userrandomuser userrandomuser - 10 months ago 47
Javascript Question

Drawing on canvas using touchscreen devices

I've been doing a bit of searching to see if it is possible to allow touch screen users to draw on the canvas tag through raw JavaScript. So far, I am not having much luck and end up going back to square one again of having a canvas that cannot be interacted with. I have the canvas inside of a div. Any advice as to how I could accomplish this? I've tried some tutorials and they only seem to work for desktop devices and click events, not when the user is moving their finger across the canvas.



<script type="text/javascript">
    // Variables for referencing the canvas and 2dcanvas context
    var canvas,ctx;

    // Variables to keep track of the mouse position and left-button status 
    var mouseX,mouseY,mouseDown=0;

    // Draws a dot at a specific position on the supplied canvas name
    // Parameters are: A canvas context, the x position, the y position, the size of the dot
    function drawDot(ctx,x,y,size) {
        // Let's use black by setting RGB values to 0, and 255 alpha (completely opaque)
        r=0; g=0; b=0; a=255;

        // Select a fill style
        ctx.fillStyle = "rgba("+r+","+g+","+b+","+(a/255)+")";

        // Draw a filled circle
        ctx.arc(x, y, size, 0, Math.PI*2, true); 

    // Clear the canvas context using the canvas width and height
    function clearCanvas(canvas,ctx) {
        ctx.clearRect(0, 0, canvas.width, canvas.height);

    // Keep track of the mouse button being pressed and draw a dot at current location
    function sketchpad_mouseDown() {

    // Keep track of the mouse button being released
    function sketchpad_mouseUp() {

    // Keep track of the mouse position and draw a dot if mouse button is currently pressed
    function sketchpad_mouseMove(e) { 
        // Update the mouse co-ordinates when moved

        // Draw a dot if the mouse button is currently being pressed
        if (mouseDown==1) {

    // Get the current mouse position relative to the top-left of the canvas
    function getMousePos(e) {
        if (!e)
            var e = event;

        if (e.offsetX) {
            mouseX = e.offsetX;
            mouseY = e.offsetY;
        else if (e.layerX) {
            mouseX = e.layerX;
            mouseY = e.layerY;

    // Set-up the canvas and add our event handlers after the page has loaded
    function init() {
        // Get the specific canvas element from the HTML document
        canvas = document.getElementById('sketchpad');

        // If the browser supports the canvas tag, get the 2d drawing context for this canvas
        if (canvas.getContext)
            ctx = canvas.getContext('2d');

        // Check that we have a valid context to draw on/with before adding event handlers
        if (ctx) {
            canvas.addEventListener('mousedown', sketchpad_mouseDown, false);
            canvas.addEventListener('mousemove', sketchpad_mouseMove, false);
            window.addEventListener('mouseup', sketchpad_mouseUp, false);

/* Some CSS styling */
#sketchpadapp {
    /* Prevent nearby text being highlighted when accidentally dragging mouse outside confines of the canvas */
    -webkit-touch-callout: none;
    -webkit-user-select: none;
    -khtml-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;

#sketchpad {
    border:2px solid #888;
    position:relative; /* Necessary for correct mouse co-ords in Firefox */

<body onload="init()">
    <div id="sketchpadapp">
        <div class="leftside1">

             <input type="submit" value="Clear Sketchpad" onclick="clearCanvas(canvas,ctx);">
        <div class="rightside">
            <canvas id="sketchpad" height="300" width="400">