What s the best way to set a single pixel in an HTML5 canvas

Question

The HTML5 Canvas has no method for explicitly setting a single pixel   It might be possible to set a pixel using a very short line  but then antialising and line caps might interfere   Another way might be to create a small ImageData object and using   context putImageData data  x  y    to put it in place   Can anyone describe an efficient and reliable way of doing this

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Hmm  you could also just make a 1 pixel wide line with a length of 1 pixel and make it s direction move along a single axis               ctx beginPath                ctx lineWidth   1     one pixel wide             ctx strokeStyle   rgba                   ctx moveTo 50 25      positioned at 50 25             ctx lineTo 51 25      one pixel long             ctx stroke

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It seems strange  but nonetheless HTML5 supports drawing lines  circles  rectangles and many other basic shapes  it does not have anything suitable for drawing the basic point  The only way to do so is to simulate point with whatever you have   So basically there are 3 possible solutions    draw point as a line draw point as a polygon draw point as a circle   Each of them has their drawbacks    Line  function point x  y  canvas     canvas beginPath      canvas moveTo x  y     canvas lineTo x 1  y 1     canvas stroke        Keep in mind that we are drawing to South-East direction  and if this is the edge  there can be a problem  But you can also draw in any other direction     Rectangle  function point x  y  canvas     canvas strokeRect x y 1 1       or in a faster way using fillRect because render engine will just fill one pixel   function point x  y  canvas     canvas fillRect x y 1 1       Circle    One of the problems with circles is that it is harder for an engine to render them   function point x  y  canvas     canvas beginPath      canvas arc x  y  1  0  2   Math PI  true     canvas stroke        the same idea as with rectangle you can achieve with fill   function point x  y  canvas     canvas beginPath      canvas arc x  y  1  0  2   Math PI  true     canvas fill        Problems with all these solutions    it is hard to keep track of all the points you are going to draw  when you zoom in  it looks ugly    If you are wondering   What is the best way to draw a point    I would go with filled rectangle  You can see my jsperf here with comparison tests

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function setPixel imageData  x  y  r  g  b  a        var index   4    x   y   imageData width       imageData data index 0    r      imageData data index 1    g      imageData data index 2    b      imageData data index 3    a

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Since different browsers seems to prefer different methods  maybe it would make sense to do a smaller test with all three methods as a part of the loading process to find out which is best to use and then use that throughout the application

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There are two best contenders    Create a 1  1 image data  set the color  and putImageData at the location   var id   myContext createImageData 1 1      only do this once per page var d    id data                            only do this once per page d 0      r  d 1      g  d 2      b  d 3      a  myContext putImageData  id  x  y          Use fillRect   to draw a pixel  there should be no aliasing issues    ctx fillStyle    rgba   r     g     b      a 255       ctx fillRect  x  y  1  1       You can test the speed of these here  http   jsperf com setting-canvas-pixel 9 or here https   www measurethat net Benchmarks Show 1664 1   I recommend testing against browsers you care about for maximum speed  As of July 2017  fillRect   is 5-6   faster on Firefox v54 and Chrome v59  Win7x64    Other  sillier alternatives are    using getImageData   putImageData   on the entire canvas  this is about 100   slower than other options  creating a custom image using a data url and using drawImage   to show it   var img   new Image  img src    data image png base64     myPNGEncoder r g b a      Writing the PNGEncoder is left as an exercise for the reader  creating another img or canvas filled with all the pixels you want and use drawImage   to blit just the pixel you want across  This would probably be very fast  but has the limitation that you need to pre-calculate the pixels you need    Note that my tests do not attempt to save and restore the canvas context fillStyle  this would slow down the fillRect   performance  Also note that I am not starting with a clean slate or testing the exact same set of pixels for each test

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One method that hasnt been mentioned is using getImageData and then putImageData  This method is good for when you want to draw a lot in one go  fast  http   next plnkr co edit mfNyalsAR2MWkccr    var canvas   document getElementById  canvas      var ctx   canvas getContext  2d      var canvasWidth   canvas width    var canvasHeight   canvas height    ctx clearRect 0  0  canvasWidth  canvasHeight     var id   ctx getImageData 0  0  canvasWidth  canvasHeight     var pixels   id data       var x   Math floor Math random     canvasWidth       var y   Math floor Math random     canvasHeight       var r   Math floor Math random     256       var g   Math floor Math random     256       var b   Math floor Math random     256       var off    y   id width   x    4      pixels off    r      pixels off   1    g      pixels off   2    b      pixels off   3    255     ctx putImageData id  0  0

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What about a rectangle  That s got to be more efficient than creating an ImageData object

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To complete Phrogz very thorough answer  there is a critical difference between fillRect   and putImageData     The first uses context to draw over by adding a rectangle  NOT a pixel   using the fillStyle alpha value AND the context globalAlpha and the transformation matrix  line caps etc   The second replaces an entire set of pixels  maybe one  but why    The result is different as you can see on jsperf   Nobody wants to set one pixel at a time  meaning drawing it on screen   That is why there is no specific API to do that  and rightly so   Performance wise  if the goal is to generate a picture  for example a ray-tracing software   you always want to use an array obtained by getImageData   which is an optimized Uint8Array  Then you call putImageData   ONCE or a few times per second using setTimeout seTInterval

