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Get a pixel from HTML Canvas?

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


M
Magmatic

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);

Pixel manipulation section is now available here : w3.org/TR/2dcontext2/#pixel-manipulation
R
Rifky Niyas

Try the getImageData method:

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

shouldn't this be context.getImageData() and not canvas.getImageData()?
@Crashalot depends on what the var "canvas" contains, it could simply be the context of a canvas with a crappy var name.
Wow, very elegant! I thought about searching for the point in the entire context, but this is much smarter.
This is clever, but if you're going to be calling getPixel a lot, it is much faster to cache the ImageData object for the whole image (0,0,width,height), and then compute the index using idx = (y * width + x) * 4 like Georg's answer. However, don't forget to refresh that cached object every time the image changes.
What's that Color() constructor? That doesn't seem to exist anywhere
Q
Qwerty

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.


Yay! Thanks it works very well and it is waay faster than calling context.getImageData(x, y, 1, 1);
It is faster indeed. Thanks!
That's ok if there is no credits for my (very) small contribution, but thanks for willing to. ^^ Your answer is already a pro move and deserved another upvote by who you know, so my joy is complete. :-)
Just as a side note, your answer helped me to find a way to write a flood fill of a closed area. Even if my code is still not good (really a stack overflow, too much recursion for a given amount of pixel to fill) your answer is a cobblestone of the way leading to my goal. So big thanks!
@Amessihel Nice, thanks for the heads-up and praise 😊. Anyway, sometimes it is better to use while with a storage to avoid stack overflow with too much recursion.
a
anton-tchekov
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);
}

ctx should be set as a attribute in GetPixel though
Nice rgbToHex return statement
An explanation would be in order.
R
Rifky Niyas

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

OCR and Neural Nets in JavaScript


R
Rifky Niyas

Note that getImageData returns a snapshot. Implications are:

Changes will not take effect until subsequent putImageData

getImageData and putImageData calls are relatively slow


R
Rifky Niyas
//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];

u
user9342572809

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]
    });
}

More explanation would be in order.
K
Kamil Kiełczewski

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();

Move mouse on black box


P
Peter Mortensen

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.