Canvas

interface Canvas
androidx.ui.graphics.Canvas

Summary

Public methods

abstract Unit
clipPath(path: Path)

Reduces the clip region to the intersection of the current clip and the given Path.

abstract Unit
clipRRect(rrect: RRect)

Reduces the clip region to the intersection of the current clip and the given rounded rectangle.

abstract Unit
clipRect(rect: Rect, clipOp: ClipOp = ClipOp.intersect)

Reduces the clip region to the intersection of the current clip and the given rectangle.

abstract Unit
concat(matrix4: Matrix4)

Multiply the current transform by the specified 4⨉4 transformation matrix specified as a list of values in column-major order.

abstract Unit
drawArc(rect: Rect, startAngle: Float, sweepAngle: Float, useCenter: Boolean, paint: Paint)

Draw an arc scaled to fit inside the given rectangle.

open Unit
drawArcRad(rect: Rect, startAngleRad: Float, sweepAngleRad: Float, useCenter: Boolean, paint: Paint)

Draw an arc scaled to fit inside the given rectangle.

abstract Unit
drawCircle(center: Offset, radius: Float, paint: Paint)

Draws a circle centered at the point given by the first argument and that has the radius given by the second argument, with the Paint given in the third argument.

abstract Unit
drawDoubleRoundRect(outer: RRect, inner: RRect, paint: Paint)

Draws a shape consisting of the difference between two rounded rectangles with the given Paint.

abstract Unit
drawImage(image: Image, topLeftOffset: Offset, paint: Paint)

Draws the given Image into the canvas with its top-left corner at the given Offset.

abstract Unit
drawImageRect(image: Image, src: Rect, dst: Rect, paint: Paint)

Draws the subset of the given image described by the src argument into the canvas in the axis-aligned rectangle given by the dst argument.

abstract Unit
drawLine(p1: Offset, p2: Offset, paint: Paint)

Draws a line between the given points using the given paint.

abstract Unit
drawOval(rect: Rect, paint: Paint)

Draws an axis-aligned oval that fills the given axis-aligned rectangle with the given Paint.

abstract Unit
drawPath(path: Path, paint: Paint)

Draws the given Path with the given Paint.

abstract Unit
drawPicture(picture: Picture)

Draw the given picture onto the canvas.

abstract Unit
drawPoints(pointMode: PointMode, points: List<Offset>, paint: Paint)

Draws a sequence of points according to the given PointMode.

abstract Unit
drawRRect(rrect: RRect, paint: Paint)

Draws a rounded rectangle with the given Paint.

abstract Unit
drawRawPoints(pointMode: PointMode, points: FloatArray, paint: Paint)

Draws a sequence of points according to the given PointMode.

abstract Unit
drawRect(rect: Rect, paint: Paint)

Draws a rectangle with the given Paint.

abstract Unit
drawVertices(vertices: Vertices, blendMode: BlendMode, paint: Paint)

abstract Unit

Pops the current save stack, if there is anything to pop.

abstract Unit
rotate(degrees: Float)

Add a rotation to the current transform.

open Unit
rotateRad(radians: Float)

Add a rotation to the current transform.

abstract Unit

Saves a copy of the current transform and clip on the save stack.

abstract Unit
saveLayer(bounds: Rect, paint: Paint)

Saves a copy of the current transform and clip on the save stack, and then creates a new group which subsequent calls will become a part of.

abstract Unit
scale(sx: Float, sy: Float = sx)

Add an axis-aligned scale to the current transform, scaling by the first argument in the horizontal direction and the second in the vertical direction.

abstract Unit
skew(sx: Float, sy: Float)

Add an axis-aligned skew to the current transform, with the first argument being the horizontal skew in degrees clockwise around the origin, and the second argument being the vertical skew in degrees clockwise around the origin.

open Unit
skewRad(sxRad: Float, syRad: Float)

Add an axis-aligned skew to the current transform, with the first argument being the horizontal skew in radians clockwise around the origin, and the second argument being the vertical skew in radians clockwise around the origin.

abstract Unit
translate(dx: Float, dy: Float)

Add a translation to the current transform, shifting the coordinate space horizontally by the first argument and vertically by the second argument.

Extension functions

From androidx.ui.engine.geometry
Unit
Canvas.drawOutline(outline: Outline, paint: Paint)

Draws the Outline on a Canvas.

From androidx.ui.graphics
Unit
Canvas.withSave(block: () -> Unit)

Saves a copy of the current transform and clip on the save stack and executes the provided lambda with the current transform applied.

Unit
Canvas.withSaveLayer(bounds: Rect, paint: Paint, block: () -> Unit)

Saves a copy of the current transform and clip on the save stack, and then creates a new group which subsequent calls will become a part of.

Properties

abstract NativeCanvas

Return an instance of the native primitive that implements the Canvas interface

Public methods

clipPath

abstract fun clipPath(path: Path): Unit

Reduces the clip region to the intersection of the current clip and the given Path.

