Canvas
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interface Canvas
Summary
Public functions |
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Unit |
Reduces the clip region to the intersection of the current clip and the given |
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open Unit |
Reduces the clip region to the intersection of the current clip and the given rectangle. |
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Reduces the clip region to the intersection of the current clip and the given bounds. |
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Unit |
Multiply the current transform by the specified 4⨉4 transformation matrix specified as a list of values in column-major order. |
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disableZ()Disables Z support, preventing any layers drawn after this point from being visually reordered or having shadows rendered. |
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open Unit |
Draw an arc scaled to fit inside the given rectangle. |
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Unit |
drawArc(Draw an arc scaled to fit inside the given rectangle. |
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open Unit |
drawArcRad(Draw an arc scaled to fit inside the given rectangle. |
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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 |
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drawImage(image: ImageBitmap, topLeftOffset: Offset, paint: Paint)Draws the given |
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drawImageRect(Draws the subset of the given image described by the |
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Draws a line between the given points using the given paint. |
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open Unit |
Draws an axis-aligned oval that fills the given axis-aligned rectangle with the given |
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Draws an axis-aligned oval that fills the given bounds provided with the given |
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Unit |
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drawPoints(pointMode: PointMode, points: List<Offset>, paint: Paint)Draws a sequence of points according to the given |
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drawRawPoints(pointMode: PointMode, points: FloatArray, paint: Paint)Draws a sequence of points according to the given |
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open Unit |
Draws a rectangle with the given |
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Draws a rectangle with the given |
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drawRoundRect(Draws a rounded rectangle with the given |
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Unit |
drawVertices(vertices: Vertices, blendMode: BlendMode, paint: Paint) |
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Unit |
enableZ()Enables Z support which defaults to disabled. |
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Unit |
restore()Pops the current save stack, if there is anything to pop. |
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Unit |
Add a rotation (in degrees clockwise) to the current transform |
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Unit |
save()Saves a copy of the current transform and clip on the save stack. |
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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. |
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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. |
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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. |
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open 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. |
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Unit |
Add a translation to the current transform, shifting the coordinate space horizontally by the first argument and vertically by the second argument. |
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Extension functions |
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Unit |
Add a rotation (in degrees clockwise) to the current transform at the given pivot point. |
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Unit |
Add a rotation (in radians clockwise) to the current transform at the given pivot point. |
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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 at the given pivot coordinate. |
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inline Unit |
Saves a copy of the current transform and clip on the save stack and executes the provided lambda with the current transform applied. |
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inline 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. |
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Unit |
Canvas.drawOutline(outline: Outline, paint: Paint) |
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Extension properties |
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NativeCanvas |
Return an instance of the native primitive that implements the Canvas interface |
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Public functions
clipPath
fun clipPath(path: Path, clipOp: ClipOp = ClipOp.Intersect): Unit
Reduces the clip region to the intersection of the current clip and the given Path.
clipRect
open fun clipRect(rect: Rect, clipOp: ClipOp = ClipOp.Intersect): Unit
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.
clipRect
fun clipRect(
left: Float,
top: Float,
right: Float,
bottom: Float,
clipOp: ClipOp = ClipOp.Intersect
): Unit
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.
concat
fun concat(matrix: Matrix): Unit
Multiply the current transform by the specified 4⨉4 transformation matrix specified as a list of values in column-major order.
disableZ
fun disableZ(): Unit
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.
| See also | |
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enableZ |
drawArc
open 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.
drawArc
fun drawArc(
left: Float,
top: Float,
right: Float,
bottom: Float,
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.
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left: Float |
Left bound of the arc |
top: Float |
Top bound of the arc |
right: Float |
Right bound of the arc |
bottom: Float |
Bottom bound of the arc |
startAngle: Float |
Starting angle of the arc relative to 3 o'clock |
sweepAngle: Float |
Sweep angle in degrees clockwise |
useCenter: Boolean |
Flag indicating whether or not to include the center of the oval in the |
paint: Paint |
Paint used to draw the arc. arc, and close it if it is being stroked. This will draw a wedge. |
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
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.
drawImage
fun drawImage(image: ImageBitmap, topLeftOffset: Offset, paint: Paint): Unit
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.
drawImageRect
fun drawImageRect(
image: ImageBitmap,
srcOffset: IntOffset = IntOffset.Zero,
srcSize: IntSize = IntSize(image.width, image.height),
dstOffset: IntOffset = IntOffset.Zero,
dstSize: IntSize = srcSize,
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.
