userrandomuser userrandomuser - 1 year ago 58
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.

Answer Source


<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">