The HTMLCanvasElement.getContext()
method returns a drawing context on the canvas, or null
if the context identifier is not supported.
Later calls to this method on the same canvas element return the same drawing context instance as was returned the last time the method was invoked with the same contextType
argument. To get a different drawing context object you need to pass a different contextType
or call the method on a different canvas element.
Syntax
var ctx = canvas.getContext(contextType); var ctx = canvas.getContext(contextType, contextAttributes);
Parameters
contextType
- Is a
DOMString
containing the context identifier defining the drawing context associated to the canvas. Possible values are:"2d"
, leading to the creation of aCanvasRenderingContext2D
object representing a two-dimensional rendering context."webgl"
(or"experimental-webgl"
) which will create aWebGLRenderingContext
object representing a three-dimensional rendering context. This context is only available on browsers that implement WebGL version 1 (OpenGL ES 2.0)."webgl2"
which will create aWebGL2RenderingContext
object representing a three-dimensional rendering context. This context is only available on browsers that implement WebGL version 2 (OpenGL ES 3.0)."bitmaprenderer"
which will create anImageBitmapRenderingContext
which only provides functionality to replace the content of the canvas with a givenImageBitmap
.
Note: The identifier
"experimental-webgl"
is used in new implementations of WebGL. These implementations have either not reached test suite conformance, or the graphics drivers on the platform are not yet stable. The Khronos Group certifies WebGL implementations under certain conformance rules. contextAttributes
-
You can use several context attributes when creating your rendering context, for example:
const gl = canvas.getContext('webgl', { antialias: false, depth: false });
2d context attributes:alpha
: Boolean that indicates if the canvas contains an alpha channel. If set tofalse
, the browser now knows that the backdrop is always opaque, which can speed up drawing of transparent content and images.desynchronized
: Boolean that hints the user agent to reduce the latency by desynchronizing the canvas paint cycle from the event loop- (Gecko only)
willReadFrequently
: Boolean that indicates whether or not a lot of read-back operations are planned. This will force the use of a software (instead of hardware accelerated) 2D canvas and can save memory when callinggetImageData()
frequently. This option is only available, if the flaggfx.canvas.willReadFrequently.enable
is set totrue
(which, by default, is only the case for B2G/Firefox OS). - (Blink only)
storage
: String that indicates which storage is used ("persistent" by default).
alpha
: Boolean that indicates if the canvas contains an alpha buffer.desynchronized
: Boolean that hints the user agent to reduce the latency by desynchronizing the canvas paint cycle from the event loopantialias
: Boolean that indicates whether or not to perform anti-aliasing.depth
: Boolean that indicates that the drawing buffer has a depth buffer of at least 16 bits.failIfMajorPerformanceCaveat
: Boolean that indicates if a context will be created if the system performance is low or if no hardware GPU is available.powerPreference
: A hint to the user agent indicating what configuration of GPU is suitable for the WebGL context. Possible values are:"default"
: Let the user agent decide which GPU configuration is most suitable. This is the default value."high-performance"
: Prioritizes rendering performance over power consumption."low-power"
: Prioritizes power saving over rendering performance.
premultipliedAlpha
: Boolean that indicates that the page compositor will assume the drawing buffer contains colors with pre-multiplied alpha.preserveDrawingBuffer
: If the value is true the buffers will not be cleared and will preserve their values until cleared or overwritten by the author.stencil
: Boolean that indicates that the drawing buffer has a stencil buffer of at least 8 bits.
Return value
A RenderingContext
which is either a
CanvasRenderingContext2D
for"2d"
,WebGLRenderingContext
for"webgl"
and"experimental-webgl"
,WebGL2RenderingContext
for"webgl2"
orImageBitmapRenderingContext
for"bitmaprenderer"
.
If the contextType
doesn't match a possible drawing context, null
is returned.
