OverlayEffect
class OverlayEffect : CameraEffect, AutoCloseable
A CameraEffect for drawing overlay on top of the camera frames.
This class manages and processes camera frames with OpenGL. Upon arrival, frames are enqueued into an array of GL textures for deferred rendering. Calling drawFrameAsync dequeues frames and renders them to the output. Additionally, when the texture queue reaches its capacity, the oldest frame is automatically dequeued and rendered. The size of the texture queue can be defined in the constructor.
The queuing mechanism provides the flexibility to postpone frame rendering until analysis results are available. For instance, to highlight on a QR code in a preview, one can apply a QR code detection algorithm using ImageAnalysis. Once the frame's analysis result is ready, invoke drawFrameAsync and pass in the getTimestamp to release the frame and draw overlay. If the app doesn't render real-time analysis results, set the queue depth to 0 to avoid unnecessary buffer copies. For example, when laying over a static watermark.
Prior to rendering a frame, the OverlayEffect invokes the listener set in setOnDrawListener. This listener provides a Frame object, which contains both a Canvas object for drawing the overlay and the metadata like crop rect, rotation degrees, etc to calculate how the overlay should be positioned. Once the listener returns, OverlayEffect updates the SurfaceTexture behind the Canvas and blends it with the camera frame before rendering to the output.
This class is thread-safe. The methods can be invoked on any thread. The app provides a Handler object in the constructor which is used for listening for Surface updates, performing OpenGL operations and invoking app provided listeners.
Summary
Constants |
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const Int |
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const Int |
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const Int |
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const Int |
RESULT_SUCCESS = 1The |
Public constructors |
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OverlayEffect(Creates an |
Public functions |
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Unit |
Clears the listener set in |
Unit |
close()Closes the |
ListenableFuture<Int!> |
drawFrameAsync(timestampNs: Long)Draws the queued frame with the given timestamp. |
Handler |
Gets the |
Int |
Gets the depth of the queue set in the constructor. |
Unit |
setOnDrawListener(onDrawListener: Function<Frame!, Boolean!>)Sets the listener for drawing overlay. |
Inherited Constants |
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Constants
RESULT_CANCELLED_BY_CALLER
const val RESULT_CANCELLED_BY_CALLER = 4: Int
The drawFrameAsync call failed because the caller cancelled the drawing. This happens when the listener in setOnDrawListener returned false.
RESULT_FRAME_NOT_FOUND
const val RESULT_FRAME_NOT_FOUND = 2: Int
The drawFrameAsync call failed because the frame with the exact timestamp was not found in the queue. It could be one of the following reasons:
- the timestamp provided was incorrect, or
- the frame was removed because
drawFrameAsynchad been called with a newer timestamp, or - the frame was removed due to the queue being full.
RESULT_INVALID_SURFACE
const val RESULT_INVALID_SURFACE = 3: Int
The drawFrameAsync call failed because the output surface is missing, or the output surface no longer matches the frame. It can happen when the output Surface is replaced or disabled. For example, when the UseCase was unbound.
RESULT_SUCCESS
const val RESULT_SUCCESS = 1: Int
The drawFrameAsync call was successful. The frame with the exact timestamp was drawn to the output surface.
Public constructors
OverlayEffect
OverlayEffect(
targets: Int,
queueDepth: Int,
handler: Handler,
errorListener: Consumer<Throwable!>
)
Creates an OverlayEffect.
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targets: Int |
The targets the effect applies to. For example, |
queueDepth: Int |
The depth of the queue. This value indicates how many frames can be queued before the oldest frame being automatically released. |
handler: Handler |
The Handler for listening for the input Surface updates and for performing OpenGL operations. |
errorListener: Consumer<Throwable!> |
invoked if the effect runs into unrecoverable errors. The |
Public functions
clearOnDrawListener
fun clearOnDrawListener(): Unit
Clears the listener set in setOnDrawListener.
close
fun close(): Unit
Closes the OverlayEffect.
Once closed, the OverlayEffect can no longer be used.
drawFrameAsync
fun drawFrameAsync(timestampNs: Long): ListenableFuture<Int!>
Draws the queued frame with the given timestamp.
Once invoked, OverlayEffect retrieves the queued frame with the given timestamp and draws it to the output Surface. If the frame is successfully drawn, ListenableFuture completes with RESULT_SUCCESS. Otherwise, it completes with one of the following results: RESULT_FRAME_NOT_FOUND, RESULT_INVALID_SURFACE or RESULT_CANCELLED_BY_CALLER. If this method is called after the OverlayEffect is released, the ListenableFuture completes with an IllegalStateException.
This method is thread safe. When calling from the getHandler thread, it's executed right away; otherwise, it posts the execution on the getHandler. It's recommended to call this method from the getHandler thread to avoid thread hopping.
getQueueDepth
fun getQueueDepth(): Int
Gets the depth of the queue set in the constructor.
setOnDrawListener
fun setOnDrawListener(onDrawListener: Function<Frame!, Boolean!>): Unit
Sets the listener for drawing overlay.
Each time before OverlayEffect draws a frame to the output, the listener receives a Frame object, which contains the necessary APIs for drawing overlay.
To draw an overlay, first call getOverlayCanvas ()} to get a Canvas object. The Canvas object is backed by a SurfaceTexture with the size of getSize. getSensorToBufferTransform represents the mapping from camera sensor coordinates to the frame's coordinates. To draw objects in the sensor coordinates, call setMatrix with the value of getSensorToBufferTransform.
Once the drawing is done, the listener should return true for the OverlayEffect to draw it to the output Surface. If it returns false, the frame will be dropped.
OverlayEffect invokes the listener on the getHandler thread.
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Frame |