How to write dynamically to whole stencil buffer in Unity - unity3d

Question

What do I want to achieve ?
I'd like to achieve an effect in Unity3D, where I superpose a few cameras on top of each other. Each cameras would draw to a specific area of the screen. If possible, I'd like these areas to change dynamically.
I am using unity (latest version), and URP.
How technically I see it :
For implementation and performances reasons, it seems writing to the stencil buffer is the way to go. That way, I can only render what part of the screen I want for each camera. It is also quite easy once the stencil is made, cause the ForwardRendering settings in Unity offer such capabilities out of the box.
What I can't figure out :
The problem is, I don't know to efficiently write to the whole stencil buffer (each frame). The best way would be to use a compute shader (or maybe a simple script), that directly write the values after some calculations. Is there a way for that ? If yes, How ?
Another alternative may be to use a transparent quad in front of one of each camera, and to write to the stencil buffers like that. But 1) It seems there exist a SV_StencilRef keyword in the fragment buffer, but not supported by Unity yet ? 2) I will still lose performance nevertheless.
Thanks for any help / ideas about how to tackle this problem.
Edit (Clarification) : I'd like to be able to render free shapes, and not only rects, which prevent the use of the standard ViewportRect.
After some search, I found the Voronoi split screen to be quite similar (with a technical view) to what I'd like to achieve (See here)

Answer 1

If I understand correctly, you only need to play with the different camera Viewport Rect (https://docs.unity3d.com/ScriptReference/Camera-rect.html) to determine what camera should render what part of the screen.
Response to comment: no, it's not stretched. Here is an example with four cameras:
Create a scene with four cameras, add this script to it and add the cameras to the array on the script. I added the _movingObject just to see something moving, but it's not necessary.
using UnityEngine;
public class CameraHandler : MonoBehaviour
{
[SerializeField] private Transform _movingObject;
[SerializeField] private float _posMod = 10.0f;
[SerializeField] private float _cameraPosMod = 0.1f;
[SerializeField] private Camera[] _cameras;
private void Update()
{
float t = Time.time;
float x = Mathf.Sin(t);
float y = Mathf.Cos(t);
if (_movingObject) _movingObject.position = new(x * _posMod, 1.0f, y * _posMod);
Vector2 center = new(0.5f + x * _cameraPosMod, 0.5f + y * _cameraPosMod);
// bottom left camera
_cameras[0].rect = new(0.0f, 0.0f, center.x, center.y);
// bottom right camera
_cameras[1].rect = new(center.x, 0.0f, 1.0f - center.x, center.y);
// upper left camera
_cameras[2].rect = new(0.0f, center.y, center.x, 1.0f - center.y);
// upper right camera
_cameras[3].rect = new(center.x, center.y, 1.0f - center.x, 1.0f - center.y);
}
}

Answer 2

Not exactly an answer to your question about stencil buffer but I had a (hopefully) similar use case recently.
The main issue: In the URP Camera stack
If your camera is set to Base it will overdraw the entire screen
you can not adjust the Viewport on any Overlay camera
You can actually try to set the viewport via code -> result your camera renders only the correct part of the scene ... but it gets stretched to the entire screen ^^
What I did in the end was
Leave all content and cameras at the origin position
Apply according masks to filter the content per camera
Make your camera Overlay (as usual)
go through a custom Camera.projectionMatrix
m_Camera.projectionMatrix = Matrix4x4.Translate(projectionOffset) * Matrix4x4.Perspective(m_Camera.fieldOfView, m_Camera.aspect, m_Camera.nearClipPlane, m_Camera.farClipPlane);
where the projectionOffset is an offset in viewport space (normalized 0 - 1) from the bottom left corner.
For example in my case I wanted a minimap at 400, 400 pixels from the top-right corner so I did
var topRightOffsetPixels = new Vector2(400, 400);
var topRightOffsetViewport = Vector2.one - new Vector2(topRightOffsetPixels.x * 2 / Screen.width, topRightOffsetPixels.y * 2 / Screen.height);
m_Camera.projectionMatrix = Matrix4x4.Translate(topRightOffsetViewport) * Matrix4x4.Perspective(m_Camera.fieldOfView, m_Camera.aspect, m_Camera.nearClipPlane, m_Camera.farClipPlane);
See also Matrix4x4.Perspective