Android OpenCV + camera is just showing black screen

edit

I've spent days (well, nights) trying to work this out. So many examples online are for different versions of Android Studio, different versions of Android, different versions of OpenCV and I can't get any of them to the final 'working' stage.

This example (based on a youtube tutorial, I got to the point where I needed permissions. That's fine, I added that in and a check for them, and it pops up asking the user for camera permissions. But the screen stays blank. I've put in logcat debug, all the right methods seem to be getting called. Would appreciate any assistance.

EDIT: I've found other 'working' examples, copied them over and run them too, and black screen still. I tried shrinking the cameraview to half view, and indeed, it's the camera view going black. Cannot figure it out.

Code:

Manifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.mytestopencvapp" >

    <uses-permission android:name="android.permission.CAMERA"/>


    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme" >
        <activity android:name=".MainActivity" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

MainActivity.java

package com.example.mytestopencvapp;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.util.Log;
import android.view.SurfaceView;

import org.opencv.android.BaseLoaderCallback;
import org.opencv.android.CameraBridgeViewBase;
import org.opencv.android.JavaCamera2View;
import org.opencv.android.JavaCameraView;
import org.opencv.android.OpenCVLoader;
import org.opencv.core.Core;
import org.opencv.core.CvType;
import org.opencv.core.Mat;
import org.opencv.imgproc.Imgproc;

public class MainActivity extends AppCompatActivity implements CameraBridgeViewBase.CvCameraViewListener2 {

    private static String TAG = "MainActivity";

    JavaCameraView javaCameraView;
    Mat mRGBA, mRGBAT;

    private final int PERMISSIONS_READ_CAMERA=1;

    BaseLoaderCallback baseLoaderCallback = new BaseLoaderCallback(MainActivity.this) {
        @Override
        public void onManagerConnected(int status) {
            Log.d(TAG, "callbacksuccess");
            switch (status)
            {
                case BaseLoaderCallback.SUCCESS:
                {
                    Log.d(TAG, "case success");
                    javaCameraView.enableView();
                    break;
                }
                default:
                {
                    Log.d(TAG, "case default");
                    super.onManagerConnected(status);
                    break;
                }

            }

        }
    };

    static
    {
        if (OpenCVLoader.initDebug())
        {
            Log.d(TAG, "OpenCV is intialised");
        }
        else
        {
            Log.d(TAG, "OpenCV is not initialised");
        }
    }


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d(TAG, "onCreate");
        setContentView(R.layout.activity_main);
        javaCameraView = (JavaCameraView)findViewById(R.id.my_camera_view);
        javaCameraView.setVisibility(SurfaceView.VISIBLE);
        javaCameraView.setCvCameraViewListener(MainActivity.this);

// Here, thisActivity is the current activity
        if (ContextCompat.checkSelfPermission(this,
                Manifest.permission.CAMERA)
                != PackageManager.PERMISSION_GRANTED) {

            // Permission is not granted
            // Should we show an explanation?
            if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                    Manifest.permission.CAMERA)) {
                // Show an explanation to the user *asynchronously* -- don't block
                // this thread waiting for the user's response! After the user
                // sees the explanation, try again to request the permission.
            } else {
                // No explanation needed, we can request the permission.
                ActivityCompat.requestPermissions ...