PHP + CodeIgniter + Canvas // PARTE FRONTEND

    <div class="row">
        <div class="col-lg-12">
            <h1 class="page-header">
                Modifica sketch
            </h1>
        </div>
    </div>


    <div class="row">
        <div class="col-md-6" style="position: inherit!important;">
            <canvas id="can" width="800" height="600" style="position:relative; background: url('<?=$background?>') no-repeat; background-size: cover;"></canvas>
        </div>

        <div class="col-md-6">
            <h2>Seleziona il colore</h2>
            <a href="#" id="green" onclick="color(this)"><i style="color: green;" class="fa fa-paint-brush fa-2x"></i></a>
            <a href="#" id="blue" onclick="color(this)"><i style="color: blue;" class="fa fa-paint-brush fa-2x"></i></a>
            <a href="#" id="red" onclick="color(this)"><i style="color: red;" class="fa fa-paint-brush fa-2x"></i></a>
            <a href="#" id="yellow" onclick="color(this)"><i style="color: yellow;" class="fa fa-paint-brush fa-2x"></i></a>
            <a href="#" id="orange" onclick="color(this)"><i style="color: orange;" class="fa fa-paint-brush fa-2x"></i></a>
            <a href="#" id="black" onclick="color(this)"><i style="color: black;" class="fa fa-paint-brush fa-2x"></i></a>
            <a href="#" id="white" onclick="color(this)"><i style="color: black;" class="fa fa-eraser fa-2x"></i></a><br /><br />

            <?php echo form_open("pages/save_sketch"); ?>
                <img id="canvasimg" style="display:none;">
                <input type="hidden" name="segment" id="segment" value="<?=$this->uri->segment('3')?>" />
                <input type="hidden" name="sketch" id="sketch" value="" />
                <input id="btn" name="btn_update" type="submit" class="btn btn-success" value="Salva" />
                <!-- <button id="btn" class="btn btn-success" onclick="save()"><i class="fa fa-check"></i> Salva</button> -->
                <!-- <button id="clr" class="btn btn-danger" onclick="erase()"><i class="fa fa-ban"></i> Cancella</button> -->
            <?php echo form_close(); ?>
            <?php echo $this->session->flashdata('msg'); ?>
        </div>
    </div>

 <script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.4/jquery.min.js"></script>

 <script type="text/javascript">
    $( document ).ready(function() {
        init();

        var saved = "<?=$saved?>";

        if(saved != "") {
            loadCanvas(saved);
        }

        $("#btn").click(function(e) {
//          e.preventDefault();
            var dataURL = canvas.toDataURL();
            $("#canvasimg").attr("src", dataURL);
            $("#sketch").attr("value", $("#canvasimg").attr("src"));
        });
    });

var canvas, ctx, flag = false,
    prevX = 0,
    currX = 0,
    prevY = 0,
    currY = 0,
    dot_flag = false;

var x = "red",
    y = 2;


function loadCanvas(dataURL) {
    var canvas = document.getElementById('can');
    var context = canvas.getContext('2d');

    var imageObj = new Image();
    imageObj.onload = function() {
      context.drawImage(this, 0, 0);
    };

    imageObj.src = dataURL;
 }

function init() {
    canvas = document.getElementById('can');
    ctx = canvas.getContext("2d");
    w = canvas.width;
    h = canvas.height;

    canvas.addEventListener("mousemove", function (e) {
        findxy('move', e)
    }, false);
    canvas.addEventListener("mousedown", function (e) {
        findxy('down', e)
    }, false);
    canvas.addEventListener("mouseup", function (e) {
        findxy('up', e)
    }, false);
    canvas.addEventListener("mouseout", function (e) {
        findxy('out', e)
    }, false);
}

function color(obj) {
    switch (obj.id) {
        case "green":
            x = "green";
            break;
        case "blue":
            x = "blue";
            break;
        case "red":
            x = "red";
            break;
        case "yellow":
            x = "yellow";
            break;
        case "orange":
            x = "orange";
            break;
        case "black":
            x = "black";
            break;
        case "white":
            x = "white";
            break;
    }
    if (x == "white") y = 14;
    else y = 2;

}

function draw() {
    ctx.beginPath();
    ctx.moveTo(prevX, prevY);
    ctx.lineTo(currX, currY);
    ctx.strokeStyle = x;
    ctx.lineWidth = y;
    ctx.stroke();
    ctx.closePath();
}

function erase() {
    var m = confirm("Want to clear");
    if (m) {
        ctx.clearRect(0, 0, w, h);
        document.getElementById("canvasimg").style.display = "none";
    }
}

function save() {
    document.getElementById("canvasimg").style.border = "2px solid";
    var dataURL = canvas.toDataURL();
    document.getElementById("canvasimg").src = dataURL;
    $("#sketch").attr("value", $("#canvasimg").attr("src"));
//     document.getElementById("canvasimg").style.display = "inline";
}

function findxy(res, e) {
    if (res == 'down') {
        prevX = currX;
        prevY = currY;
        currX = e.clientX - canvas.offsetLeft;
        currY = e.clientY - canvas.offsetTop;

        flag = true;
        dot_flag = true;
        if (dot_flag) {
            ctx.beginPath();
            ctx.fillStyle = x;
            ctx.fillRect(currX, currY, 2, 2);
            ctx.closePath();
            dot_flag = false;
        }
    }
    if (res == 'up' || res == "out") {
        flag = false;
    }
    if (res == 'move') {
        if (flag) {
            prevX = currX;
            prevY = currY;
            currX = e.clientX - canvas.offsetLeft;
            currY = e.clientY - canvas.offsetTop;
            draw();
        }
    }
}
</script>