CanvasRenderingContext2D

La interfaz CanvasRenderingContext2D proporciona el contexto de renderizado 2D para la superficie de dibujo de un elemento <canvas>.

Para obtener un objeto de esta interfaz, llama a getContext() en un <canvas>, proporcionando "2d" como argumento:

var canvas = document.getElementById('mycanvas');
var ctx = canvas.getContext('2d');

Una vez que tengas el contexto de renderizado 2D para un canvas, podrás dibujar en ella. Por ejemplo:

ctx.fillStyle = "rgb(200,0,0)";  
ctx.fillRect(10, 10, 55, 50);  

Mira las propiedades y métodos en la barra lateral. El tutorial de canvas tiene más información, ejemplos y recursos también.

Dibujando rectángulos

Hay 3 métodos que inmediatamente dibujan rectángulos en el bitmap.

CanvasRenderingContext2D.clearRect()

Establece todos los píxeles del rectangulo definido por los puntos (x, y) y tamaños (ancho, alto) a negro transparente, borrando cualquier contenido previo.

CanvasRenderingContext2D.fillRect()

Dibuja un rectángulo relleno en la posición (x, y) cuyo tamaño está determinado por la anchura y la altura.

CanvasRenderingContext2D.strokeRect()

Dibuja un rectángulo que tiene como posición inicial (x, y) con un ancho w y altura h sobre el canvas, utilizando el estilo de trazo actual.

Dibujando texto

Los siguientes métodos  se proporcionan para dibujar texto. Mira también el objeto TextMetrics para propiedades de texto.

CanvasRenderingContext2D.fillText()

Dibuja (llena) un texto dado en una posición (x, y) dada.

CanvasRenderingContext2D.strokeText()

Dibuja (trazas) un texto dado en una posición (x, y) dada.

CanvasRenderingContext2D.measureText()

Devuelve el objeto TextMetrics.

Los estilos de línea

Los siguientes métodos y propiedades controlan como las líneas son dibujadas.

CanvasRenderingContext2D.lineWidth

Ancho de líneas. Por defecto 1.0

CanvasRenderingContext2D.lineCap

Tipo de terminaciones en el final de las líneas. Posibles valores: butt (defecto), round, square.

CanvasRenderingContext2D.lineJoin

Define el tipo de esquinas donde dos líneas se encuentran. Pislbes valores: round, bevel, miter (defecto).

CanvasRenderingContext2D.miterLimit

Relación límite angular. Defecto 10.

CanvasRenderingContext2D.getLineDash()

Devuelve la matriz patrón de trazos actual que contiene un número de par de números no negativos.

CanvasRenderingContext2D.setLineDash()

Establece el patrón de trazos linea actual.

CanvasRenderingContext2D.lineDashOffset

Especifica donde inicia una gama de instrumentos en una línea.

Estilos de texto

Las siguientes propiedades control como el texto es presentado.

CanvasRenderingContext2D.font

Ajuste de fuente. Valor por defecto 10px sans-serif.

CanvasRenderingContext2D.textAlign

Text alignment setting. Possible values: start (default), end, left, right or center.

CanvasRenderingContext2D.textBaseline

Baseline alignment setting. Possible values: top, hanging, middle, alphabetic (default), ideographic, bottom.

CanvasRenderingContext2D.direction

Directionality. Possible values: ltr, rtl, inherit (default).

Fill and stroke styles

Fill styling is used for colors and styles inside shapes and stroke styling is used for the lines around shapes.

CanvasRenderingContext2D.fillStyle

Color or style to use inside shapes. Default #000 (black).

CanvasRenderingContext2D.strokeStyle

Color or style to use for the lines around shapes. Default #000 (black).

Gradients and patterns

CanvasRenderingContext2D.createLinearGradient()

Creates a linear gradient along the line given by the coordinates represented by the parameters.

CanvasRenderingContext2D.createRadialGradient()

Creates a radial gradient along the line given by the coordinates represented by the parameters.