Examples
Given this <canvas>
element:
<canvas id="canvas" width="300" height="300"></canvas>
You can get a 2d
context of the canvas with the following code:
var canvas = document.getElementById('canvas'); var ctx = canvas.getContext('2d'); console.log(ctx); // CanvasRenderingContext2D { ... }
Now you have the 2D rendering context for a canvas and you can draw within it.
Specifications
Specification | Status | Comment |
---|---|---|
HTML Living Standard The definition of 'HTMLCanvasElement.getContext' in that specification. |
Living Standard | No change since the latest snapshot, HTML5 |
HTML 5.1 The definition of 'HTMLCanvasElement.getContext' in that specification. |
Recommendation | |
HTML5 The definition of 'HTMLCanvasElement.getContext' in that specification. |
Recommendation | Snapshot of the HTML Living Standard containing the initial definition. |
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
getContext | Chrome Full support 1 | Edge Full support 12 | Firefox Full support 1.5 | IE Full support 9 | Opera Full support 9 | Safari Full support 2 | WebView Android Full support 1 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support 10.1 | Safari iOS Full support 1 | Samsung Internet Android Full support 1.0 |
bitmaprenderer context | Chrome Full support 56 | Edge Full support ≤79 | Firefox Full support 46 | IE No support No | Opera Full support 43 | Safari No support No | WebView Android No support No | Chrome Android Full support 56 | Firefox Android Full support 46 | Opera Android Full support 43 | Safari iOS No support No | Samsung Internet Android Full support 6.0 |
desynchronized attribute | Chrome
Full support
81
| Edge No support No | Firefox No support No | IE No support No | Opera No support No | Safari No support No | WebView Android No support No | Chrome Android No support No | Firefox Android No support No | Opera Android No support No | Safari iOS No support No | Samsung Internet Android No support No |
failIfMajorPerformanceCaveat attribute | Chrome Full support Yes | Edge Full support 12 | Firefox Full support 41 | IE Full support Yes | Opera Full support Yes | Safari ? | WebView Android No support No | Chrome Android No support No | Firefox Android Full support 41 | Opera Android ? | Safari iOS ? | Samsung Internet Android No support No |
powerPreference attribute | Chrome No support No | Edge No support No | Firefox Full support 63 | IE No support No | Opera No support No | Safari No support No | WebView Android No support No | Chrome Android No support No | Firefox Android Full support 63 | Opera Android No support No | Safari iOS No support No | Samsung Internet Android No support No |
WebGL context | Chrome
Full support
33
| Edge
Full support
79
| Firefox
Full support
24
| IE
Full support
11
| Opera
Full support
15
| Safari
Full support
5.1
| WebView Android
Full support
37
| Chrome Android
Full support
33
| Firefox Android
Full support
Yes
| Opera Android
Full support
14
| Safari iOS ? | Samsung Internet Android
Full support
2.0
|
WebGL2 context | Chrome Full support 56 | Edge Full support 79 | Firefox
Full support
51
| IE No support No | Opera No support No | Safari No support No | WebView Android Full support 56 | Chrome Android No support No | Firefox Android No support No | Opera Android No support No | Safari iOS No support No | Samsung Internet Android No support No |
2d alpha context attribute | Chrome Full support 32 | Edge Full support ≤79 | Firefox Full support 30 | IE No support No | Opera Full support Yes | Safari No support No | WebView Android No support No | Chrome Android No support No | Firefox Android Full support 30 | Opera Android No support No | Safari iOS No support No | Samsung Internet Android No support No |
Legend
- Full support
- Full support
- No support
- No support
- Compatibility unknown
- Compatibility unknown
- See implementation notes.
- See implementation notes.
- Uses a non-standard name.
- Uses a non-standard name.
See also
- The interface defining it,
HTMLCanvasElement
. OffscreenCanvas.getContext()
- Available rendering contexts:
CanvasRenderingContext2D
,WebGLRenderingContext
andWebGL2RenderingContext
andImageBitmapRenderingContext
.