HTMLCanvasElement.getContext()
The
HTMLCanvasElement.getContext()
method returns a drawing
context on the canvas, or null
if the context identifier is not
supported, or the canvas has already been set to a different context mode.
Later calls to this method on the same canvas element, with the same
contextType
argument, will always return the same drawing context instance
as was returned the first time the method was invoked. It is not possible to get a
different drawing context object on a given 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).
WebGL context attributes:
-
alpha
: Boolean that indicates if the canvas contains an alpha buffer. -
depth
: Boolean that indicates that the drawing buffer is requested to have a depth buffer of at least 16 bits. -
stencil
: Boolean that indicates that the drawing buffer is requested to have a stencil buffer of at least 8 bits. -
desynchronized
: Boolean that hints the user agent to reduce the latency by desynchronizing the canvas paint cycle from the event loop -
antialias
: Boolean that indicates whether or not to perform anti-aliasing if possible. -
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. -
xrCompatible
: Boolean that hints to the user agent to use a compatible graphics adapter for an immersive XR device. Setting this synchronous flag at context creation is discouraged; rather call the asynchronousWebGLRenderingContext.makeXRCompatible()
method the moment you intend to start an XR session.
-
Return value
A rendering context 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, or differs
from the first contextType
requested, 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 |
---|
HTML Standard # dom-canvas-getcontext-dev |
Browser compatibility
BCD tables only load in the browser
See also
CanvasRenderingContext2D.getContextAttributes()
WebGLRenderingContext.getContextAttributes()
- The interface defining it,
HTMLCanvasElement
. OffscreenCanvas.getContext()
-
Available rendering contexts:
CanvasRenderingContext2D
,WebGLRenderingContext
andWebGL2RenderingContext
andImageBitmapRenderingContext
.