Javascript image manipulation? Pixel by pixel

Question

Is there a way or a library to obtain an images color data pixel by pixel in javascript? I need to read lots of OMR form, and i want to do it on clients not on my server :D. So either i can build it the system with javascript if i can reach raw data or i have to build a desktop application which i don't like at all. Thanks

Answer 1

Sure, no need to use libraries. It's really simple through canvases.

img -> canvas -> magic -> canvas -> img

What you need is to:

1. create <canvas>
2. get canvas context
3. copy img to canvas
4. get canvas image data (an array of values [r,g,b,a,r,g,b,a,r,g,..])
5. do `The magic`®
    
6. put image data back
7. put changed canvas back to <img>

the code to deal with canvas

var cvs = document.createElement('canvas'),
    img = document.getElementsByTagName("img")[0];   // your image goes here
    // img = $('#yourImage')[0];                     // can use jquery for selection
cvs.width = img.width; cvs.height = img.height;
var ctx = cvs.getContext("2d");
ctx.drawImage(img,0,0,cvs.width,cvs.height);
var idt = ctx.getImageData(0,0,cvs.width,cvs.height);

// usage
getPixel(idt, 852);  // returns array [red, green, blue, alpha]
getPixelXY(idt, 1,1); // same pixel using x,y

setPixelXY(idt, 1,1, 0,0,0,255); // a black pixel

6,7, modify the image ...

ctx.putImageData(idt, 0,0);  // 0,0 is xy coordinates
img.src = cvs.toDataURL();

wondered where the canvas is?

document.body.appendChild(cvs);

some magic functions

Since the image data are an array of consecutive r,g,b,a,r,g,.. values and one pixel consists of 4 values, you need to recalculate indexes. The formula is simple: data[(y*width + x)*4 + ch], where ch is between 0 and 3 for r,g,b,a channels.

_{Note that there is more concise and also faster solution below.}

function getPixel(imgData, index) {
  var i = index*4, d = imgData.data;
  return [d[i],d[i+1],d[i+2],d[i+3]] // [R,G,B,A]
}

function getPixelXY(imgData, x, y) {
  return getPixel(imgData, y*imgData.width+x);
}

function setPixel(imgData, index, r, g, b, a) {
  var i = index*4, d = imgData.data;
  d[i]   = r;
  d[i+1] = g;
  d[i+2] = b;
  d[i+3] = a;
}

function setPixelXY(imgData, x, y, r, g, b, a) {
  return setPixel(imgData, y*imgData.width+x, r, g, b, a);
}

---

update 2019 Uint8ClampedArray

You can manipulate pixel data directly with this subarray "view". subarray is different from slice or from previous example in that it does not create a shallow copy of the array but an array that shares the same store.

Read more at MDN:Uint8ClampedArray.

function getPixel(imgData, index) {
  return imgData.data.subarray(index*4, index*4+4) // Uint8ClampedArray(4) [R,G,B,A]
}

let px = getPixelXY(idt, 0, 0)
px[0] = 255

// ctx.putImageData(idt,0,0); // makes the pixel reddish

or

function getPixel(imgData, index) {
  return imgData.data.slice(index*4, index*4+4) // [R,G,B,A]
}

function setPixel(imgData, index, pixelData /*[R,G,B,A]*/) {
  imgData.data.set(pixelData, index*4)
}

Answer 2

HTML5 Canvas element is the way to go if you don't have browser limitations.