CanvasRenderingContext2D.createPattern()

Creates a pattern using the specified image (a CanvasImageSource). It repeats the source in the directions specified by the repetition argument. This method returns a CanvasPattern.

Shadows

CanvasRenderingContext2D.shadowBlur

Specifies the blurring effect. Default 0

CanvasRenderingContext2D.shadowColor

Color of the shadow. Default fully-transparent black.

CanvasRenderingContext2D.shadowOffsetX

Horizontal distance the shadow will be offset. Default 0.

CanvasRenderingContext2D.shadowOffsetY

Vertical distance the shadow will be offset. Default 0.

Paths

The following methods can be used to manipulate paths of objects.

CanvasRenderingContext2D.beginPath()

Starts a new path by emptying the list of sub-paths. Call this method when you want to create a new path.

CanvasRenderingContext2D.closePath()

Causes the point of the pen to move back to the start of the current sub-path. It tries to draw a straight line from the current point to the start. If the shape has already been closed or has only one point, this function does nothing.

CanvasRenderingContext2D.moveTo()

Moves the starting point of a new sub-path to the (x, y) coordinates.

CanvasRenderingContext2D.lineTo()

Connects the last point in the subpath to the x, y coordinates with a straight line.

CanvasRenderingContext2D.bezierCurveTo()

Adds a cubic Bézier curve to the path. It requires three points. The first two points are control points and the third one is the end point. The starting point is the last point in the current path, which can be changed using moveTo() before creating the Bézier curve.

CanvasRenderingContext2D.quadraticCurveTo()

Adds a quadratic Bézier curve to the current path.

CanvasRenderingContext2D.arc()

Adds an arc to the path which is centered at (x, y) position with radius r starting at startAngle and ending at endAngle going in the given direction by anticlockwise (defaulting to clockwise).

CanvasRenderingContext2D.arcTo()

Adds an arc to the path with the given control points and radius, connected to the previous point by a straight line.

CanvasRenderingContext2D.ellipse()

Adds an ellipse to the path which is centered at (x, y) position with the radii radiusX and radiusY starting at startAngle and ending at endAngle going in the given direction by anticlockwise (defaulting to clockwise).

CanvasRenderingContext2D.rect()

Creates a path for a rectangle at position (x, y) with a size that is determined by width and height.

Drawing paths

CanvasRenderingContext2D.fill()

Fills the subpaths with the current fill style.

CanvasRenderingContext2D.stroke()

Strokes the subpaths with the current stroke style.

CanvasRenderingContext2D.drawFocusIfNeeded()

If a given element is focused, this method draws a focus ring around the current path.

CanvasRenderingContext2D.scrollPathIntoView()

Scrolls the current path or a given path into the view.

CanvasRenderingContext2D.clip()

Creates a clipping path from the current sub-paths. Everything drawn after clip() is called appears inside the clipping path only. For an example, see Clipping paths in the Canvas tutorial.

CanvasRenderingContext2D.isPointInPath()

Reports whether or not the specified point is contained in the current path.

CanvasRenderingContext2D.isPointInStroke()

Reports whether or not the specified point is inside the area contained by the stroking of a path.

Transformations

Objects in the CanvasRenderingContext2D rendering context have a current transformation matrix and methods to manipulate it. The transformation matrix is applied when creating the current default path, painting text, shapes and Path2D objects. The methods listed below remain for historical and compatibility reasons as SVGMatrix objects are used in most parts of the API nowadays and will be used in the future instead.

CanvasRenderingContext2D.currentTransform

Current transformation matrix (SVGMatrix object).

CanvasRenderingContext2D.rotate()

Adds a rotation to the transformation matrix. The angle argument represents a clockwise rotation angle and is expressed in radians.

CanvasRenderingContext2D.scale()

Adds a scaling transformation to the canvas units by x horizontally and by y vertically.

CanvasRenderingContext2D.translate()

Adds a translation transformation by moving the canvas and its origin x horzontally and y vertically on the grid.

CanvasRenderingContext2D.transform()

Multiplies the current transformation matrix with the matrix described by its arguments.