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Draw a rectangle like sdleihssirhc said   ctx fillRect  10  10  1  1      -- should draw a 1x1 rectangle at x 10  y 10

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putImageData is probably faster than fillRect natively  I think this because the fifth parameter can have different ways to be assigned  the rectangle color   using a string that must be interpreted   Suppose you re doing that   context fillRect x  y  1  1    fff   context fillRect x  y  1  1   rgba 255  255  255  0 5     context fillRect x  y  1  1   rgb 255 255 255     context fillRect x  y  1  1   blue      So  the line  context fillRect x  y  1  1   rgba 255  255  255  0 5       is the most heavy between all  The fifth argument in the fillRect call is a bit longer string

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Fast and handy Following class implements 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                      initialise  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            read pixel        0         let v  a 1 99446-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             read pixel       s putPixel x y  v 0 0                     put pixel              s refresh       frame      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 square lt  div gt  x000D   x000D   x000D

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If you are concerned about the speed then you could also consider WebGL

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I hadn t considered fillRect    but the answers spurred me to benchmark it against putImage     Putting 100 000 randomly coloured pixels in random locations  with Chrome 9 0 597 84 on an  old  MacBook Pro  takes less than 100ms with putImage    but nearly 900ms using fillRect      Benchmark code at http   pastebin com 4ijVKJcC    If instead I choose a single colour outside of the loops and just plot that colour at random locations  putImage   takes 59ms vs 102ms for fillRect     It seems that the overhead of generating and parsing a CSS colour specification in rgb      syntax is responsible for most of the difference   Putting raw RGB values straight into an ImageData block on the other hand requires no string handling or parsing

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Fast HTML Demo code  Based on what I know about SFML C   graphics library   Save this as an HTML file with UTF-8 Encoding and run it   Feel free to refactor  I just like using japanese variables because they are concise and don t take up much space  Rarely are you going to want to set ONE arbitrary pixel and display it on the screen  So use the   PutPix x y  r g b a     method to draw numerous arbitrary pixels to a back-buffer   cheap calls    Then when ready to show  call the   Apply      method to display the changes   expensive call   Full  HTML file code below    lt  DOCTYPE HTML  gt   lt html lang  en  gt   lt head gt       lt title gt  back-buffer demo  lt  title gt   lt  head gt   lt body gt    lt  body gt    lt script gt    Main function to execute once    all script is loaded  function main           Create a canvas      var canvas      canvas   attachCanvasToDom           Do the pixel setting test      var test type   FAST TEST      backBufferTest canvas  test type        Constants  var SLOW TEST   1  var FAST TEST   2    function attachCanvasToDom          Canvas Creation        cccccccccccccccccccccccccccccccccccccccccc         Create Canvas and append to body      var can   document createElement  canvas        document body appendChild can          Make canvas non-zero in size         so we can see it      can width   800      can height  600         Get the context  fill canvas to get visual      var ctx   can getContext  2d        ctx fillStyle    rgba 0  0  200  0 5        ctx fillRect 0 0 can width-1  can height-1         cccccccccccccccccccccccccccccccccccccccccc          Return the canvas that was created      return can       THIS OBJECT IS SLOOOOOWW           pen    T      Type Pen  function T  canvas           Publicly Exposed Functions       PEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPE       this PutPix    putPix        PEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPE        if  canvas           throw   NilCanvasGivenToPenConstruct                var  ctx   canvas getContext  2d           Pixel Setting Test         only do this once per page             image              data             image data              pen      var        ctx createImageData 1 1           only do this once per page     var           data           function  putPix x y   r g b a              0      r             1      g             2      b             3      a           ctx putImageData       x  y                 Back-buffer object  for fast pixel setting         butt rear  using to mean  back-buffer    T    type  back-buffer  function T  canvas          Publicly Exposed Functions       PEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPE       this PutPix    putPix      this Apply     apply        PEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPEPE        if  canvas           throw   NilCanvasGivenToPenConstruct                var  can   canvas      var  ctx   canvas getContext  2d           Pixel Setting Test         only do this once per page             image              data             image data              pen      var  w    can width      var  h    can height      var        ctx createImageData  w  h           only do this once per page     var           data           function  putPix x y   r g b a              Convert XY to index          var dex      y 4    w     x 4               dex 0      r             dex 1      g             dex 2      b             dex 3      a              function  apply             ctx putImageData       0 0                function backBufferTest canvas input  test type       var can   canvas input    shorthand var       if test type  SLOW TEST           var t    new T   can               Iterate over entire canvas             and set pixels          var x0   0          var x1   can width - 1           var y0   0          var y1   can height -1           for var x   x0  x  lt   x1  x             for var y   y0  y  lt   y1  y                 t  PutPix                  x y                   x 256  y 256  x y  256  255                            next X Y       else     if test type  FAST TEST           var t    new T   can               Iterate over entire canvas             and set pixels          var x0   0          var x1   can width - 1           var y0   0          var y1   can height -1           for var x   x0  x  lt   x1  x             for var y   y0  y  lt   y1  y                 t  PutPix                  x y                   x 256  y 256  x y  256  255                            next X Y            When done setting arbitrary pixels            use the apply method to show them            on screen          t  Apply               main     lt  script gt   lt  html gt

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