DEPTH_BUFFER_BIT);

    // turn on depth testing
    gl.enable(gl.DEPTH_TEST);

    // tell webgl to cull faces
    gl.enable(gl.CULL_FACE);

    // Tell it to use our program (pair of shaders)
    gl.useProgram(program);

    // Bind the attribute/buffer set we want.
    gl.bindVertexArray(vao);

    // Compute the matrix
    var aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
    var zNear = 1;
    var zFar = 2000;
    var projectionMatrix = m4.perspective(fieldOfViewRadians, aspect, zNear, zFar)