[eigen] Compiling Eigen to the Web

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Hi,

This is to share a simple example of using Eigen in a Web page:

http://people.mozilla.org/~bjacob/bezier/bezier.html

Here, a linear solver from Eigen is used to build a small _javascript_ power series library, which is used to do some calculus to find a constant-speed parametrization of a Bezier curve.

The compiler used to compile Eigen to JS is Emscripten:

https://github.com/kripken/emscripten/wiki

The Eigen-using C++ code being compiled is:

http://people.mozilla.org/~bjacob/bezier/solve.cpp

The compilation command line is:

emcc solve.cpp -O2 -o solve.js -I /hack/eigen -std=c++11 -DEIGEN_ASM_COMMENT -s EXPORTED_FUNCTIONS="['_solve']"

This yields a JS file which you can see here:

http://people.mozilla.org/~bjacob/bezier/solve.js

(If you want it to be more readable, so you can see what Eigen is doing in JS, pass -g to the compiler)

To use that JS code, user code uses Emscripten's pseudo malloc() function and writes/reads directly on the Typed Array that serves as Emscripten's heap, see e.g. the code implementing power series division using Eigen's solver:

function PowerSeriesDiv(a, b) {
  var size = a.length;

  var matrix_data = Module._malloc(4 * size * size);
  var rhs_data = Module._malloc(4 * size);
  var result_data = Module._malloc(4 * size);

  for (var i = 0; i < size; i++) {
    HEAPF32[(rhs_data>>2) + i] = a[i];
  }

  for (var col = 0; col < size; col++) {
    for (var row = 0; row < size; row++) {
      HEAPF32[(matrix_data>>2) + col * size + row] = row >= col ? b[row - col] : 0;
    }
  }

  _solve(size, matrix_data, rhs_data, result_data);

  var result = new Float32Array(size);
  for (var i = 0; i < size; i++) {
    result[i] = HEAPF32[(result_data>>2) + i];
  }

  return result;
}

Cheers,

Benoit