Get a pixel from HTML Canvas?

Question

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

Answer 1

There's a section about pixel manipulation in the W3C documentation.

Here's an example on how to invert an image:

var context = document.getElementById('myCanvas').getContext('2d');

// Get the CanvasPixelArray from the given coordinates and dimensions.
var imgd = context.getImageData(x, y, width, height);
var pix = imgd.data;

// Loop over each pixel and invert the color.
for (var i = 0, n = pix.length; i < n; i += 4) {
    pix[i  ] = 255 - pix[i  ]; // red
    pix[i+1] = 255 - pix[i+1]; // green
    pix[i+2] = 255 - pix[i+2]; // blue
    // i+3 is alpha (the fourth element)
}

// Draw the ImageData at the given (x,y) coordinates.
context.putImageData(imgd, x, y);

Answer 2

Try the getImageData method:

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

Answer 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(i, i+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.

Answer 4

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);
}

Answer 5

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

OCR and Neural Nets in JavaScript

Answer 6

Note that getImageData returns a snapshot. Implications are:

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

Answer 7

// 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];

Answer 8

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]
    });
}

Answer 9

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.

Answer 10

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>