER_FONT

Support for custom fonts defined by callback functions. If true then the user_font_face_create function should be available in the main module table.

Copyright

This software and documentation is Copyright © 2008 Geoff Richards <geoff at this domain dot co dot uk>. It is free software; you can redistribute it and/or modify it under the terms of the Lua 5.0 license. The full terms are given in the file COPYRIGHT supplied with the source code package, and are also available here: http://www.lua.org/license.html