Converting rgba values into one integer in Javascript

Question

I can already convert 32bit integers into their rgba values like this:

pixelData[i] = {
        red: pixelValue >> 24 & 0xFF,
        green: pixelValue >> 16 & 0xFF,
        blue: pixelValue >> 8 & 0xFF,
        alpha: pixelValue & 0xFF
    };

But I don't really know how to reverse it.

MightyPork · Accepted Answer · 2016-01-27 18:07:45Z

21

To reverse it, you just have to combine the bytes into an integer. Simply use left-shift and add them, and it will work.

var rgb = (red << 24) + (green << 16) + (blue << 8) + (alpha);

Alternatively, to make it safer, you could first AND each of them with 0xFF:

var r = red & 0xFF;
var g = green & 0xFF;
var b = blue & 0xFF;
var a = alpha & 0xFF;

var rgb = (r << 24) + (g << 16) + (b << 8) + (a);

(You may use bitwise OR | instead of + here, the outcome will be the same).

edited Jan 27, 2016 at 18:07

answered Jul 30, 2013 at 11:41

MightyPork

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4 Comments

Jelle De Loecker Over a year ago

That does work, indeed. I thought it didn't because the input value was not the same as the output value. Still isn't. Don't know what extra bits are stored in there, but I guess they don't matter.

lunatix Over a year ago

Keep in mind that this does not work with r/g/b/a being integers. One must cast them explicitly into byte before adding them together.

Ali Over a year ago

JS bitwise operators get and return 32 bit signed integer, so 255 << 24 become -16777216, the r part should be (r * (2 ** 24)), and you can't use | instead of +

Philippe Over a year ago

One can use >>> 0 to get the signed integer, i.e.: 255 << 24 >>> 0 = 4278190080

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