GetBoundsAtScreenSpace

using System;
using System.Collections.Generic;
using UnityEngine;

public static class GetBoundsAtScreenSpace
{
    private static readonly List<Vector3> _localSpaceVerticesBuffer = new List<Vector3>(); 
    
    public static Bounds Get(MeshFilter meshFilter, Camera camera)
    {
        var mesh = meshFilter.sharedMesh;
        mesh.GetVertices(_localSpaceVerticesBuffer);
        var worldToClipMatrix = camera.projectionMatrix * camera.worldToCameraMatrix * meshFilter.transform.localToWorldMatrix;

        Span<Vector3> screenSpaceVertices = stackalloc Vector3[mesh.vertexCount];

        for (int i = 0; i < mesh.vertexCount; i++)
        {
            var clipSpaceVertex = worldToClipMatrix.MultiplyPoint(_localSpaceVerticesBuffer[i]);
            screenSpaceVertices[i] = ClipToScreenPoint(clipSpaceVertex);
        }

        var minX = float.MaxValue;
        var maxX = float.MinValue;
        var minY = float.MaxValue;
        var maxY = float.MinValue;

        foreach (var vertex in screenSpaceVertices)
        {
            minX = Mathf.Min(minX, vertex.x);
            maxX = Mathf.Max(maxX, vertex.x);
            minY = Mathf.Min(minY, vertex.y);
            maxY = Mathf.Max(maxY, vertex.y);
        }

        var center = new Vector2((minX + maxX) / 2f, (minY + maxY) / 2f);
        var size = new Vector2(maxX - minX, maxY - minY);
        return new Bounds(center, size);
    }

    private static Vector3 ClipToScreenPoint(Vector3 point)
    {
        // Clip -> Viewport / (-1, +1) -> (0, 1)
        var viewportSpaceX = (point.x + 1f) / 2f;
        var viewportSpaceY = (point.y + 1f) / 2f;
        var viewportSpaceZ = (point.z + 1f) / 2f;
        
        // Viewport -> Screen / (0, 1) -> (0, Resolution)
        var screenSpaceX = viewportSpaceX * Screen.width;
        var screenSpaceY = viewportSpaceY * Screen.height;
        
        return new Vector3(screenSpaceX, screenSpaceY, viewportSpaceZ); // (0, Resolution)
    }
}