CanvasRenderingContext2D.setTransform()

Resets the current transform to the identity matrix, and then invokes the transform() method with the same arguments.

CanvasRenderingContext2D.resetTransform()

Resets the current transform by the identity matrix.

Compositing

CanvasRenderingContext2D.globalAlpha

Alpha value that is applied to shapes and images before they are composited onto the canvas. Default 1.0 (opaque).

CanvasRenderingContext2D.globalCompositeOperation

With globalAlpha applied this sets how shapes and images are drawn onto the existing bitmap.

Drawing images

CanvasRenderingContext2D.drawImage()

Draws the specified image. This method is available in multiple formats, providing a great deal of flexibility in its use.

Pixel manipulation

See also the ImageData object.

CanvasRenderingContext2D.createImageData()

Creates a new, blank ImageData object with the specified dimensions. All of the pixels in the new object are transparent black.

CanvasRenderingContext2D.getImageData()

Returns an ImageData object representing the underlying pixel data for the area of the canvas denoted by the rectangle which starts at (sx, sy) and has an sw width and sh height.

CanvasRenderingContext2D.putImageData()

Paints data from the given ImageData object onto the bitmap. If a dirty rectangle is provided, only the pixels from that rectangle are painted.

Image smoothing

CanvasRenderingContext2D.imageSmoothingEnabled

Image smoothing mode; if disabled, images will not be smoothed if scaled.

The canvas state

The CanvasRenderingContext2D rendering context contains a variety of drawing style states (attributes for line styles, fill styles, shadow styles, text styles). The following methods help you to work with that state:

CanvasRenderingContext2D.save()

Saves the current drawing style state using a stack so you can revert any change you make to it using restore().

CanvasRenderingContext2D.restore()

Restores the drawing style state to the last element on the 'state stack' saved by save().

CanvasRenderingContext2D.canvas

A read-only back-reference to the HTMLCanvasElement. Might be null if it is not associated with a <canvas> element.

Hit regions

CanvasRenderingContext2D.addHitRegion()

Adds a hit region to the canvas.

CanvasRenderingContext2D.removeHitRegion()

Removes the hit region with the specified id from the canvas.

CanvasRenderingContext2D.clearHitRegions()

Removes all hit regions from the canvas.

Non-standard APIs

Blink and WebKit

Most of these APIs are deprecated and will be removed in the future.

CanvasRenderingContext2D.clearShadow()

Removes all shadow settings like CanvasRenderingContext2D.shadowColor and CanvasRenderingContext2D.shadowBlur.

CanvasRenderingContext2D.drawImageFromRect()

This is redundant with an equivalent overload of drawImage.

CanvasRenderingContext2D.setAlpha()

Use CanvasRenderingContext2D.globalAlpha instead.

CanvasRenderingContext2D.setCompositeOperation()

Use CanvasRenderingContext2D.globalCompositeOperation instead.

CanvasRenderingContext2D.setLineWidth()

Use CanvasRenderingContext2D.lineWidth instead.

CanvasRenderingContext2D.setLineJoin()

Use CanvasRenderingContext2D.lineJoin instead.

CanvasRenderingContext2D.setLineCap()

Use CanvasRenderingContext2D.lineCap instead.

CanvasRenderingContext2D.setMiterLimit()

Use CanvasRenderingContext2D.miterLimit instead.

CanvasRenderingContext2D.setStrokeColor()

Use CanvasRenderingContext2D.strokeStyle instead.

CanvasRenderingContext2D.setFillColor()

Use CanvasRenderingContext2D.fillStyle instead.

CanvasRenderingContext2D.setShadow()

Use CanvasRenderingContext2D.shadowColor and CanvasRenderingContext2D.shadowBlur instead.

CanvasRenderingContext2D.webkitLineDash

Use CanvasRenderingContext2D.getLineDash() and CanvasRenderingContext2D.setLineDash() instead.

CanvasRenderingContext2D.webkitLineDashOffset

Use CanvasRenderingContext2D.lineDashOffset instead.

