将字节转换为图像以在HTML5画布上绘制

Question

Anyone know how I would convert bytes which are sent via a websocket (from a C# app) to an image? I then want to draw the image on a canvas. I can see two ways of doing this:

• Somehow draw the image on the canvas in byte form without converting it.
• Convert the bytes to a base64 string somehow in javascript then draw.

Here's my function which receives the bytes for drawing:

function draw(imgData) {

    var img=new Image();
    img.onload = function() {
        cxt.drawImage(img, 0, 0, canvas.width, canvas.height);
    };

// What I was using before...
img.src = "data:image/jpeg;base64,"+imgData;

}

I was receiving the image already converted as a base64 string before, but after learning that sending the bytes is smaller in size (30% smaller?) I would prefer to get this working. I should also mention that the image is a jpeg.

Anyone know how I would do it? Thanks for the help. :)

Answer 1

If your image is really a jpeg already, you can just convert the received data to a base64 stream. Firefox and Webkit browsers (as I recall) have a certain function, btoa() . It converts the input string to a base64 encoded string. Its counterpart is atob() that does the opposite.

You could use it like the following:

function draw(imgData){
    var b64imgData = btoa(imgData); //Binary to ASCII, where it probably stands for
    var img = new Image();
    img.src = "data:image/jpeg;base64," + b64imgData;
    document.body.appendChild(img); //or append it to something else, just an example
}

If the browser you target (IE, for example) isn't Firefox or a Webkit one, you can use one of the multiple conversion function lying around the internet ( good one , it also provides statistics of performances in multiple browsers, if you're interested :)

Answer 2

I used this in the end:

function draw(imgData, frameCount) {
    var r = new FileReader();
    r.readAsBinaryString(imgData);
    r.onload = function(){ 
        var img=new Image();
        img.onload = function() {
            cxt.drawImage(img, 0, 0, canvas.width, canvas.height);
        }
        img.src = "data:image/jpeg;base64,"+window.btoa(r.result);
    };
}

I needed to read the bytes into a string before using btoa().