How can I draw an image from the HTML5 File API on Canvas?

问题

I would like to draw an image opened with the HTML5 File API on a canvas.

In the handleFiles(e) method, I can access the File with e.target.files[0] but I can't draw that image directly using drawImage. How do I draw an image from the File API on HTML5 canvas?

Here is the code I have used:

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"/>
<script>
window.onload = function() {
    var input = document.getElementById('input');
    input.addEventListener('change', handleFiles);
}

function handleFiles(e) {
    var ctx = document.getElementById('canvas').getContext('2d');
    ctx.drawImage(e.target.files[0], 20,20);
    alert('the image is drawn');
}
</script>
</head>
<body>
<h1>Test</h1>
<input type="file" id="input"/>
<canvas width="400" height="300" id="canvas"/>
</body>
</html>

回答1:

You have a File instance which is not an image.

~~To get contents of a File, use a FileReader. Then pass the contents to an Image instance, which can be drawn on a canvas: http://jsfiddle.net/t7mv6/.~~

To get an image, use new Image(). The src needs to be an URL referencing to the selected File. You can use URL.createObjectURL to get an URL referencing to a Blob (a File is also a Blob): http://jsfiddle.net/t7mv6/86/.

var ctx = document.getElementById('canvas').getContext('2d');
var img = new Image;
img.onload = function() {
    ctx.drawImage(img, 20,20);
    alert('the image is drawn');
}
img.src = URL.createObjectURL(e.target.files[0]);

Note: be sure to revoke the object url when you are done with it otherwise you'll leak memory. If you're not doing anything too crazy, you can just stick a URL.revokeObjectURL(img.src) in the img.onload function.

References:

https://developer.mozilla.org/en/DOM/File
http://html5demos.com/file-api

回答2:

Live Example

function handleFiles(e) {
    var ctx = document.getElementById('canvas').getContext('2d');
    var url = URL.createObjectURL(e.target.files[0]);
    var img = new Image();
    img.onload = function() {
        ctx.drawImage(img, 20, 20);    
    }
    img.src = url;   
}

window.URL.createObjectUrldocs

You could also use the FileReader instead to create the object URL.

The FileReader has slightly better browser support.

The FileReader approach works in FF6 / Chrome. I'm not certain whether setting Img.src to a Blob is valid and cross-browser though.

Creating object urls is the correct way to do it.

Edit:

As mentioned in the commment window.URL support whilst offline seems unavailable in FF6/Chrome.

回答3:

Here is a complete example (Fiddle) using FileReader (which has better browser support as mentioned by Raynos). In this example I also scale Canvas to fit the image.

In real life example you might scale the image to some maximum so that your form will not blow up ;-). Here is an example with scaling (Fiddle).

来源：https://stackoverflow.com/questions/6775767/how-can-i-draw-an-image-from-the-html5-file-api-on-canvas

标签

javascript

html5-canvas

fileapi

drawimage