CanvasRenderingContext2D.webkitImageSmoothingEnabled

Use CanvasRenderingContext2D.imageSmoothingEnabled instead.

Blink only

CanvasRenderingContext2D.getContextAttributes()

Inspired by the same WebGLRenderingContext method it returns an Canvas2DContextAttributes object that contains the attributes "storage" to indicate which storage is used ("persistent" by default) and the attribute "alpha" (true by default) to indicate that transparency is used in the canvas.

CanvasRenderingContext2D.isContextLost()

Inspired by the same WebGLRenderingContext method it returns true if the Canvas context has been lost, or false if not.

WebKit only

CanvasRenderingContext2D.webkitBackingStorePixelRatio

The backing store size in relation to the canvas element. See High DPI Canvas.

CanvasRenderingContext2D.webkitGetImageDataHD

Intended for HD backing stores, but removed from canvas specifications.

CanvasRenderingContext2D.webkitPutImageDataHD

Intended for HD backing stores, but removed from canvas specifications.

Gecko only

CanvasRenderingContext2D.filter

CSS and SVG filters as Canvas APIs. Likely to be standardized in a new version of the specification.

Prefixed APIs

CanvasRenderingContext2D.mozCurrentTransform

Sets or gets the current transformation matrix, see CanvasRenderingContext2D.currentTransform. 

CanvasRenderingContext2D.mozCurrentTransformInverse

Sets or gets the current inversed transformation matrix. 

CanvasRenderingContext2D.mozFillRule

The fill rule to use. This must be one of evenodd or nonzero (default).

CanvasRenderingContext2D.mozImageSmoothingEnabled

See CanvasRenderingContext2D.imageSmoothingEnabled.

CanvasRenderingContext2D.mozDash

An array which specifies the lengths of alternating dashes and gaps . Use CanvasRenderingContext2D.getLineDash() and CanvasRenderingContext2D.setLineDash() instead.

CanvasRenderingContext2D.mozDashOffset

Specifies where to start a dash array on a line. . Use CanvasRenderingContext2D.lineDashOffset instead.

CanvasRenderingContext2D.mozTextStyle

Introduced in in Gecko 1.9, deprecated in favor of the CanvasRenderingContext2D.font property.

CanvasRenderingContext2D.mozDrawText()

This method was introduced in Gecko 1.9 and is removed starting with Gecko 7.0. Use CanvasRenderingContext2D.strokeText() or CanvasRenderingContext2D.fillText() instead.

CanvasRenderingContext2D.mozMeasureText()

This method was introduced in Gecko 1.9 and is unimplemented starting with Gecko 7.0. Use CanvasRenderingContext2D.measureText() instead.

CanvasRenderingContext2D.mozPathText()

This method was introduced in Gecko 1.9 and is removed starting with Gecko 7.0.

CanvasRenderingContext2D.mozTextAlongPath()

This method was introduced in Gecko 1.9 and is removed starting with Gecko 7.0.

Internal APIs (chrome-context only)

CanvasRenderingContext2D.asyncDrawXULElement()

Renders a region of a XUL element into the canvas.

CanvasRenderingContext2D.drawWindow()

Renders a region of a window into the canvas. The contents of the window's viewport are rendered, ignoring viewport clipping and scrolling.

CanvasRenderingContext2D.demote()

This causes a context that is currently using a hardware-accelerated backend to fallback to a software one. All state should be preserved.

Internet Explorer

CanvasRenderingContext2D.msFillRule

The fill rule to use. This must be one of evenodd or nonzero (default).

Specifications

Browser compatibility

Compatibility notes

• Starting with Gecko 5.0 (Firefox 5.0 / Thunderbird 5.0 / SeaMonkey 2.2), specifying invalid values are now silently ignored for the following methods and properties: translate(), transform(), rotate(), scale(), rect(), clearRect(), fillRect(), strokeRect(), lineTo(), moveTo(), quadraticCurveTo(), arc(), shadowOffsetX, shadowOffsetY,  shadowBlur.

See also

• HTMLCanvasElement