Canvas was initially introduced by Apple for use inside their own Mac OS X WebKit component in 2004, powering applications like Dashboard widgets and the Safari browser. Later, in 2005 it was adopted in version 1.8 of Gecko browsers, and Opera in 2006, and standardized by the Web Hypertext Application Technology Working Group (WHATWG) on new proposed specifications for next generation web technologies.
The following code creates a Canvas element in an HTML page:
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.
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 on setting element size and drawing surface size to different values:
SVG is an earlier standard for drawing shapes in browsers. However, unlike canvas, which is raster-based, SVG is vector-based, i.e., 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.
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.
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, e.g., the absence of a namespace.
On March 14, 2007, WebKit developer Dave Hyatt forwarded an email from Apple's Senior Patent Counsel, Helene Plotka Workman, 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”, 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. 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.
The element is supported by the current versions of Mozilla Firefox, Google Chrome, Internet Explorer, Safari, Konqueror and Opera.