[javascript] Get a pixel from HTML Canvas?

Is it possible to query a HTML Canvas object to get the color at a specific location?

You can use i << 2.

const data = context.getImageData(x, y, width, height).data;
const pixels = [];

for (let i = 0, dx = 0; dx < data.length; i++, dx = i << 2) {
    pixels.push({
        r: data[dx  ],
        g: data[dx+1],
        b: data[dx+2],
        a: data[dx+3]
    });
}

Yes sure, provided you have its context. (See how to get canvas context here.)

var imgData = context.getImageData(0,0,canvas.width,canvas.height);
// { data: [r,g,b,a,r,g,b,a,r,g,..], ... }

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

// AND/OR

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

PS: If you plan to mutate the data and draw them back on the canvas, you can use subarray

var
  idt = imgData, // See previous code snippet
  a = getPixel(idt, 188411), // Array(4) [0, 251, 0, 255]
  b = idt.data.subarray(188411*4, 188411*4 + 4) // Uint8ClampedArray(4) [0, 251, 0, 255]

a[0] = 255 // Does nothing
getPixel(idt, 188411), // Array(4) [0, 251, 0, 255]

b[0] = 255 // Mutates the original imgData.data
getPixel(idt, 188411), // Array(4) [255, 251, 0, 255]

// Or use it in the function
function getPixel(imgData, index) {
  var i = index*4, d = imgData.data;
  return imgData.data.subarray(index, index+4) // Returns subarray [R,G,B,A]
}

You can experiment with this on http://qry.me/xyscope/, the code for this is in the source, just copy/paste it in the console.

Yup, check out getImageData(). Here's an example of breaking CAPTCHA with JavaScript using canvas:

OCR and Neural Nets in JavaScript

// Get pixel data
var imageData = context.getImageData(x, y, width, height);

// Color at (x,y) position
var color = [];
color['red'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 0];
color['green'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 1];
color['blue'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 2];
color['alpha'] = imageData.data[((y*(imageData.width*4)) + (x*4)) + 3];

If you want to extract a particular color of pixel by passing the coordinates of pixel into the function, this will come in handy:

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
function detectColor(x, y){
  data=ctx.getImageData(x, y, 1, 1).data;
  col={
    r:data[0],
    g:data[1],
    b:data[2]
  };
  return col;
}

x, y is the coordinate you want to filter out color.

var color = detectColor(x, y)

The color is the object, you will get the RGB value by color.r, color.g, color.b.

function GetPixel(context, x, y)
{
    var p = context.getImageData(x, y, 1, 1).data; 
    var hex = "#" + ("000000" + rgbToHex(p[0], p[1], p[2])).slice(-6);  
    return hex;
}

function rgbToHex(r, g, b) {
    if (r > 255 || g > 255 || b > 255)
        throw "Invalid color component";
    return ((r << 16) | (g << 8) | b).toString(16);
}

Try the getImageData method:

var data = context.getImageData(x, y, 1, 1).data;
var rgb = [ data[0], data[1], data[2] ];

Fast and handy

Use following class which implement fast method described in this article and contains all you need: readPixel, putPixel, get width/height. Class update canvas after calling refresh() method. Example solve simple case of 2d wave equation

class Screen{
  constructor(canvasSelector) {
    this.canvas = document.querySelector(canvasSelector);
    this.width  = this.canvas.width;
    this.height = this.canvas.height;
    this.ctx = this.canvas.getContext('2d');
    this.imageData = this.ctx.getImageData(0, 0, this.width, this.height);
    this.buf = new ArrayBuffer(this.imageData.data.length);
    this.buf8 = new Uint8ClampedArray(this.buf);
    this.data = new Uint32Array(this.buf);  
  }
  
  // r,g,b,a - red, gren, blue, alpha components in range 0-255
  putPixel(x,y,r,g,b,a=255) {
    this.data[y * this.width + x] = (a << 24) | (b << 16) | (g <<  8) | r;
  }
  
  readPixel(x,y) {
    let p= this.data[y * this.width + x]
    return [p&0xff, p>>8&0xff, p>>16&0xff, p>>>24];
  }

  refresh() {
    this.imageData.data.set(this.buf8);
    this.ctx.putImageData(this.imageData, 0, 0);
  }
}




// --------
// TEST
// --------

let s=new Screen('#canvas');

function draw() {

  for (var y = 1; y < s.height-1; ++y) {
    for (var x = 1; x < s.width-1; ++x) {      
      let a = [[1,0],[-1,0],[0,1],[0,-1]].reduce((a,[xp,yp])=> 
        a+= s.readPixel(x+xp,y+yp)[0]
      ,0);
      let v=a/2-tmp[x][y];
      tmp[x][y]=v<0 ? 0:v;
    }
  }

  for (var y = 1; y < s.height-1; ++y) {
    for (var x = 1; x < s.width-1; ++x) {  
      let v=tmp[x][y];
      tmp[x][y]= s.readPixel(x,y)[0];
      s.putPixel(x,y, v,v,v);
    }
  }

  s.refresh();
  window.requestAnimationFrame(draw)
}

// temporary 2d buffer ()for solving wave equation)
let tmp = [...Array(s.width)].map(x => Array(s.height).fill(0));

function move(e) { s.putPixel(e.x-10, e.y-10, 255,255,255);}

draw();

<canvas id="canvas" height="150" width="512" onmousemove="move(event)"></canvas>
<div>Move mouse on black box</div>

Note that getImageData returns a snapshot. Implications are:

• Changes will not take effect until subsequent putImageData
• getImageData and putImageData calls are relatively slow