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Canvas element

January 01, 1970

This article is about the HTML element. For the general element in user interfaces, see Canvas (GUI).

The canvas element is part of HTML5 and allows for dynamic, scriptable rendering of 2D shapes and bitmap images. It is a low level, procedural model that updates a bitmap. HTML5 Canvas also helps in making 2D games.

While the HTML5 canvas offers its own 2D drawing API, it also supports the WebGL API to allow 3D rendering with OpenGL ES.

History edit

Canvas was initially introduced by Apple for use in their own Mac OS X WebKit component in 2004,[1] powering applications like Dashboard widgets and the Safari browser. Later, in 2005, it was adopted in version 1.8 of Gecko browsers,[2] and Opera in 2006,[3] and standardized by the Web Hypertext Application Technology Working Group (WHATWG) on new proposed specifications for next generation web technologies.[citation needed]

Usage edit

A canvas consists of a drawable region defined in HTML code with height and width attributes. JavaScript code may access the area through a full set of drawing functions similar to those of other common 2D APIs, thus allowing for dynamically generated graphics. Some anticipated uses of canvas include building graphs, animations, games, and image composition.

Interacting with the canvas involves obtaining the canvas' rendering context, which determines whether to use the canvas API, WebGL, or WebGL2 rendering context.

Example edit

The following code creates a Canvas element in an HTML page: 

<canvas id="example" width="200" height="200"> This text is displayed if your browser does not support HTML5 Canvas. </canvas>

Using JavaScript, you can draw on the canvas: 

var example = document.getElementById('example'); var context = example.getContext('2d'); context.fillStyle = 'red'; context.fillRect(30, 30, 50, 50);

This code draws a red rectangle on the screen.

The Canvas API also provides save() and restore(), for saving and restoring all the canvas context's attributes.

Canvas element size versus drawing surface size edit

A canvas actually has two sizes: the size of the element itself and the size of the element's drawing surface. Setting the element's width and height attributes sets both of these sizes; CSS attributes affect only the element's size and not the drawing surface.

By default, both the canvas element's size and the size of its drawing surface is 300 screen pixels wide and 150 screen pixels high. In the listing shown in the example, which uses CSS to set the canvas element's size, the size of the element is 600 pixels wide and 300 pixels high, but the size of the drawing surface remains unchanged at the default value of 300 pixels × 150 pixels. When a canvas element's size does not match the size of its drawing surface, the browser scales the drawing surface to fit the element (which may result in surprising and unwanted effects).

Example setting element size and drawing surface size to different values: 

<!DOCTYPE html> <html> <head> <title>Canvas element size: 600 x 300, Canvas drawing surface size: 300 x 150</title> <style>  body {  background: #dddddd;  }  #canvas {  margin: 20px;  padding: 20px;  background: #ffffff;  border: thin inset #aaaaaa;  width: 600px;  height: 300px;  }  </style> </head> <body> <canvas id="canvas"> Canvas not supported </canvas> </body> </html>

Canvas versus Scalable Vector Graphics (SVG) edit

SVG is an alternative approach to drawing shapes in browsers.[4] Unlike canvas, which is raster-based, SVG is vector-based, so that each drawn shape is remembered as an object in a scene graph or Document Object Model, which is subsequently rendered to a bitmap. This means that if attributes of an SVG object are changed, the browser can automatically re-render the scene.

Canvas objects, on the other hand, are drawn in immediate mode. In the canvas example above, the rectangle draw operation modifies the canvas, and its representation as a rectangle is forgotten by the system. If the rectangle's position were to be changed, the canvas would need to be redrawn, including any objects that might have been covered by the rectangle. In the equivalent SVG case, one could simply change the position attributes of the rectangle and the browser would determine how to repaint it. There are additional JavaScript libraries that abstract the canvas model to have svg-like scene capabilities within the canvas element. Multiple canvas layers can also be used, meaning that only specific layers need to be recreated when changes are required.

SVG images are represented in XML, and complex scenes can be created and maintained with XML editing tools.

The SVG scene graph enables event handlers to be associated with objects, so a rectangle may respond to an onClick event. To get the same functionality with canvas, one must manually match the coordinates of the mouse click with the coordinates of the drawn rectangle to determine whether it was clicked.

Conceptually, canvas is a lower-level API upon which higher-level interfaces might be built (for example, SVG support). There are JavaScript libraries that provide partial SVG implementations using canvas for browsers that do not provide SVG but support canvas, such as the browsers in Android 2.x. However, this is not normally the case—they are independent standards. The situation is complicated because there are scene graph libraries for canvas, and SVG has some bitmap manipulation functionality.

