seup', function(e){
        canvas_rect = canvas.getBoundingClientRect();
        var j = e.clientX - canvas_rect.left;
        var i = e.clientY - canvas_rect.top;
        var in_pixels_i = Math.floor(i / pixel_size);
        var in_pixels_j = Math.floor(j / pixel_size);
        pixels[in_pixels_i][in_pixels_j] = 1 - pixels[in_pixels_i][in_pixels_j];
        draw_canvas();
    }, false);
    
    draw_canvas();
}


function draw_canvas(){
    var canvas = document.getElementById("canvas");
    var context 