How to resize html canvas element

Question

I have a canvas element defined statically in the html with a width and height  If I attempt to use JavaScript to resize it dynamically  setting a new width and height - either on the attributes of the canvas or via the style properties  I get the following error in Firefox      uncaught exception   Exception     Illegal operation on WrappedNative prototype object   nsresult   0x8057000c  NS ERROR XPC BAD OP ON WN PROTO    location   JS frame    file    home russh Desktop test html    onclick    line 1   data  no    Is it possible to resize this element or do I have to destroy it and create a new element on the fly

User · Answer

You didn t publish your code  and I suspect you do something wrong  it is possible to change the size by assigning width and height attributes using numbers   canvasNode width    200     in pixels canvasNode height   100     in pixels   At least it works for me  Make sure you don t assign strings  e g    2cm    3in   or  2 5px    and don t mess with styles   Actually this is a publicly available knowledge  mdash  you can read all about it in the HTML canvas spec  mdash  it is very small and unusually informative  This is the whole DOM interface   interface HTMLCanvasElement   HTMLElement              attribute unsigned long width             attribute unsigned long height     DOMString toDataURL      DOMString toDataURL in DOMString type   Variadic  in any args      DOMObject getContext in DOMString contextId        As you can see it defines 2 attributes width and height  and both of them are unsigned long

User · Answer

Here s my effort to give a more complete answer  building on  john s answer    The initial issue I encountered was changing the width and height of a canvas node  using styles   resulted in the contents just being  zoomed  or  shrunk   This was not the desired effect   So  say you want to draw two rectangles of arbitrary size in a canvas that is 100px by 100px    lt canvas width  100  height  100  gt  lt  canvas gt    To ensure that the rectangles will not exceed the size of the canvas and therefore not be visible  you need to ensure that the canvas is big enough   var  canvas      canvas        oldCanvas      context    canvas 0  getContext  2d     function drawRects x  y  width  height      if    canvas width    lt  x width      canvas height    lt  y height          oldCanvas    canvas 0  toDataURL  image png        canvas 0  width   x width       canvas 0  height   y height       var img   new Image        img src   oldCanvas      img onload   function           context drawImage img  0  0                context strokeRect x  y  width  height       drawRects 5 5  10  10   drawRects 15 15  20  20   drawRects 35 35  40  40   drawRects 75  75  80  80     Finally  here s the jsfiddle for this  http   jsfiddle net Rka6D 4

User · Answer

Prototypes can be a hassle to work with  and from the  PROTO part of the error it appears your error is caused by  say  HTMLCanvasElement prototype width  possibly as an attempt to resize all the canvases at once   As a suggestion  if you are trying to resize a number of canvases at once  you could try    lt canvas gt  lt  canvas gt   lt canvas gt  lt  canvas gt   lt canvas gt  lt  canvas gt   lt script type  text javascript  gt           lt  script gt    In the JavaScript  instead of invoking a prototype  try this        function        return document querySelectorAll apply document arguments     for var i in     canvas         canvas       canvas   i       canvas width   canvas width 100      canvas height   canvas height 100      This would resize all the canvases by adding 100 px to their size  as is demonstrated in this example  Hope this helped

User · Answer

This worked for me just now    lt canvas id  c  height  100  width  100  style  border 1px solid red  gt  lt  canvas gt   lt script gt  var c   document getElementById  c    alert c height         c width   c height   200  c width   200  alert c height         c width    lt  script gt

User · Answer

This worked for me just now    lt canvas id  c  height  100  width  100  style  border 1px solid red  gt  lt  canvas gt   lt script gt  var c   document getElementById  c    alert c height         c width   c height   200  c width   200  alert c height         c width    lt  script gt

User · Answer

lt div id  canvasdiv  style  margin  5px  height  100   width  100    gt       lt canvas id  mycanvas  style  border  1px solid red   gt  lt  canvas gt   lt  div gt   lt script gt    function        InitContext        function InitContext     var  canvasDiv       canvasdiv     var canvas   document getElementById  mycanvas    canvas height    canvasDiv innerHeight    canvas width    canvasDiv innerWidth       lt  script gt

User · Answer

Note that if your canvas is statically declared you should use the width and height attributes  not the style  eg  this will work    lt canvas id  c  height  100  width  100  style  border 1px  gt  lt  canvas gt   lt script gt      document getElementById  c   width   200   lt  script gt    But this will not work    lt canvas id  c  style  width  100px  height  100px  border 1px  gt  lt  canvas gt   lt script gt      document getElementById  c   width   200   lt  script gt

User · Answer

Note that if your canvas is statically declared you should use the width and height attributes  not the style  eg  this will work    lt canvas id  c  height  100  width  100  style  border 1px  gt  lt  canvas gt   lt script gt      document getElementById  c   width   200   lt  script gt    But this will not work    lt canvas id  c  style  width  100px  height  100px  border 1px  gt  lt  canvas gt   lt script gt      document getElementById  c   width   200   lt  script gt

User · Answer

I just had the same problem as you  but found out about the toDataURL method  which proved useful   The gist of it is to turn your current canvas into a dataURL  change your canvas size  and then draw what you had back into your canvas from the dataURL you saved   So here s my code   var oldCanvas   canvas toDataURL  image png    var img   new Image    img src   oldCanvas  img onload   function         canvas height    100      ctx drawImage img  0  0