Reactions edit

At the time of its introduction, the canvas element was met with mixed reactions from the web standards community. There have been arguments against Apple's decision to create a new proprietary element instead of supporting the SVG standard. There are other concerns about syntax, such as the absence of a namespace.[5]

Intellectual property over canvas edit

On March 14, 2007, WebKit developer Dave Hyatt forwarded an email from Apple's Senior Patent Counsel, Helene Plotka Workman,[6] which stated that Apple reserved all intellectual property rights relative to WHATWG's Web Applications 1.0 Working Draft, dated March 24, 2005, Section 10.1, entitled “Graphics: The bitmap canvas”,[7] but left the door open to licensing the patents should the specification be transferred to a standards body with a formal patent policy. This caused considerable discussion among web developers and raised questions concerning the WHATWG's lack of a policy on patents in comparison to the World Wide Web Consortium (W3C)'s explicit favoring of royalty-free licenses. Apple later disclosed the patents under the W3C's royalty-free patent licensing terms.[8] The disclosure means that Apple is required to provide royalty-free licensing for the patent whenever the Canvas element becomes part of a future W3C recommendation created by the HTML working group.[9]

Privacy concerns edit

Canvas fingerprinting is one of a number of browser fingerprinting techniques for tracking online users that allow websites to identify and track visitors using HTML5 canvas element. The technique received wide media coverage in 2014,[10][11][12][13] after researchers from Princeton University and KU Leuven University described it in their paper The Web never forgets.[14] The privacy concerns regarding canvas fingerprinting center around the fact that even deleting cookies and clearing the cache will not be sufficient for users to avoid online tracking.

Browser support edit

The element is supported by the current versions of Mozilla Firefox, Google Chrome, Internet Explorer, Safari, Konqueror, Opera[15] and Microsoft Edge.[16]

See also edit

References edit

  1. ^ Ian Hixie (2004-07-12). "Extending HTML". Retrieved 2011-06-13.
  2. ^ Mozilla Developer Connection. . Archived from the original on 2011-06-04. Retrieved 2011-06-13.
  3. ^ "Opera 9.0 changelog". Archived from the original on 2012-09-10. Retrieved 2006-06-20.
  4. ^ "Scalable Vector Graphics | CorelDRAW". www.coreldraw.com. Retrieved 2022-09-23.
  5. ^ Ian Hickson remarks regarding canvas and other Apple extensions to HTML
  6. ^ . Archived from the original on 2007-05-02. Retrieved 2007-05-01.
  7. ^ Web Applications 1.0 Early Working Draft - Dynamic graphics: The bitmap canvas
  8. ^ HTML Working Group Patent Policy Status – Known Disclosures
  9. ^ W3C patent policy in use by HTML working group
  10. ^ Knibbs, Kate (July 21, 2014). "What You Need to Know About the Sneakiest New Online Tracking Tool". Gizmodo. Retrieved July 21, 2014.
  11. ^ Joseph Steinberg (July 23, 2014). "You Are Being Tracked Online By A Sneaky New Technology -- Here's What You Need To Know". Forbes. Retrieved November 15, 2014.
  12. ^ Angwin, Julia (July 21, 2014). "Meet the Online Tracking Device That is Virtually Impossible to Block". ProPublica. Retrieved July 21, 2014.
  13. ^ Kirk, Jeremy (July 21, 2014). "Stealthy Web tracking tools pose increasing privacy risks to users". PC World. Retrieved July 21, 2014.
  14. ^ Acar, Gunes; Eubank, Christian; Englehardt, Steven; Juarez, Marc; Narayanan, Arvind; Diaz, Claudia (July 24, 2014). "The Web never forgets: Persistent tracking mechanisms in the wild". Retrieved July 24, 2014.
  15. ^ Sucan, Mihai (4 Feb 2010). . Opera Software. Archived from the original on 23 June 2010. Retrieved 3 May 2010.
  16. ^ "Canvas, Microsoft Edge documentation".