If Paint.isAntiAlias is true, then the clip will be anti-aliased. If multiple draw commands intersect with the clip boundary, this can result in incorrect blending at the clip boundary. See saveLayer for a discussion of how to address that.

clipRRect

abstract fun clipRRect(rrect: RRect): Unit

Reduces the clip region to the intersection of the current clip and the given rounded rectangle.

If Paint.isAntiAlias is true, then the clip will be anti-aliased. If multiple draw commands intersect with the clip boundary, this can result in incorrect blending at the clip boundary. See saveLayer for a discussion of how to address that and some examples of using clipRRect.

clipRect

abstract fun clipRect(
    rect: Rect,
    clipOp: ClipOp = ClipOp.intersect
): Unit

Reduces the clip region to the intersection of the current clip and the given rectangle.

If the clip is not axis-aligned with the display device, and Paint.isAntiAlias is true, then the clip will be anti-aliased. If multiple draw commands intersect with the clip boundary, this can result in incorrect blending at the clip boundary. See saveLayer for a discussion of how to address that.

Use ClipOp.difference to subtract the provided rectangle from the current clip.

concat

abstract fun concat(matrix4: Matrix4): Unit

Multiply the current transform by the specified 4⨉4 transformation matrix specified as a list of values in column-major order.

drawArc

abstract fun drawArc(
    rect: Rect,
    startAngle: Float,
    sweepAngle: Float,
    useCenter: Boolean,
    paint: Paint
): Unit

Draw an arc scaled to fit inside the given rectangle. It starts from startAngle degrees around the oval up to startAngle + sweepAngle degrees around the oval, with zero degrees being the point on the right hand side of the oval that crosses the horizontal line that intersects the center of the rectangle and with positive angles going clockwise around the oval. If useCenter is true, the arc is closed back to the center, forming a circle sector. Otherwise, the arc is not closed, forming a circle segment.

This method is optimized for drawing arcs and should be faster than Path.arcTo.

drawArcRad

open fun drawArcRad(
    rect: Rect,
    startAngleRad: Float,
    sweepAngleRad: Float,
    useCenter: Boolean,
    paint: Paint
): Unit

Draw an arc scaled to fit inside the given rectangle. It starts from startAngle radians around the oval up to startAngle + sweepAngle radians around the oval, with zero radians being the point on the right hand side of the oval that crosses the horizontal line that intersects the center of the rectangle and with positive angles going clockwise around the oval. If useCenter is true, the arc is closed back to the center, forming a circle sector. Otherwise, the arc is not closed, forming a circle segment.

This method is optimized for drawing arcs and should be faster than Path.arcTo.

drawCircle

abstract fun drawCircle(
    center: Offset,
    radius: Float,
    paint: Paint
): Unit

Draws a circle centered at the point given by the first argument and that has the radius given by the second argument, with the Paint given in the third argument. Whether the circle is filled or stroked (or both) is controlled by Paint.style.

drawDoubleRoundRect

abstract fun drawDoubleRoundRect(
    outer: RRect,
    inner: RRect,
    paint: Paint
): Unit

Draws a shape consisting of the difference between two rounded rectangles with the given Paint. Whether this shape is filled or stroked (or both) is controlled by Paint.style.

This shape is almost but not quite entirely unlike an annulus.

drawImage

abstract fun drawImage(
    image: Image,
    topLeftOffset: Offset,
    paint: Paint
): Unit

Draws the given Image into the canvas with its top-left corner at the given Offset. The image is composited into the canvas using the given Paint.

drawImageRect

abstract fun drawImageRect(
    image: Image,
    src: Rect,
    dst: Rect,
    paint: Paint
): Unit

Draws the subset of the given image described by the src argument into the canvas in the axis-aligned rectangle given by the dst argument.

This might sample from outside the src rect by up to half the width of an applied filter.

drawLine

abstract fun drawLine(
    p1: Offset,
    p2: Offset,
    paint: Paint
): Unit

Draws a line between the given points using the given paint. The line is stroked, the value of the Paint.style is ignored for this call.

The p1 and p2 arguments are interpreted as offsets from the origin.

drawOval

abstract fun drawOval(
    rect: Rect,
    paint: Paint
): Unit

Draws an axis-aligned oval that fills the given axis-aligned rectangle with the given Paint. Whether the oval is filled or stroked (or both) is controlled by Paint.style.

drawPath

abstract fun drawPath(
    path: Path,
    paint: Paint
): Unit

Draws the given Path with the given Paint. Whether this shape is filled or stroked (or both) is controlled by Paint.style. If the path is filled, then subpaths within it are implicitly closed (see Path.close).

drawPicture

abstract fun drawPicture(picture: Picture): Unit

Draw the given picture onto the canvas. To create a picture, see PictureRecorder.

drawPoints

abstract fun drawPoints(
    pointMode: PointMode,
    points: List<Offset>,
    paint: Paint
): Unit

Draws a sequence of points according to the given PointMode.

