Java: How to draw on CameraX previewView?

Question

Can anybody show me how to draw a rectangle over cameraX preview? I tried implementing a custom view as suggested by past stack overflow answers that were written in kotlin but it does not seemed to be working for me when I attempted to convert it into Java.

XML File

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:id="@+id/my_root">

    <com.example.cameraxxx.RectOverlay
        android:id="@+id/rectOverlay"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <androidx.camera.view.PreviewView
        android:id="@+id/previewView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:contentDescription="preview_area"
        android:importantForAccessibility="no"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0">

    </androidx.camera.view.PreviewView>


    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="13dp"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="@+id/previewView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        tools:visibility="visible" />


</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity Extract

imageAnalysis.setAnalyzer(executor, new ImageAnalysis.Analyzer() {
            @SuppressLint("UnsafeExperimentalUsageError")
            @Override
            public void analyze(@NonNull ImageProxy imageProxy) {
                int rotationDegrees = imageProxy.getImageInfo().getRotationDegrees();
                FaceDetectorOptions realTimeOpts =
                        new FaceDetectorOptions.Builder()
                                .setPerformanceMode(1).enableTracking()
                                .build();
                
                Image mediaImage = imageProxy.getImage();
                if(mediaImage != null) {
                    InputImage image = InputImage.fromMediaImage(mediaImage,imageProxy.getImageInfo().getRotationDegrees());
                    FaceDetector detector = FaceDetection.getClient(realTimeOpts);
                    Task<List<Face>> result =
                            detector.process(image)
                                    .addOnSuccessListener(
                                            new OnSuccessListener<List<Face>>() {
                                                @Override
                                                public void onSuccess(List<Face> faces) {
                                                    // Task completed successfully
                                                    // ...
                                                    if(faces.size() !=0){
                                                        rectOverlay.setAlpha(1f);
                                                        Toast.makeText(MainActivity.this,"Face detected",Toast.LENGTH_SHORT).show();
                                                        Canvas canvas = new Canvas();
                                                        for (Face face: faces){
                                                            rectOverlay.drawOverlay(face,canvas);
                                                        }

                                                    }
                                                }
                                            })
                                    .addOnFailureListener(
                                            new OnFailureListener() {
                                                @Override
                                                public void onFailure(@NonNull Exception e) {
                                                    // Task failed with an exception
                                                    // ...
                                                    Log.d("y123","nno face");
                                                }
                                            })
                                    .addOnCompleteListener(
                                            new OnCompleteListener<List<Face>>() {
                                                @Override
                                                public void onComplete(@NonNull Task<List<Face>> task) {
                                                    imageProxy.close();
                                                }
                                            });
                }
            }
        });
        cameraProvider.bindToLifecycle((LifecycleOwner) this, cameraSelector, imageAnalysis, preview);
        preview.setSurfaceProvider(mPreviewView.createSurfaceProvider());
    }

RectOverlay Class

package com.example.cameraxxx;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;

import androidx.annotation.Nullable;

import com.google.mlkit.vision.face.Face;

public class RectOverlay extends View {

    private Paint paint;

    public RectOverlay(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        paint = new Paint();
        paint.setColor(Color.RED);
        paint.setStrokeWidth(10);
        paint.setStyle(Paint.Style.FILL);
    }

    public void drawOverlay(Face face, Canvas canvas){
        canvas.drawRect(face.getBoundingBox().left,face.getBoundingBox().top, face.getBoundingBox().right, face.getBoundingBox().bottom, paint);
        invalidate();

    }
}

Answer 1

1. Use match_parent in width and height of RectOverlay in XML
2. Place the RectOverlay below the PreviewView in XML