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I am trying to alter an image by highlighting an area defined by coordinates.

I've been using two canvases, on top of each other. Now I don't know if this is the best way to do it in the first place. /

<canvas id='canvas'>Your browser does not support HTML5 canvas</canvas>
<canvas id='canvas2'>Your browser does not support HTML5 canvas</canvas>

Currently, I am using two images, but I wonder if there is any way to use masks on canvas.

Second of all, I'd like to save the output of the stacked canvases.

I am trying to alter an image by highlighting an area defined by coordinates.

I've been using two canvases, on top of each other. Now I don't know if this is the best way to do it in the first place. http://jsfiddle/9wJu8/

<canvas id='canvas'>Your browser does not support HTML5 canvas</canvas>
<canvas id='canvas2'>Your browser does not support HTML5 canvas</canvas>

Currently, I am using two images, but I wonder if there is any way to use masks on canvas.

Second of all, I'd like to save the output of the stacked canvases.

  • javascript
  • html
  • canvas
Share Improve this question asked Mar 7, 2014 at 0:17 jonaslljonasll 4933 gold badges9 silver badges20 bronze badges 2
  • deepliquid./content/Jcrop.html – a Commented Mar 7, 2014 at 0:28
  • This is a cropping library, I need to highlight and output an image. – jonasll Commented Mar 7, 2014 at 0:33
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2 Answers 2

Reset to default 3

What @Ken said but I think some of his code example was accidentally omitted:

A Demo: http://jsfiddle/m1erickson/Spkhz/

<!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./jquery.min.js"></script>
<style>
    body{ background-color: ivory; }
    canvas{border:1px solid red;}
</style>
<script>
$(function(){

    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");

    var cw=700;
    var ch=438;

    var img=new Image();
    img.onload=start;
    img.src="cat.jpg";
    function start(){
        canvas.width=cw;
        canvas.height=ch;

        // draw the image on the canvas
        ctx.drawImage(img,0,0,cw,ch);

        // darken the image with a 50% black fill
        ctx.save();
        ctx.globalAlpha=.50;
        ctx.fillStyle="black";
        ctx.fillRect(0,0,cw,ch);
        ctx.restore();

        // ctx.clip() the area to highlight
        // and redraw the whole image
        // (the image will draw only in the clipping region)
        ctx.save();
        ctx.beginPath();
        ctx.clearRect(300,100,200,100);
        ctx.rect(300,100,200,100);
        ctx.clip();
        ctx.drawImage(img,0,0,cw,ch);
        ctx.restore();

    }

}); // end $(function(){});
</script>
</head>
<body>
    <canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

You can use a single canvas for this:

➔ Draw plete image
➔ Draw a transparent black rectangle on top
➔ Use drawImage() with clipping settings

For example:

// draw full image
ctx.drawImage(img, 0, 0);

// cover with a darkened overlay
ctx.fillStyle = 'rgba(0,0,0, 0.5);
ctx.fillRect(0, 0, canvas.width, canvas.height);

// draw region of image
ctx.drawImage(img, x, y, w, h,  x, y, w, h);

where x, y, w and h is the region you want to highlight.

To save out as an image just use toDataURL() on the canvas element.

本文标签： javascriptHighlight area of image with canvasJSStack Overflow

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