Canvas

@JvmDefaultWithCompatibility
interface Canvas

Functions

fun save()

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.

fun restore()

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.

fun 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. 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 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 clipPath with a rounded rectangle defined in a path to draw rounded rectangles like this, prefer the drawRoundRect 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:

• save, which saves the current state, but does not create a new layer for subsequent commands.
• BlendMode, which discusses the use of Paint.blendMode with saveLayer.

fun 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.

fun 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.

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

Parameters

fun rotate(degrees: Float)

Add a rotation (in degrees clockwise) to the current transform

Parameters

fun 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.

fun 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.

fun concat(matrix: Matrix)

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

fun clipRect(rect: Rect, clipOp: ClipOp = ClipOp.Intersect) =
        clipRect(rect.left, rect.top, rect.right, rect.bottom, clipOp)

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

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

fun clipRect(
        left: Float,
        top: Float,
        right: Float,
        bottom: Float,
        clipOp: ClipOp = ClipOp.Intersect,
    )

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

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

Parameters

fun clipPath(path: Path, clipOp: ClipOp = ClipOp.Intersect)

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

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

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.

fun drawRect(rect: Rect, paint: Paint) =
        drawRect(
            left = rect.left,
            top = rect.top,
            right = rect.right,
            bottom = rect.bottom,
            paint = paint,
        )

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

fun drawRect(left: Float, top: Float, right: Float, bottom: Float, paint: Paint)

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

Parameters

fun drawRoundRect(
        left: Float,
        top: Float,
        right: Float,
        bottom: Float,
        radiusX: Float,
        radiusY: Float,
        paint: Paint,
    )

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

fun drawOval(rect: Rect, paint: Paint) =
        drawOval(
            left = rect.left,
            top = rect.top,
            right = rect.right,
            bottom = rect.bottom,
            paint = paint,
        )

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.

fun drawOval(left: Float, top: Float, right: Float, bottom: Float, paint: Paint)

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

Parameters

fun 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. Whether the circle is filled or stroked (or both) is controlled by Paint.style.

fun drawArc(
        rect: Rect,
        startAngle: Float,
        sweepAngle: Float,
        useCenter: Boolean,
        paint: Paint,
    ) =
        drawArc(
            left = rect.left,
            top = rect.top,
            right = rect.right,
            bottom = rect.bottom,
            startAngle = startAngle,
            sweepAngle = sweepAngle,
            useCenter = useCenter,
            paint = paint,
        )

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.

fun drawArc(
        left: Float,
        top: Float,
        right: Float,
        bottom: Float,
        startAngle: Float,
        sweepAngle: Float,
        useCenter: Boolean,
        paint: Paint,
    )

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.

Parameters

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

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.

fun drawPath(path: Path, paint: Paint)

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).

fun drawImage(image: ImageBitmap, topLeftOffset: Offset, paint: Paint)

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

fun drawImageRect(
        image: ImageBitmap,
        srcOffset: IntOffset = IntOffset.Zero,
        srcSize: IntSize = IntSize(image.width, image.height),
        dstOffset: IntOffset = IntOffset.Zero,
        dstSize: IntSize = srcSize,
        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.

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

Parameters

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

Draws a sequence of points according to the given PointMode.

The points argument is interpreted as offsets from the origin.

See also:

• drawRawPoints, which takes points as a FloatArray rather than a List<Offset>.

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

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:

• drawPoints, which takes points as a List<Offset> rather than a List<Float32List>.

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

fun enableZ()

Enables Z support which defaults to disabled. This allows layers drawn with different elevations to be rearranged based on their elevation. It also enables rendering of shadows.

fun disableZ()

Disables Z support, preventing any layers drawn after this point from being visually reordered or having shadows rendered. This is not impacted by any save or restore calls as it is not considered part of the matrix or clip.