External links edit

  • The canvas element, W3C, 2014-10-28, retrieved 2015-01-09
  • HTML Canvas 2D Context, W3C, 2014-08-21, retrieved 2015-01-09
  • Basic Canvas Tutorial on Opera Developer Community
  • Canvas tutorial and introductory page on Mozilla Developer center 2012-08-03 at the Wayback Machine

January 01, 1970
canvas, element, this, article, about, html, element, general, element, user, interfaces, canvas, canvas, element, part, html5, allows, dynamic, scriptable, rendering, shapes, bitmap, images, level, procedural, model, that, updates, bitmap, html5, canvas, also. This article is about the HTML element For the general element in user interfaces see Canvas GUI The canvas element is part of HTML5 and allows for dynamic scriptable rendering of 2D shapes and bitmap images It is a low level procedural model that updates a bitmap HTML5 Canvas also helps in making 2D games While the HTML5 canvas offers its own 2D drawing API it also supports the WebGL API to allow 3D rendering with OpenGL ES Contents 1 History 2 Usage 3 Example 4 Canvas element size versus drawing surface size 5 Canvas versus Scalable Vector Graphics SVG 6 Reactions 6 1 Intellectual property over canvas 6 2 Privacy concerns 7 Browser support 8 See also 9 References 10 External linksHistory editCanvas was initially introduced by Apple for use in their own Mac OS X WebKit component in 2004 1 powering applications like Dashboard widgets and the Safari browser Later in 2005 it was adopted in version 1 8 of Gecko browsers 2 and Opera in 2006 3 and standardized by the Web Hypertext Application Technology Working Group WHATWG on new proposed specifications for next generation web technologies citation needed Usage editA canvas consists of a drawable region defined in HTML code with height and width attributes JavaScript code may access the area through a full set of drawing functions similar to those of other common 2D APIs thus allowing for dynamically generated graphics Some anticipated uses of canvas include building graphs animations games and image composition Interacting with the canvas involves obtaining the canvas rendering context which determines whether to use the canvas API WebGL or WebGL2 rendering context Example editThe following code creates a Canvas element in an HTML page lt canvas id example width 200 height 200 gt This text is displayed if your browser does not support HTML5 Canvas lt canvas gt Using JavaScript you can draw on the canvas var example document getElementById example var context example getContext 2d context fillStyle red context fillRect 30 30 50 50 This code draws a red rectangle on the screen The Canvas API also provides save and restore for saving and restoring all the canvas context s attributes Canvas element size versus drawing surface size editA canvas actually has two sizes the size of the element itself and the size of the element s drawing surface Setting the element s width and height attributes sets both of these sizes CSS attributes affect only the element s size and not the drawing surface By default both the canvas element s size and the size of its drawing surface is 300 screen pixels wide and 150 screen pixels high In the listing shown in the example which uses CSS to set the canvas element s size the size of the element is 600 pixels wide and 300 pixels high but the size of the drawing surface remains unchanged at the default value of 300 pixels 150 pixels When a canvas element s size does not match the size of its drawing surface the browser scales the drawing surface to fit the element which may result in surprising and unwanted effects Example setting element size and drawing surface size to different values lt DOCTYPE html gt lt html gt lt head gt lt title gt Canvas element size 600 x 300 Canvas drawing surface size 300 x 150 lt title gt lt style gt body background dddddd canvas margin 20 px padding 20 px background ffffff border thin inset aaaaaa width 600 px height 300 px lt style gt lt head gt lt body gt lt canvas id canvas gt Canvas not supported lt canvas gt lt body gt lt html gt Canvas versus Scalable Vector Graphics SVG editThis section does not cite any sources Please help improve this section by adding citations to reliable sources Unsourced material may be challenged and removed October 2017 Learn how and when to remove this message SVG is an alternative approach to drawing shapes in browsers 4 Unlike canvas which is raster based SVG is vector based so that each drawn shape is remembered as an object in a scene graph or Document Object Model which is subsequently rendered to a bitmap This means that if attributes of an SVG object are changed the browser can automatically re render the scene Canvas objects on the other hand are drawn in immediate mode In the canvas example above the rectangle draw operation modifies the canvas and its representation as a rectangle is forgotten by the system If the rectangle s position were to be changed the canvas would need to be redrawn including any objects that might have been covered by the rectangle In the equivalent SVG case one could simply change the position attributes of the rectangle and the browser would determine how to repaint it There are additional JavaScript libraries that abstract the canvas model to have svg like scene capabilities within the canvas element Multiple canvas layers can also be used