Save PNG Canvas Image to HTML5 Storage (JAVASCRIPT

问题:

I am developing a chrome extension.

I open an image file in canvas, I apply some changes to it, then I am trying to save it to the HTML5 filesystem api.

First I get the dataURL from the canvas:

    var dataURL = canvas.toDataURL('image/png;base64'); 

Then just the data:

    var image64 = dataURL.replace(/data:image\/png;base64,/, '');

Then I make a Blob.

    var bb = new BlobBuilder();
    bb.append(image64);
    var blob = bb.getBlob('image/png');

Then I request the file system with the following function onInitFs();

    function onInitFs(fs) {
      fs.root.getFile('image.png', {create: true}, function(fileEntry) {
        // Create a FileWriter object for our FileEntry (log.txt).
        fileEntry.createWriter(function(fileWriter) {
        //WRITING THE BLOB TO FILE
        fileWriter.write(blob);
        }, errorHandler);
      }, errorHandler);
    }

    window.requestFileSystem(window.PERSISTENT, 5*1024*1024, onInitFs, errorHandler);

This results in a corrupted file being written to the file system.

I don't know what else I can do to make this work.

Could someone please guide me in the right direction.

The following are some of the sources to the functions I am using to accomplish this task.

http://dev.w3.org/html5/canvas-api/canvas-2d-api.html#todataurl-method

http://www.html5rocks.com/en/tutorials/file/filesystem/#toc-file-creatingempty

Thank You!

回答1:

I've found a function that converts a data URL to a blob.

Great for when you need to save a canvas image to the sandboxed FileSystem. Works in Chrome 13.

function dataURItoBlob(dataURI, callback) {
    // convert base64 to raw binary data held in a string
    // doesn't handle URLEncoded DataURIs
    var byteString = atob(dataURI.split(',')[1]);

    // separate out the mime component
    var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0]

    // write the bytes of the string to an ArrayBuffer
    var ab = new ArrayBuffer(byteString.length);
    var ia = new Uint8Array(ab);
    for (var i = 0; i < byteString.length; i++) {
        ia[i] = byteString.charCodeAt(i);
    }

    // write the ArrayBuffer to a blob, and you're done
    var bb = new window.WebKitBlobBuilder(); // or just BlobBuilder() if not using Chrome
    bb.append(ab);
    return bb.getBlob(mimeString);
};

Usage:

window.requestFileSystem(window.PERSISTENT, 1024*1024, function(fs){
    fs.root.getFile("image.png", {create:true}, function(fileEntry) {
        fileEntry.createWriter(function(fileWriter) {
            fileWriter.write(dataURItoBlob(myCanvas.toDataURL("image/png")));
        }, errorHandler);
    }, errorHandler);
}, errorHandler);

Source trail:

http://mustachified.com/master.js
via http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2011-April/031243.html
via https://bugs.webkit.org/show_bug.cgi?id=51652
via http://code.google.com/p/chromium/issues/detail?id=67587

回答2:

There is now a canvas.toBlob() polyfill: https://github.com/eligrey/canvas-toBlob.js
Other interesting link: https://github.com/eligrey/BlobBuilder.js

So with these it become easy to save image from canvas:

<script type="text/javascript" src="https://raw.github.com/eligrey/BlobBuilder.js/master/BlobBuilder.min.js"></script>
<script type="text/javascript" src="https://raw.github.com/eligrey/canvas-toBlob.js/master/canvas-toBlob.min.js"></script>
...
canvas.toBlob(function(blob) {
  fileWriter.write(blob);
}, "image/png");

Here is a little example using the FileSaver: http://jsfiddle.net/JR5XE/

回答3:

You seem to be directly writing the base64 representation to disk. You need to decode it first.

回答4:

I had been wondering the same thing and had a look at finding an answer.
From what I can tell you have to convert the raw string you get from an atob to an unit8array and then you can append it to a blob.
Here's an example that converts an image to a canvas and then the data from the canvas to a blob and then the third images src is set to the uri of the blob....you should be able to add the fs stuff from there....
http://jsfiddle.net/PAEz/XfDUS/
..sorry to not put the code here, but to be honest I couldnt figure out how to ;) and anyways, JSFiddle is a nice way to play with it.

References
https://gist.github.com/1032746
http://code.google.com/p/html5wow/source/browse/src/demos/photo-gallery/js/utils.js
Get image data in JavaScript?
http://jsdo.it/tsmallfield/typed_array

回答5:

You want HTMLCanvasElement.toBlob and then write the blob to a filesystem with the W3C filesystem API, but most browsers haven't implemented it yet.