User · Answer

You didn t publish your code  and I suspect you do something wrong  it is possible to change the size by assigning width and height attributes using numbers   canvasNode width    200     in pixels canvasNode height   100     in pixels   At least it works for me  Make sure you don t assign strings  e g    2cm    3in   or  2 5px    and don t mess with styles   Actually this is a publicly available knowledge  mdash  you can read all about it in the HTML canvas spec  mdash  it is very small and unusually informative  This is the whole DOM interface   interface HTMLCanvasElement   HTMLElement              attribute unsigned long width             attribute unsigned long height     DOMString toDataURL      DOMString toDataURL in DOMString type   Variadic  in any args      DOMObject getContext in DOMString contextId        As you can see it defines 2 attributes width and height  and both of them are unsigned long

User · Answer

I just had the same problem as you  but found out about the toDataURL method  which proved useful   The gist of it is to turn your current canvas into a dataURL  change your canvas size  and then draw what you had back into your canvas from the dataURL you saved   So here s my code   var oldCanvas   canvas toDataURL  image png    var img   new Image    img src   oldCanvas  img onload   function         canvas height    100      ctx drawImage img  0  0

User · Answer

lt div id  canvasdiv  style  margin  5px  height  100   width  100    gt       lt canvas id  mycanvas  style  border  1px solid red   gt  lt  canvas gt   lt  div gt   lt script gt    function        InitContext        function InitContext     var  canvasDiv       canvasdiv     var canvas   document getElementById  mycanvas    canvas height    canvasDiv innerHeight    canvas width    canvasDiv innerWidth       lt  script gt

User · Answer

This worked for me just now    lt canvas id  c  height  100  width  100  style  border 1px solid red  gt  lt  canvas gt   lt script gt  var c   document getElementById  c    alert c height         c width   c height   200  c width   200  alert c height         c width    lt  script gt

User · Answer

You didn t publish your code  and I suspect you do something wrong  it is possible to change the size by assigning width and height attributes using numbers   canvasNode width    200     in pixels canvasNode height   100     in pixels   At least it works for me  Make sure you don t assign strings  e g    2cm    3in   or  2 5px    and don t mess with styles   Actually this is a publicly available knowledge  mdash  you can read all about it in the HTML canvas spec  mdash  it is very small and unusually informative  This is the whole DOM interface   interface HTMLCanvasElement   HTMLElement              attribute unsigned long width             attribute unsigned long height     DOMString toDataURL      DOMString toDataURL in DOMString type   Variadic  in any args      DOMObject getContext in DOMString contextId        As you can see it defines 2 attributes width and height  and both of them are unsigned long

User · Answer

Here s my effort to give a more complete answer  building on  john s answer    The initial issue I encountered was changing the width and height of a canvas node  using styles   resulted in the contents just being  zoomed  or  shrunk   This was not the desired effect   So  say you want to draw two rectangles of arbitrary size in a canvas that is 100px by 100px    lt canvas width  100  height  100  gt  lt  canvas gt    To ensure that the rectangles will not exceed the size of the canvas and therefore not be visible  you need to ensure that the canvas is big enough   var  canvas      canvas        oldCanvas      context    canvas 0  getContext  2d     function drawRects x  y  width  height      if    canvas width    lt  x width      canvas height    lt  y height          oldCanvas    canvas 0  toDataURL  image png        canvas 0  width   x width       canvas 0  height   y height       var img   new Image        img src   oldCanvas      img onload   function           context drawImage img  0  0                context strokeRect x  y  width  height       drawRects 5 5  10  10   drawRects 15 15  20  20   drawRects 35 35  40  40   drawRects 75  75  80  80     Finally  here s the jsfiddle for this  http   jsfiddle net Rka6D 4

User · Answer

Prototypes can be a hassle to work with  and from the  PROTO part of the error it appears your error is caused by  say  HTMLCanvasElement prototype width  possibly as an attempt to resize all the canvases at once   As a suggestion  if you are trying to resize a number of canvases at once  you could try    lt canvas gt  lt  canvas gt   lt canvas gt  lt  canvas gt   lt canvas gt  lt  canvas gt   lt script type  text javascript  gt           lt  script gt    In the JavaScript  instead of invoking a prototype  try this        function        return document querySelectorAll apply document arguments     for var i in     canvas         canvas       canvas   i       canvas width   canvas width 100      canvas height   canvas height 100      This would resize all the canvases by adding 100 px to their size  as is demonstrated in this example  Hope this helped

User · Answer

This worked for me just now    lt canvas id  c  height  100  width  100  style  border 1px solid red  gt  lt  canvas gt   lt script gt  var c   document getElementById  c    alert c height         c width   c height   200  c width   200  alert c height         c width    lt  script gt

User · Answer

You didn t publish your code  and I suspect you do something wrong  it is possible to change the size by assigning width and height attributes using numbers   canvasNode width    200     in pixels canvasNode height   100     in pixels   At least it works for me  Make sure you don t assign strings  e g    2cm    3in   or  2 5px    and don t mess with styles   Actually this is a publicly available knowledge  mdash  you can read all about it in the HTML canvas spec  mdash  it is very small and unusually informative  This is the whole DOM interface   interface HTMLCanvasElement   HTMLElement              attribute unsigned long width             attribute unsigned long height     DOMString toDataURL      DOMString toDataURL in DOMString type   Variadic  in any args      DOMObject getContext in DOMString contextId        As you can see it defines 2 attributes width and height  and both of them are unsigned long

[javascript] How to resize html canvas element?

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