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OCaml library to reuse Cairo-based drawing code in browser
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JsOfOCairo is an OCaml (4.02.3+) library to reuse Cairo-based drawing code in web browsers. It's an adapter, implementing (a reasonable subset of) the interface of Cairo OCaml targeting HTML5 canvas elements as exposed to OCaml by js_of_ocaml (3.0.0+).

It's licensed under the MIT license. It's available on OPAM. Its source code is on GitHub.

Here is DrawGrammar, a real-life aplication of JsOfOCairo.

There is no real documentation besides this README.rst file. See below what is implemented, what behaves differently from Cairo, and what is not implemented.

Questions? Remarks? Bugs? Want to contribute? Open an issue!


A breaking change was introduced in OCaml Cairo version 0.6.

JsOfOCairo version 2 implements a subset of the interface of OCaml Cairo version 0.6, while JsOfOCairo version 1 implements a subset of the interface of OCaml Cairo version 0.5.

Quick start

Install from OPAM:

$ opam install JsOfOCairo

The files described below are available as a demo directory. Have a look at this directory for the details about compiling. In particular see the dune file and the call to dune.

Create a functor implementing your drawing code against the JsOfOCairo.S signature. File

module Make(C: JsOfOCairo.S) = struct
  let draw ctx = ctx;
    C.arc ctx 50. 50. ~r:40. ~a1:0. ~a2:5.;
    C.stroke ctx;
    C.restore ctx

Instantiate this functor with Cairo to create a command-line program. File

module Drawings = Drawings.Make(Cairo)

let () = begin
  let image = Cairo.Image.create Cairo.Image.ARGB32 ~w:100 ~h:100 in
  Drawings.draw (Cairo.create image);
  Cairo.PNG.write image "draw_on_command_line.png";

Instantiate the same functor with JsOfOCairo and compile it using js_of_ocaml to create a Javascript file. File

module Drawings = Drawings.Make(JsOfOCairo)

let () = Js.export "draw" (fun canvas ->
  Drawings.draw (JsOfOCairo.create canvas)

And call this javascript file in an HTML document. File draw_in_browser.html:

<!DOCTYPE html>
<html lang="en">
  <meta charset="utf-8">
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">

  <title>JsOfOCairo demo</title>
  <h1>PNG image from command-line</h1>
  <img src="draw_on_command_line.png" />
  <h1>HTML5 canvas</h1>
  <canvas id="drawings" width="100" height="100"></canvas>
  <script src="_build/default/draw_in_browser.bc.js"></script>

As a bonus, JsOfOCairo comes with CairoMock, which implements the JsOfOCairo.S signature and simply records the calls made on the context object. You can use it to automate some tests on your drawing code:

module Drawings = Drawings.Make(CairoMock)

let () = begin
  let ctx = CairoMock.create () in
  Drawings.draw ctx;
  assert (CairoMock.calls ctx = ["save"; "arc 50.00 50.00 ~r:40.00 ~a1:0.00 ~a2:5.00"; "stroke"; "restore"])

CairoMock itself is split into CairoMock.Mock, an actual mock implementation of JsOfOCairo.S that does nothing, and CairoMock.Decorate, that can be used to record calls made on any implementation of JsOfOCairo.S. So, you can draw and record calls at the same time.

What is implemented

See the interface file ( If a function is present, it should behave as described in the Cairo OCaml Tutorial.

How to avoid pitfalls

There are limitations however: text-related functions, arcs, re-use of the same canvas... Details of the limitations identified so far are available with the tests. We believe they are small enough for the library to be useful anyway.

Here is a set of rules to follow to stay on the safe side of using JsOfOCairo:

  • Always call save just after creating a context, and restore just before stopping using it.
  • Never create two contexts from the same canvas at the same time: wait until you have restore-d a context before creating another.
  • Never draw arcs of more than one full turn.
  • Use only the width``returned by ``text_extents.
  • Use only the ascent and descent returned by font_extents.

What is not implemented

Contributions in this area are welcome. Please start a discussion before doing anything to avoid wasting time.

Everything involving a Surface.t has been dismissed. This doesn't make much sense in an HTML5 context. An attempt has been made to implement set_source_for_image using a hidden canvas but it's been unsuccessful.

A few other functions commented out at the beginning of have been dismissed as well.

Testing strategy

There are three sets of tests:

universal tests
They are run on Cairo to check their validity, and then on JsOfOCairo and CairoMock to actually test the library. They verify that getters return the value that was last set, that the current point is updated, and that all this is saved and restored consistently.
drawing tests
They are run on Cairo to generate reference bitmaps, and then on JsOfOCairo to verify that both libraries produce very similar drawings.
decoration tests
They verify the strings generated by CairoMock.

All these tests are run automatically as OCaml bytecode and in Node.js (through js_of_ocaml) and are available in web browsers.

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