meaning that only specific layers need to be recreated when changes are required SVG images are represented in XML and complex scenes can be created and maintained with XML editing tools The SVG scene graph enables event handlers to be associated with objects so a rectangle may respond to an onClick event To get the same functionality with canvas one must manually match the coordinates of the mouse click with the coordinates of the drawn rectangle to determine whether it was clicked Conceptually canvas is a lower level API upon which higher level interfaces might be built for example SVG support There are JavaScript libraries that provide partial SVG implementations using canvas for browsers that do not provide SVG but support canvas such as the browsers in Android 2 x However this is not normally the case they are independent standards The situation is complicated because there are scene graph libraries for canvas and SVG has some bitmap manipulation functionality Reactions editAt the time of its introduction the canvas element was met with mixed reactions from the web standards community There have been arguments against Apple s decision to create a new proprietary element instead of supporting the SVG standard There are other concerns about syntax such as the absence of a namespace 5 Intellectual property over canvas edit On March 14 2007 WebKit developer Dave Hyatt forwarded an email from Apple s Senior Patent Counsel Helene Plotka Workman 6 which stated that Apple reserved all intellectual property rights relative to WHATWG s Web Applications 1 0 Working Draft dated March 24 2005 Section 10 1 entitled Graphics The bitmap canvas 7 but left the door open to licensing the patents should the specification be transferred to a standards body with a formal patent policy This caused considerable discussion among web developers and raised questions concerning the WHATWG s lack of a policy on patents in comparison to the World Wide Web Consortium W3C s explicit favoring of royalty free licenses Apple later disclosed the patents under the W3C s royalty free patent licensing terms 8 The disclosure means that Apple is required to provide royalty free licensing for the patent whenever the Canvas element becomes part of a future W3C recommendation created by the HTML working group 9 Privacy concerns edit Canvas fingerprinting is one of a number of browser fingerprinting techniques for tracking online users that allow websites to identify and track visitors using HTML5 canvas element The technique received wide media coverage in 2014 10 11 12 13 after researchers from Princeton University and KU Leuven University described it in their paper The Web never forgets 14 The privacy concerns regarding canvas fingerprinting center around the fact that even deleting cookies and clearing the cache will not be sufficient for users to avoid online tracking Browser support editThe element is supported by the current versions of Mozilla Firefox Google Chrome Internet Explorer Safari Konqueror Opera 15 and Microsoft Edge 16 See also editAnti Grain Geometry AGG Cairo graphics Display PostScript Graphics Device Interface GDI Quartz 2D WebGLReferences edit Ian Hixie 2004 07 12 Extending HTML Retrieved 2011 06 13 Mozilla Developer Connection HTMLCanvasElement Archived from the original on 2011 06 04 Retrieved 2011 06 13 Opera 9 0 changelog Archived from the original on 2012 09 10 Retrieved 2006 06 20 Scalable Vector Graphics CorelDRAW www coreldraw com Retrieved 2022 09 23 Ian Hickson remarks regarding canvas and other Apple extensions to HTML whatwg Web Applications 1 0 Draft David Hyatt Wed Mar 14 14 31 53 PDT 2007 Archived from the original on 2007 05 02 Retrieved 2007 05 01 Web Applications 1 0 Early Working Draft Dynamic graphics The bitmap canvas HTML Working Group Patent Policy Status Known Disclosures W3C patent policy in use by HTML working group Knibbs Kate July 21 2014 What You Need to Know About the Sneakiest New Online Tracking Tool Gizmodo Retrieved July 21 2014 Joseph Steinberg July 23 2014 You Are Being Tracked Online By A Sneaky New Technology Here s What You Need To Know Forbes Retrieved November 15 2014 Angwin Julia July 21 2014 Meet the Online Tracking Device That is Virtually Impossible to Block ProPublica Retrieved July 21 2014 Kirk Jeremy July 21 2014 Stealthy Web tracking tools pose increasing privacy risks to users PC World Retrieved July 21 2014 Acar Gunes Eubank Christian Englehardt Steven Juarez Marc Narayanan Arvind Diaz Claudia July 24 2014 The Web never forgets Persistent tracking mechanisms in the wild Retrieved July 24 2014 Sucan Mihai 4 Feb 2010 SVG or Canvas Shoosing between the two Opera Software Archived from the original on 23 June 2010 Retrieved 3 May 2010 Canvas Microsoft Edge documentation External links editThe canvas element W3C 2014 10 28 retrieved 2015 01 09 HTML Canvas 2D Context W3C 2014 08 21 retrieved 2015 01 09 Canvas description in WHATWG Web Applications draft specifications Canvas reference page in Apple Developers Connection Basic Canvas Tutorial on Opera Developer Community Canvas tutorial and introductory page on Mozilla Developer center Archived 2012 08 03 at the Wayback Machine Retrieved from https en wikipedia org w index php title Canvas element amp oldid 1189223350, wikipedia, wiki, book, books, library,

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