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image: ImageBitmap |
ImageBitmap to draw |
srcOffset: IntOffset = IntOffset.Zero |
: Optional offset representing the top left offset of the source image to draw, this defaults to the origin of |
srcSize: IntSize = IntSize(image.width, image.height) |
: Optional dimensions of the source image to draw relative to |
dstOffset: IntOffset = IntOffset.Zero |
: Offset representing the top left offset of the destination image to draw |
dstSize: IntSize = srcSize |
: Dimensions of the destination to draw |
paint: Paint |
Paint used to composite the |
drawLine
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
open 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.
drawOval
fun drawOval(left: Float, top: Float, right: Float, bottom: Float, paint: Paint): Unit
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.
drawPath
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).
drawPoints
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:
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drawRawPoints, which takespointsas aFloatArrayrather than a List.
drawRawPoints
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:
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drawPoints, which takespointsas a Listrather than a List .
drawRect
open 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.
drawRect
fun drawRect(left: Float, top: Float, right: Float, bottom: Float, paint: Paint): Unit
Draws a rectangle with the given Paint. Whether the rectangle is filled or stroked (or both) is controlled by Paint.style.
drawRoundRect
fun drawRoundRect(
left: Float,
top: Float,
right: Float,
bottom: Float,
radiusX: Float,
radiusY: Float,
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.
enableZ
fun enableZ(): Unit
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.
| See also | |
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disableZ |
restore
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
fun rotate(degrees: Float): Unit
Add a rotation (in degrees clockwise) to the current transform
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degrees: Float |
to rotate clockwise |
save
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
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 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:
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save, which saves the current state, but does not create a new layer for subsequent commands. -
BlendMode, which discusses the use ofPaint.blendModewithsaveLayer.
scale
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
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.
Extension functions
rotate
fun Canvas.rotate(degrees: Float, pivotX: Float, pivotY: Float): Unit
Add a rotation (in degrees clockwise) to the current transform at the given pivot point. The pivot coordinate remains unchanged by the rotation transformation
rotateRad
fun Canvas.rotateRad(radians: Float, pivotX: Float = 0.0f, pivotY: Float = 0.0f): Unit
Add a rotation (in radians clockwise) to the current transform at the given pivot point. The pivot coordinate remains unchanged by the rotation transformation
scale
fun Canvas.scale(sx: Float, sy: Float = sx, pivotX: Float, pivotY: Float): 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 at the given pivot coordinate. The pivot coordinate remains unchanged by the scale transformation.
If sy is unspecified, sx will be used for the scale in both directions.
withSave
inline fun Canvas.withSave(block: () -> Unit): Unit
Saves a copy of the current transform and clip on the save stack and executes the provided lambda with the current transform applied. Once the lambda has been executed, the transformation is popped from the stack, undoing the transformation.
See also:
Canvas.saveLayer, which does the same thing but additionally also groups the commands
withSaveLayer
inline fun Canvas.withSaveLayer(bounds: Rect, paint: Paint, block: () -> Unit): 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 lambda is executed and the save stack is 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 Canvas.saveLayer, each part of the group would be painted individually, so where they overlap would be darker than where they do not. By using Canvas.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 Canvas.saveLayer's paint.
Using saveLayer with clips
When a rectangular clip operation (from Canvas.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 Canvas.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 Canvas.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 Canvas.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 lambda is finished executing).
Performance considerations
Generally speaking, Canvas.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:
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Canvas.save, which saves the current state, but does not create a new layer for subsequent commands. -
BlendMode, which discusses the use ofPaint.blendModewith saveLayer.
Extension properties
nativeCanvas
val Canvas.nativeCanvas: NativeCanvas
Return an instance of the native primitive that implements the Canvas interface