Get a pixel from HTML Canvas

Question

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

User · Accepted Answer

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  lt  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

User · Answer

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

User · Answer

Note that getImageData returns a snapshot  Implications are   Changes will not take effect until subsequent putImageData getImageData and putImageData calls are relatively slow

User · Answer

Try the getImageData method  var data   context getImageData x  y  1  1  data  var rgb     data 0   data 1   data 2

User · Answer

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

User · Answer

You can use i  lt  lt  2   const data   context getImageData x  y  width  height  data  const pixels        for  let i   0  dx   0  dx  lt  data length  i    dx   i  lt  lt  2        pixels push           r  data dx             g  data dx 1           b  data dx 2           a  data dx 3

User · Answer

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  gt  255    g  gt  255    b  gt  255          throw  Invalid color component       return   r  lt  lt  16     g  lt  lt  8    b  toString 16

User · Answer

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  quot canvas quot    var ctx   canvas getContext  quot 2d quot    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

User · Answer

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  x000D   x000D  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  lt  lt  24     b  lt  lt  16     g  lt  lt   8    r           readPixel x y        let p  this data y   this width   x      return  p amp 0xff  p gt  gt 8 amp 0xff  p gt  gt 16 amp 0xff  p gt  gt  gt 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  lt  s height-1    y        for  var x   1  x  lt  s width-1    x                let a     1 0   -1 0   0 1   0 -1   reduce  a  xp yp    gt           a   s readPixel x xp y yp  0         0         let v a 2-tmp x  y         tmp x  y  v lt 0   0 v               for  var y   1  y  lt  s height-1    y        for  var x   1  x  lt  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   gt  Array s height  fill 0     function move e    s putPixel e x-10  e y-10  255 255 255     draw    x000D   lt canvas id  canvas  height  150  width  512  onmousemove  move event   gt  lt  canvas gt   lt div gt Move mouse on black box lt  div gt  x000D   x000D   x000D

User · Answer

Yup  check out getImageData    Here s an example of breaking CAPTCHA with JavaScript using canvas  OCR and Neural Nets in JavaScript

[javascript] Get a pixel from HTML Canvas?

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