Uint8ClampedArray替代或使Uint8Array产生一个钳位的字节值字符串

Question

I'm working on this real head scratcher!

I'm successfully adding byte values to a Uint8ClampedArray and using the array to generate a byte string using this function:

this.utf8ArrayToStr = function(array) {
        var s = '';
        for (var i = 0; i < array.length; i++) {
            s += String.fromCharCode(array[i]);
        }
        return s;
};

This works a treat and successfully generates the correct byte sequence.

The problem is that not all browsers support Uint8ClampedArray and I specifically need it to work on Android browsers.

I tried using a standard Uint8Array but it results in extra byte values in the resulting string, causing erroneous behaviour.

I also found a small shim here, but it did not work, again producing extra output: https://gist.github.com/gengkev/2295355

Is there anyway I could make Uint8Array act as Uint8ClampedArray, or more specifically, clamp the values while generating the string in the function above?

UPDATE: It looks like I'm getting somewhere using an standard array but there's still some messed up characters causing problems.

Answer 1

I think you shouldn't be using Uint8Array to store your data - it simply doesn't work like a Uint8ClampedArray. For example:

a = new Uint8Array(1);
b = new Uint8ClampedArray(1);
a[0] = 256;
b[0] = 256;

a[0] === 0;    // true
b[0] === 255;  // true

I would just use a regular array (if that's an option) and clamp the values when you store them into it, or when you read them to generate your string like this:

s += String.fromCharCode(Math.max(0, Math.min(255, array[i])));

Answer 2

The WhatWG has recommended the Uint8ClampedArray for storing canvas pixel data.

But, various browsers use Uint8ClampedArray (or not) depending on whether they've implemented the WhatWG recommendations.

Therefore, a more cross-browser solution is to pull an array from your canvas using .getImageData and then manipulate that array as desired. That way you know you're using a compatible array.

var imageData = context.getImageData(0,0,canvas.width,canvas.height);

var data = imageData.data;  // this array is always compatible with its browser.