guitio2002 guitio2002 - 5 months ago 69
Javascript Question

Display image from blob using javascript and websockets

I'm currently working on a WebSocket application that is displaying images send by a C++ server.
I've seen a couple of topics around there but I can't seem to get rid of this error in Firefox:


Image corrupt or truncated: data:image/png;base64,[some data]


Here's the Javascript code I'm using to display my blob:

socket.onmessage = function(msg) {
var blob = msg.data;

var reader = new FileReader();
reader.onloadend = function() {
var string = reader.result;
var buffer = Base64.encode(string);
var data = "data:image/png;base64,"+buffer;

var image = document.getElementById('image');
image.src = data;
};
reader.readAsBinaryString(blob);
}


I'm using the image of a red dot that I found on this topic: http://stackoverflow.com/a/4478878/1464608
And the Base64 class is from here: http://stackoverflow.com/a/246813/1464608

But the base64 outcome I get doesn't match and Firefox retrieves me an error of the image being corrupted.

I know this ain't much informations but I don't have a clue where to look :/
Any help is more than welcome!!

Answer

I think the cleanest solution would be to change the base64 encoder to operate directly on a Uint8Array instead of a string.

Important: You'll need to set the binaryType of the web socket to "arraybuffer" for this.

The onmessage method should look like this:

socket.onmessage = function(msg) {
    var arrayBuffer = msg.data;
    var bytes = new Uint8Array(arrayBuffer);

    var image = document.getElementById('image');
    image.src = encode(bytes);
};

The converted encoder should then look like this (based on http://stackoverflow.com/a/246813/1464608):

// public method for encoding an Uint8Array to base64
function encode (input) {
    var keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
    var output = "";
    var chr1, chr2, chr3, enc1, enc2, enc3, enc4;
    var i = 0;

    while (i < input.length) {
        chr1 = input[i++];
        chr2 = i < input.length ? input[i++] : Number.NaN; // Not sure if the index 
        chr3 = i < input.length ? input[i++] : Number.NaN; // checks are needed here

        enc1 = chr1 >> 2;
        enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
        enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
        enc4 = chr3 & 63;

        if (isNaN(chr2)) {
            enc3 = enc4 = 64;
        } else if (isNaN(chr3)) {
            enc4 = 64;
        }
        output += keyStr.charAt(enc1) + keyStr.charAt(enc2) +
                  keyStr.charAt(enc3) + keyStr.charAt(enc4);
    }
    return output;
}