CODEWITHSUNDEEP
javascriptcsscanvasposition
Langston
Langston

Reputation: 1093

Positioning a canvas below overlapping canvases

I have two canvases that overlap intentionally, to draw a ball falling on top of the other canvas. I want to put a third canvas below those two overlapping ones on the page, without any overlap. When the div element that holds the overlapping canvases has a relative position, it does not prevent other elements from overlapping it. As I understand it, that div must be positioned relatively so that the canvases inside it can be positioned absolutely and overlap.

Here is my HTML:

<div id="box" style="position: relative;"></div>
<div id="countdiv"></div>

Here is the JavaScript:

boxCanvas = document.createElement("canvas");
// the margins on the page are 3%
boxCanvas.width = window.innerWidth - (window.innerWidth * .06);
boxCanvas.height = document.getElementById("height").value;
boxCanvas.style = "border: 1px solid #808080; position: absolute; left: 0; top: 0; z-index: 0";
document.getElementById("box").appendChild(boxCanvas);

// second canvas for drawing balls falling
ballCanvas = document.createElement("canvas");
ballCanvas.width = boxCanvas.width;
ballCanvas.height = boxCanvas.height;
ballCanvas.style = "border: 1px solid #808080; position: absolute; left: 0; top: 0; z-index: 1";
document.getElementById("box").appendChild(ballCanvas);

And here is the JavaScript for the third canvas:

countCanvas = document.createElement("canvas");
countCanvas.width = window.innerWidth - (window.innerWidth * .06);
countCanvas.height = 100;
countCanvas.style = "border: 1px solid #808080;";
document.getElementById("box").appendChild(countCanvas);
countctx = countCanvas.getContext("2d");
ballCanvas.height = boxCanvas.height;
ballCanvas.style = "border: 1px solid #808080; position: absolute; left: 0; top: 0; z-index: 1";
document.getElementById("countdiv").appendChild(ballCanvas);

The problem can be seen here when you click drawbox() and then "show hitcounts". Thanks for the help! Let me know if I could provide any more information.

Upvotes: 1

Views: 3077

Answers (1)

markE
markE

Reputation: 105035

I must admit I’m not exactly sure what you’re asking.

I think you want to have 2 overlapping canvases on top followed by a separate canvas on the bottom.

And you want to do the styling in javascript instead of CSS.

Here is code and a Fiddle: http://jsfiddle.net/m1erickson/hgHBw/

<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>

<style>
#box{
  border:1px solid blue;
}
.subcanvs{
  border: 1px solid green;
}
#countCanvas{
  border: 1px solid gold;    
}

</style>

<script>
$(function(){

    var w=300;
    var h=200;

    var boxDiv=document.getElementById("box");
    boxDiv.style.width=w+"px";
    boxDiv.style.height=h+"px";
    boxDiv.style.position="relative";

    var box=document.getElementById("boxCanvas");
    var boxCtx=box.getContext("2d");
    box.width=w;
    box.height=h;
    box.style.position="absolute";
    box.style.left=0;
    box.style.top=0;

    var ball=document.getElementById("ballCanvas");
    var ballCtx=ball.getContext("2d");
    ball.width=w;
    ball.height=h;
    ball.style.position="absolute";
    ball.style.left=0;
    ball.style.top=0;

    var counter=document.getElementById("countCanvas");
    var countCtx=counter.getContext("2d");
    counter.width=w;
    counter.height=100;

    test(boxCtx,20,30,"red");
    test(ballCtx,100,30,"green");
    test(countCtx,30,30,"blue");

    function test(ctx,x,y,color){
        ctx.beginPath();
        ctx.fillStyle=color;
        ctx.rect(x,y,50,50);
        ctx.fill();
    }

});   // end $(function(){});
</script>
</head>

<body>
    <div id="box">
      <canvas id="boxCanvas" class="subcanvs"></canvas>
      <canvas id="ballCanvas" class="subcanvs"></canvas>
    </div>
    <canvas id="countCanvas"></canvas>
</body>
</html>

One thing to note: 

// this sets the canvas drawing area to 100px wide
myCanvas.width=100;

// this styles the canvas element to be 100px wide on the page
// if this isn't == myCanvas.width then the image will be horizontally distorted
myCanvas.style.width="100px";

Upvotes: 2

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