how to remove one canvas element from displaying on top of another canvas element

Tags: javascript,css,html5-canvas,popup-balloons

Problem :

I have 2 canvases. One, the main canvas. Upon which all is drawn, Second, the speech bubble canvas (balloon). Which displays information about specific regions on my main canvas upon client clicks.

I was playing around with my canvas after introducing the speech bubble and came across an issue.

This is a simple code that shows how the speech bubble is introduced:-

My canvas is a timeline, and is scrollable; has historical events plotted on it. Once a user clicks on an event a speech bubble appears.

Now what I don't want to happen is, when a client clicks on the canvas, a speech bubble appears and then scrolls, the speech bubble moves to a new position on the scrolled image, however still showing information about the previous location.

For this we have the hideballoon () which assigns css property left : -200. However this still causes inconsistencies. For example if I drag the canvas from left to right, the balloon doesn't disappear with the scroll, but reappears in a new position.

there is a .remove() function $("#balloon").remove()

This successfully removes the balloon however, the issue with this is:- it removes the balloon completely, and no future clicks will pop up any more speech bubbles. This is not what I want. I want something dynamic.

Click on event >> speech bubble appears >> scroll canvas >> speech bubble disappears >> click on canvas >> speech bubble pertaining to new click appears back >> and so on and so forth.

Solution :


Use .show() and .hide() to keep the balloon out of your way when its not needed

When the user scrolls the window then just hide the balloon.

I assume you're scrolling the window instead of the canvas. If you're scrolling the canvas, just use $("#canvas").scroll( ... ) instead.

So when you need the balloon:

        // move the balloon canvas to the target
        $("#balloon").css({left:offsetX+X, top:offsetY+Y});

        // and show it

And hide the balloon when the user clicks on it or when the window scrolls:

    // listen for clicks on the balloon and then hide the balloon
    $("#balloon").click(function(e){ $("#balloon").hide(); });

    // listen for scrolls and then hide the balloon

Here’s working sample code and a Fiddle:

<!doctype html>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src=""></script>

    body{ width:2000px; background-color: ivory; padding:10px;padding-top:100px; }
    #canvas{border:1px solid red;}
    #balloon{ position:absolute; left:-200px; }


    // get reference to our drawing canvas
    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");
    // get reference to our balloon canvas
    var balloon=document.getElementById("balloon");
    var popCtx=balloon.getContext("2d");

    // get the position of canvas relative to window
    var canvasOffset=$("#canvas").offset();
    var offsetX=canvasOffset.left;

    // define some targets and their basic info
    var count=1;
    var circles=[];
    for(var x=50;x<1900;x+=50){
            x:x,  y:120, radius:20,
            info:"I'm #"+(count++)});

    // draw the target circles on the canvas
    ctx.font="16px verdana";
    for(var i=0;i<circles.length;i++){

    // listen for clicks on the canvas and show the balloon

        // get the mouseclick position

        // account for the window scrolling
        var scrollX=$(window).scrollLeft();
        var scrollY=$(window).scrollTop();

        // see if we clicked on any targets
        for(var i=0;i<circles.length;i++){
            var circle=circles[i];
            var dx=(circle.x-scrollX)-mouseX;
            var dy=(circle.y-scrollY)-mouseY;
            var radius=circle.radius;
            // true if we clicked in the target circle

    // listen for clicks on the balloon and then hide the balloon
    $("#balloon").click(function(e){ $("#balloon").hide(); });

    // listen for scrolls and then hide the balloon

    function drawCircle(circle){;

    function drawBalloon(X,Y,theInfo){;
        // draw the balloon
        // draw theInfo
        popCtx.font="10pt arial";
        // move the balloon canvas to the target
        $("#balloon").css({left:offsetX+X, top:offsetY+Y});

}); // end $(function(){});


    <canvas id="canvas" width=1950 height=300></canvas>
    <canvas id="balloon" width=105 height=105></canvas>

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