The points argument is interpreted as offsets from the origin.

See also:

drawRRect

abstract fun drawRRect(
    rrect: RRect,
    paint: Paint
): Unit

Draws a rounded rectangle with the given Paint. Whether the rectangle is filled or stroked (or both) is controlled by Paint.style.

drawRawPoints

abstract fun drawRawPoints(
    pointMode: PointMode,
    points: FloatArray,
    paint: Paint
): Unit

Draws a sequence of points according to the given PointMode.

The points argument is interpreted as a list of pairs of floating point numbers, where each pair represents an x and y offset from the origin.

See also:

drawRect

abstract fun drawRect(
    rect: Rect,
    paint: Paint
): Unit

Draws a rectangle with the given Paint. Whether the rectangle is filled or stroked (or both) is controlled by Paint.style.

drawVertices

abstract fun drawVertices(
    vertices: Vertices,
    blendMode: BlendMode,
    paint: Paint
): Unit

restore

abstract fun restore(): Unit

Pops the current save stack, if there is anything to pop. Otherwise, does nothing.

Use save and saveLayer to push state onto the stack.

If the state was pushed with with saveLayer, then this call will also cause the new layer to be composited into the previous layer.

rotate

abstract fun rotate(degrees: Float): Unit

Add a rotation to the current transform. The argument is in degrees clockwise.

rotateRad

open fun rotateRad(radians: Float): Unit

Add a rotation to the current transform. The argument is in radians clockwise.

save

abstract fun save(): Unit

Saves a copy of the current transform and clip on the save stack.

Call restore to pop the save stack.

See also:

  • saveLayer, which does the same thing but additionally also groups the commands done until the matching restore.

saveLayer

abstract fun saveLayer(
    bounds: Rect,
    paint: Paint
): Unit

Saves a copy of the current transform and clip on the save stack, and then creates a new group which subsequent calls will become a part of. When the save stack is later popped, the group will be flattened into a layer and have the given paint's Paint.colorFilter and Paint.blendMode applied.

This lets you create composite effects, for example making a group of drawing commands semi-transparent. Without using saveLayer, each part of the group would be painted individually, so where they overlap would be darker than where they do not. By using saveLayer to group them together, they can be drawn with an opaque color at first, and then the entire group can be made transparent using the saveLayer's paint.

Call restore to pop the save stack and apply the paint to the group.

Using saveLayer with clips

When a rectangular clip operation (from clipRect) is not axis-aligned with the raster buffer, or when the clip operation is not rectalinear (e.g. because it is a rounded rectangle clip created by clipRRect or an arbitrarily complicated path clip created by clipPath), the edge of the clip needs to be anti-aliased.

If two draw calls overlap at the edge of such a clipped region, without using saveLayer, the first drawing will be anti-aliased with the background first, and then the second will be anti-aliased with the result of blending the first drawing and the background. On the other hand, if saveLayer is used immediately after establishing the clip, the second drawing will cover the first in the layer, and thus the second alone will be anti-aliased with the background when the layer is clipped and composited (when restore is called).

(Incidentally, rather than using clipRRect to draw rounded rectangles like this, prefer the drawRRect method.

Performance considerations

Generally speaking, saveLayer is relatively expensive.

There are a several different hardware architectures for GPUs (graphics processing units, the hardware that handles graphics), but most of them involve batching commands and reordering them for performance. When layers are used, they cause the rendering pipeline to have to switch render target (from one layer to another). Render target switches can flush the GPU's command buffer, which typically means that optimizations that one could get with larger batching are lost. Render target switches also generate a lot of memory churn because the GPU needs to copy out the current frame buffer contents from the part of memory that's optimized for writing, and then needs to copy it back in once the previous render target (layer) is restored.

See also:

scale

abstract fun scale(
    sx: Float,
    sy: Float = sx
): Unit

Add an axis-aligned scale to the current transform, scaling by the first argument in the horizontal direction and the second in the vertical direction.

If sy is unspecified, sx will be used for the scale in both directions.

skew

abstract fun skew(
    sx: Float,
    sy: Float
): Unit

Add an axis-aligned skew to the current transform, with the first argument being the horizontal skew in degrees clockwise around the origin, and the second argument being the vertical skew in degrees clockwise around the origin.

skewRad

open fun skewRad(
    sxRad: Float,
    syRad: Float
): Unit

Add an axis-aligned skew to the current transform, with the first argument being the horizontal skew in radians clockwise around the origin, and the second argument being the vertical skew in radians clockwise around the origin.

translate

abstract fun translate(
    dx: Float,
    dy: Float
): Unit

Add a translation to the current transform, shifting the coordinate space horizontally by the first argument and vertically by the second argument.

Properties

nativeCanvas

abstract val nativeCanvas: NativeCanvas

Return an instance of the native primitive that implements the Canvas interface