javascript - Trying to save canvas PNG data url to disk with HTML5 filesystem, but when I retrieve as URL, it's invalid

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I get the base64-encoded image form the canvas as:

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

Then I turn it into data like this:

//Remove the beginning identifier and use Chrome/Firefox?safari built int base64Decoder
var data = atob( dataURL.substring( "data:image/png;base64,".length ) );

Then I write it to the filesystem via:

event.createWriter(
    function(writerEvent)
    {
        //The success handler
        writerEvent.onwriteend = function(finishEvent)
        {
            ...
        };

        //Error handler
        writerEvent.onerror = settings.error;

        // Create a new Blob
        var blob = new Blob( [ data ], { type: "image/png" } );

        //Write it into the path
        writerEvent.write( blob );
    }
}

I try to set it as src of an image like this:

document.getElementById( "saved" ).src = event.toURL();

That writes the file and I am able to find it and get a url (by reading it and using the event: event.toURL(). But the image shows as a broken image icon on the web page. What am I doing wrong?

I get the base64-encoded image form the canvas as:

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

Then I turn it into data like this:

//Remove the beginning identifier and use Chrome/Firefox?safari built int base64Decoder
var data = atob( dataURL.substring( "data:image/png;base64,".length ) );

Then I write it to the filesystem via:

event.createWriter(
    function(writerEvent)
    {
        //The success handler
        writerEvent.onwriteend = function(finishEvent)
        {
            ...
        };

        //Error handler
        writerEvent.onerror = settings.error;

        // Create a new Blob
        var blob = new Blob( [ data ], { type: "image/png" } );

        //Write it into the path
        writerEvent.write( blob );
    }
}

I try to set it as src of an image like this:

document.getElementById( "saved" ).src = event.toURL();

That writes the file and I am able to find it and get a url (by reading it and using the event: event.toURL(). But the image shows as a broken image icon on the web page. What am I doing wrong?

• javascript
• html
• google-chrome
• html5-canvas
• data-url

Share Improve this question Follow asked Jun 26, 2013 at 23:24 Don RhummyDon Rhummy 25.8k5151 gold badges192192 silver badges360360 bronze badges 1

• What's that event object? Can you show the code you're using to read the file? – MaxArt Commented Jun 26, 2013 at 23:45

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1 Answer 1

Sorted by: Reset to default 26

data is a string, so when you pass it to blob, the binary data will be that string in UTF-8 encoding. You want binary data not a string.

You can do it like:

var canvas = document.createElement("canvas");


var dataURL = canvas.toDataURL( "image/png" );
var data = atob( dataURL.substring( "data:image/png;base64,".length ) ),
    asArray = new Uint8Array(data.length);

for( var i = 0, len = data.length; i < len; ++i ) {
    asArray[i] = data.charCodeAt(i);    
}

var blob = new Blob( [ asArray.buffer ], {type: "image/png"} );

There is also canvas.toBlob available in future but not currently in Chrome.

Demo http://jsfiddle.net/GaLRS/

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