Using Cairo and LGI

These days, Awesome’s interface is mostly based on a library called LGI. It provides access to C libraries such as GTK, GLib, Cairo, Pango, PangoCairo and RSVG using Lua code without having to write actual “glue” C code.

This is done using the GObject-introspection framework. The main advantage is the time saved and large number of features exposed for free. The downside is the lack of proper Lua centric documentation and examples. Some examples can be found in LGI’s own documentation, but this does not directly explain how to use a concrete API. Using other APIs requires some trial and error, and can be even impossible if the introspection data is missing or inaccurate. Using low-level APIs directly can easily cause crashes. It is the programmer’s responsibility to properly check return and error values.

Using LGI in Awesome

GObject and Gnome centric libraries tend to use the common C practice of emulating namespaces using underscores in function names. LGI exposes a proper namespace based API. For example, if the C function is:

cairo_image_surface_create()

Then the LGI equivalent is:

lgi.cairo.ImageSurface.create()

The same goes for enums:

CAIRO_FORMAT_ARGB32

becomes:

lgi.cairo.Format.ARGB32

LGI is also object oriented while the C API is function based. When those functions take the “class” “object”, then this:

cairo_line_to(cr, x, y)

can be expressed as:

cr:line_to(x, y)

It is however important to note some inconsistencies. For example, Cairo is called lgi.cairo while GLib is called lgi.GLib. Figuring this out will require some experimenting. The best way to do this without actually reloading Awesome is to open the lua command in a terminal and use print:

print("This will print a table address:", require("lgi").cairo)
print("This will print an error:", require("lgi").Cairo)

It is recommended to avoid using require always when using a function, but include the libraries at the top of your rc.lua or Lua module instead:

local cairo = require("lgi").cairo

The Cairo API

Cairo is a 2D graphic library used by Awesome, Gnome and XFCE. It allows to e.g. paint paths on a surface. Awesome uses it internally and being able to call it directly is a powerful feature.

The following concepts are necessary to be able to use Cairo:

Surface:

A surface is the area where the painting will be done. There are multiple types of surfaces including:

  • Color images with transparency (ARGB32) or without (RGB24)
  • Monochrome image surfaces with transparency (A8) or without (A1)
  • SVG vectorial surfaces
  • Native (XCB) surfaces
  • Framebuffers and other less interesting ones (from an Awesome’s point of view)

For more details see Surfaces.

Sources:

Sources are elements like colors, patterns or gradients. See gears.color for common sources.

For more details see Pattern.

Context and paths:

A context is the proxy between the program and the surface, and holds a path. Paths are something like a line, circle or rectangle, which may or may not be closed (a shape).

All drawing operations on a surface are done via a context. The current path is extended until it is used and reset (see next section). Until then nothing will be drawn to the surface. For example:

cr:rectangle(0 , 0 , 10, 10)
cr:rectangle(10, 10, 10, 10)

will not do anything until the operation is applied to the context.

For more details, read:

A context also holds a transformation matrix (see gears.matrix), which is used when applying an operation.

Operations:

Multiple operations can be done with the paths. The most common are:

  • fill: Fill the path with the current source.
  • stroke: Paint the path outline with the current source.
  • mask: Use the current source as an alpha mask while painting with the current operator.
  • clip: Crop the surface’s workarea so nothing outside of the clip will be affected by all following operations.

Operators:

Operators are modifiers used when applying operations.

Cairo in Awesome

All of Awesome’s wiboxes, awful.wibars, gears.wallpapers and awful.titlebars contain Cairo surfaces, which can be accessed through the drawin API. This allows widgets to use the Cairo context directly. See the declarative layout system and new widgets articles for more information and examples on how widgets work.

It is also possible to create surfaces manually. See gears.surface for some examples. Here is the most simple example you can get:

-- Create a surface
local img = cairo.ImageSurface.create(cairo.Format.ARGB32, 50, 50)

-- Create a context
local cr  = cairo.Context(img)

-- Set a red source
cr:set_source(1, 0, 0)
-- Alternative:
cr:set_source(gears.color("#ff0000"))

-- Add a 10px square path to the context at x=10, y=10
cr:rectangle(10, 10, 10, 10)

-- Actually draw the rectangle on img
cr:fill()

This can then be used as bgimage for a wibox, awful.wibar or wibox.container.background:

screen.primary.mywibox.bgimage = img
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