Compose is a vector graphics library for Julia. It forms the basis for the statistical graphics system Gadfly.

Unlike most vector graphics libraries, Compose is thoroughly declarative. Rather than issue a sequence of drawing commands, graphics are formed by sticking various things together and then letting the library figure out how to draw it. The "things" in this case fall one of three types: Property, Form, and Canvas. "Sticking together" is primary achieved with the compose function.

The semantics of composition are fairly simple, and once grasped provide a consistent and powerful means of building vector graphics.

(Work in progress) documentation is available at: composejl.org

The easiest way to get a sense of how Compose works is with an example. So, here's how to draw a Sierpinski triangle.

using Compose

function sierpinski(n)
    if n == 0
        compose(context(), polygon([(1,1), (0,1), (1/2, 0)]))
    else
        t = sierpinski(n - 1)
        compose(context(),
                (context(1/4,   0, 1/2, 1/2), t),
                (context(  0, 1/2, 1/2, 1/2), t),
                (context(1/2, 1/2, 1/2, 1/2), t))
    end
end

img = SVG("sierpinski.svg", 4inch, 4(sqrt(3)/2)inch)
draw(img, compose(sierpinski(8), linewidth(0.1mm), fill(nothing), stroke("black")))

A graphic in Compose is a tree of Context objects, each specifying a coordinate system relative to its parent canvas. One context is made a child of another with a call to compose(a::Context, b::Context).

Contexts may also have children of type Form, which are rectangles, ellipses, text, etc., and Property, which are line width, fill color, etc. Form and Property nodes are always leaf nodes.

There are fancier forms of the compose function, in particular, variadic compose, which is roughly defined as:

compose(a, b, cs...) = compose(compose(a, b), cs...)

Compose over tuples or arrays:

compose((as...)) = compose(as...)

In effect, this lets one write a complex series of compose operations as an S-expression. For example:

compose(a, b, ((c, d), (e, f), g))

Since all we are doing is building trees, this syntax tends to be pretty convenient.

These are basic constructors for the in-built forms - see src/form.jl for more constructors.

• polygon(points)
• rectangle(x0, y0, width, height)
• circle(x, y, r)
• ellipse(x, y, x_radius, y_radius)
• text(x, y, value)
• line(points)
• curve(anchor0, ctrl0, ctrl1, anchor1)
• bitmap(mime, data, x0, y0, width, height)

Besides coordinate transformations, Compose also handles mixtures of relative and absolute coordinates. For example, 1w - 10mm is a well formed expression, giving the width of the parent canvas minus ten millimeters.

Compose is intended as a futuristic version of the R library grid, and so takes a few ideas from grid. The Compose canvas is roughly equivalent to a viewport in grid, for example. The Haskell library Diagrams was another starting point with many admirable notions I hope to steal.