Rendering screenshots from Unity3d with transparent backgrounds

TransparentBackgroundScreenshotRecorder.cs

using UnityEngine;
using System.Collections;
using System.IO;

/*

Usage:
1. Attach this script to your chosen camera's game object.
2. Set that camera's Clear Flags field to Solid Color.
3. Use the inspector to set frameRate and framesToCapture
4. Choose your desired resolution in Unity's Game window (must be less than or equal to your screen resolution)
5. Turn on "Maximise on Play"
6. Play your scene. Screenshots will be saved to YourUnityProject/Screenshots by default.

*/

public class TransparentBackgroundScreenshotRecorder : MonoBehaviour {

#region public fields
[Tooltip("A folder will be created with this base name in your project root")]
public string folderBaseName = "Screenshots";
[Tooltip("How many frames should be captured per second of game time")]
public int frameRate = 24;
[Tooltip("How many frames should be captured before quitting")]
public int framesToCapture = 24;
#endregion
#region private fields
private string folderName = "";
private GameObject whiteCamGameObject;
private Camera whiteCam;
private GameObject blackCamGameObject;
private Camera blackCam;
private Camera mainCam;
private int videoFrame = 0; // how many frames we've rendered
private float originalTimescaleTime;
private bool done=false;
private int screenWidth;
private int screenHeight;
private Texture2D textureBlack;
private Texture2D textureWhite;
private Texture2D textureTransparentBackground;
#endregion

void Awake () {
  mainCam = gameObject.GetComponent<Camera>();
  CreateBlackAndWhiteCameras ();
  CreateNewFolderForScreenshots ();
  CacheAndInitialiseFields ();
  Time.captureFramerate = frameRate;
}

void LateUpdate () {
  if(!done){
    StartCoroutine(CaptureFrame());
  } else {
    Debug.Log("Complete! "+videoFrame+" videoframes rendered. File names are 0 indexed)");
  Debug.Break();
  }
}

IEnumerator CaptureFrame (){
  yield return new WaitForEndOfFrame();
  if(videoFrame < framesToCapture) {
    RenderCamToTexture(blackCam,textureBlack);
    RenderCamToTexture(whiteCam,textureWhite);
    CalculateOutputTexture ();
    SavePng ();
    videoFrame++;
    Debug.Log("Rendered frame " +videoFrame); 
    videoFrame++;
  } else {
    done=true;
    StopCoroutine("CaptureFrame");
  }
}

void RenderCamToTexture (Camera cam, Texture2D tex){
  cam.enabled=true;
  cam.Render();
  WriteScreenImageToTexture(tex);
  cam.enabled=false;
}

void CreateBlackAndWhiteCameras (){
  whiteCamGameObject = (GameObject) new GameObject();
  whiteCamGameObject.name="White Background Camera";
  whiteCam=whiteCamGameObject.AddComponent<Camera>();
  whiteCam.CopyFrom(mainCam);
  whiteCam.backgroundColor=Color.white;
  whiteCamGameObject.transform.SetParent(gameObject.transform, true);

  blackCamGameObject = (GameObject) new GameObject();
  blackCamGameObject.name="Black Background Camera";
  blackCam=blackCamGameObject.AddComponent<Camera>();
  blackCam.CopyFrom(mainCam);
  blackCam.backgroundColor=Color.black;
  blackCamGameObject.transform.SetParent(gameObject.transform, true);
}

void CreateNewFolderForScreenshots (){
  // Find a folder name that doesn't exist yet. Append number if necessary.
  folderName = folderBaseName;
  int count = 1;
  while (System.IO.Directory.Exists (folderName)) {
    folderName = folderBaseName + count;
    count++;
  }
  System.IO.Directory.CreateDirectory (folderName); // Create the folder
}

void WriteScreenImageToTexture (Texture2D tex){
  tex.ReadPixels (new Rect (0, 0, screenWidth, screenHeight), 0, 0);
  tex.Apply ();
}

void CalculateOutputTexture (){
  Color color;
  for (int y = 0; y < textureTransparentBackground.height; ++y) {
  // each row
    for (int x = 0; x < textureTransparentBackground.width; ++x) {
      // each column
      float alpha = textureWhite.GetPixel (x, y).r - textureBlack.GetPixel (x, y).r;
      alpha = 1.0f - alpha;
      if (alpha == 0) {
        color = Color.clear;
      }
      else {
        color = textureBlack.GetPixel (x, y) / alpha;
      }
      color.a = alpha;
      textureTransparentBackground.SetPixel (x, y, color);
    }
  }
}

void SavePng (){
  string name = string.Format ("{0}/{1:D04} shot.png", folderName, videoFrame);
  var pngShot = textureTransparentBackground.EncodeToPNG ();
  File.WriteAllBytes (name, pngShot);
}

void CacheAndInitialiseFields (){
  originalTimescaleTime = Time.timeScale;
  screenWidth = Screen.width;
  screenHeight = Screen.height;
  textureBlack = new Texture2D (screenWidth, screenHeight, TextureFormat.RGB24, false);
  textureWhite = new Texture2D (screenWidth, screenHeight, TextureFormat.RGB24, false);
  textureTransparentBackground = new Texture2D (screenWidth, screenHeight, TextureFormat.ARGB32, false);
  }
}

Black on URP :(