How to print canvas content using javascript

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I am using canvas for writing purpose using jsp page. I am able to write any message on canvas, (canvas message i have created..)

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日久生厌

2021-01-23 05:55

As per comment of pc-shooter my problem got sloved..

Jsp Page Code :

<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">

<script type="text/javascript" language="javascript" src="js/jquery.min.js"></script>

<script type="text/javascript">

$(document).ready(function() {
    //alert('ready');
    var w = window.innerWidth;
    var h = window.innerHeight;
    //alert('w '+w+' h '+h);

    //alert('ready end');
} );

</script>

<script type="text/javascript">
    var canvas, ctx, flag = false,
            prevX = 0,
            currX = 0,
            prevY = 0,
            currY = 0,
            dot_flag = false;

    var x = "black",
            y = 2;

    function init() {
        //alert('init start');

        w = window.innerWidth - 50;
        h = window.innerHeight - 100;
        //alert('canvas 1 w '+w+' h '+h);
        document.getElementById('can').width=w;
        document.getElementById('can').height=h;

        //document.getElementById('can').setAttribute("style","width:900px");


        canvas = document.getElementById('can');
        w = canvas.width;
        h = canvas.height;
        //alert('canvas 2 w '+w+' h '+h);

        ctx = canvas.getContext("2d");

        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);

      //  alert('init end');
    }

    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;
        document.getElementById("canvasimg").style.display = "inline";

        document.getElementById("imageDownload").href = dataURL;

    }

    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>


</head><body onload="init()">


<div style="" >
    <canvas id="can" style="border:2px solid;cursor: pointer;"></canvas>
</div>

<div style="height: auto;width: 810px;">
    <table style="width: 200px;border: 1px solid black; float: left;" >
        <tr><td></td><td colspan="7">Choose Color</td></tr>
        <tr>
            <td style="width: 12.5%">&nbsp;</td>
            <td style="width: 12.5%"><div style="width:10px;height:10px;background:green;" id="green" onclick="color(this)"></div></td>
            <td style="width: 12.5%"><div style="width:10px;height:10px;background:blue;" id="blue" onclick="color(this)"></div></td>
            <td style="width: 12.5%"><div style="width:10px;height:10px;background:red;" id="red" onclick="color(this)"></div></td>
            <td style="width: 12.5%"><div style="width:10px;height:10px;background:yellow;" id="yellow" onclick="color(this)"></div></td>
            <td style="width: 12.5%"><div style="width:10px;height:10px;background:orange;" id="orange" onclick="color(this)"></div></td>
            <td style="width: 12.5%"><div style="width:10px;height:10px;background:black;" id="black" onclick="color(this)"></div></td>
            <td style="width: 12.5%">&nbsp;</td>
        </tr>
    </table>

    <table style="margin-left:10px; width: 50px;border: 1px solid black;float: left;" >
        <tr><td>Eraser</td></tr>
        <tr>
            <td><div style="width:15px;height:15px;background:white;border:2px solid;" id="white" onclick="color(this)"></div></td>
        </tr>
    </table>

    <table style="width: 300px;border:0; float: left;margin-left: 25px;" >
        <tr><td></td><td colspan="7"></td></tr>
        <tr>
            <td style="width: 25px">&nbsp;</td>
            <td style="width: 100px"><input type="button" value="save" id="btn" size="30" onclick="save()" style=""></td>
            <td style="width: 10px">&nbsp;</td>
            <td style="width: 100px"><input type="button" value="clear" id="clr" size="23" onclick="erase()" style=""></td>
            <td style="width: 10px">&nbsp;</td>
            <td style="width: 100px"><input type="button" value="print" id="prnt" size="23" onclick="window.print()" style=""></td>
            <td style="width: 55px">&nbsp;</td>
        </tr>
    </table>

    <input type="button" id="btnSave" name="btnSave" value="Save the canvas to server" />

        <script type="text/javascript">
            // Send the canvas image to the server.
            $(function() {
                $("#btnSave").click(function() {
                    var image = document.getElementById("can").toDataURL("image/png"); 
                    image = image.replace('data:image/png;base64,', ''); 
                    $.ajax({
                        type: 'POST',
                        url: 'CanvasSave.jsp',
                        data:{'imageData':image},
                        dataType : 'json',
                        accepts: {json: 'application/json'},
                        success: function(msg) { alert('4');
                            alert('Image saved successfully !');
                        }
                    });
                });
            });
        </script>
</div>

<br /><br />

<div style="height: auto;width: 810px;">
    <a href="#" download="ImageName" title="ImageName" id="imageDownload">
        <img id="canvasimg" style="top:20px;">
    </a>
</div>



</body></html>

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北恋

2021-01-23 06:09
It can happen that your canvas is cleared when dialogs show - this happens typically in the Chrome browser.

Without having source code to try with as you didn't post any I'll make a theoretic answer that you can try out - I would suggest two possible solutions:
1. Add an event handler for resize. When triggered redraw the content (which means you need to store the points etc. that you draw or make an off-screen canvas to store a copy). I have experienced that this event triggers (in Chrome) when a dialog has cleared the canvas - if it works for print preview I am not sure - if not try next point:
2. When you click your print button, extract the canvas as an image and replace (temporary) the canvas element with an image element setting the source to the data-uri from canvas. On the image's onload handler trigger the print code:
Example for second point:
```
/// create a reference to canvas element
var canvas = document.getElementById('myCanvas');

printBtn.addEventListener('click' function() {

    /// remove it from DOM (use parent element if any, for demo I use body)
    document.body.removeChild(canvas);

    var img = new Image;

    img.id = 'tempPrintImage';    /// give an id so we can remove it in next step
    img.onload = print;           /// onload handler triggers your print method
    img.src = canvas.toDataURL(); /// set canvas image as source

    document.body.appendChild(img);

}, false);

function print() {

    ...your print code here. On return reverse the elements: ...

    var img = document.getElementById('tempPrintImage');
    document.body.removeChild(img);
    document.body.appendChild(canvas);
}
```
The code may seem a bit over-loaded but the key point here is to preserve the content of the canvas. You can place the image on top instead and so forth.
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