ThiefLightmapper/KeepersCompound.Lightmapper/PotentiallyVisibleSet.cs

using System.Numerics;
using KeepersCompound.LGS.Database.Chunks;

namespace KeepersCompound.Lightmapper;

public class PotentiallyVisibleSet
{
    private struct Edge
    {
        public int Destination;
        public Poly Poly;

        public override string ToString()
        {
            return $"<Destination: {Destination}, Poly: {Poly}";
        }
    }

    private struct Poly
    {
        public List<Vector3> Vertices;
        public readonly Plane Plane;

        public Poly(List<Vector3> vertices, Plane plane)
        {
            Vertices = vertices;
            Plane = plane;
        }

        public Poly(Poly other)
        {
            Vertices = [..other.Vertices];
            Plane = other.Plane;
        }
        
        public bool IsCoplanar(Poly other)
        {
            return MathUtils.IsCoplanar(Plane, other.Plane);
        }
        
        public override string ToString()
        {
            return $"<Plane: {Plane}, Vertices: [{string.Join(", ", Vertices)}]";
        }
    }
    
    private readonly List<int>[] _portalGraph;
    private readonly List<Edge> _edges;
    private readonly Dictionary<int, HashSet<int>> _visibilitySet;

    private const float Epsilon = 0.1f;
    
    // This is yucky and means we're not thread safe
    private readonly List<float> _clipDistances = new(32);
    private readonly List<Side> _clipSides = new(32);
    private readonly int[] _clipCounts = [0, 0, 0];

    // TODO:
    // - This is a conservative algorithm based on Matt's Ramblings Quake PVS video
    // - Build portal graph (or just use WR directly)
    // - A cell can always see it's self and any immediate neighbours
    // - The third depth cell is also visible unless the portal to it is coplanar with the second cells portal (do I need to think about this?)
    // - For all further cells:
    //   - Generate separating planes between the source cell portal and the previously passed (clipped) portal
    //   - Clip the target portal to the new cell using the separating planes
    //   - If anything is left of the clipped portal, we can see, otherwise we discard that cell
    // - The full process is a recursive depth first search

    public PotentiallyVisibleSet(WorldRep.Cell[] cells)
    {
        _edges = [];
        _visibilitySet = new Dictionary<int, HashSet<int>>();
        
        _portalGraph = new List<int>[cells.Length];
        for (var i = 0; i < cells.Length; i++)
        {
            _portalGraph[i] = [];
            var cell = cells[i];

            // If a cell is "blocks vision" flagged, we can never see out of it
            // We can see into it though, so we still want the edges coming in
            if ((cell.Flags & 8) != 0)
            {
                continue;
            }
            
            // We have to cycle through *all* polys rather than just portals to calculate the correct poly vertex offsets
            var indicesOffset = 0;
            var portalStartIdx = cell.PolyCount - cell.PortalPolyCount;
            for (var j = 0; j < cell.PolyCount; j++)
            {
                var poly = cell.Polys[j];
                if (j < portalStartIdx)
                {
                    indicesOffset += poly.VertexCount;
                    continue;
                }
                
                var other = poly.Destination;
                
                // Checking if there's already an edge is super slow. It's much faster to just add a new edge, even with
                // the duplicated poly
                var vs = new List<Vector3>(poly.VertexCount);
                for (var vIdx = 0; vIdx < poly.VertexCount; vIdx++)
                {
                    vs.Add(cell.Vertices[cell.Indices[indicesOffset + vIdx]]);
                }
                    
                var edge = new Edge
                {
                    Destination = other,
                    Poly = new Poly(vs, cell.Planes[poly.PlaneId]),
                };
                _edges.Add(edge);
                _portalGraph[i].Add(_edges.Count - 1);
                indicesOffset += poly.VertexCount;
            }
        }
    }

    public int[] GetVisible(int cellIdx)
    {
        if (_visibilitySet.TryGetValue(cellIdx, out var value))
        {
            return [..value];
        }

        var visibleCells = ComputeVisibility(cellIdx);
        _visibilitySet.Add(cellIdx, visibleCells);
        return [..visibleCells];
    }
    
    private HashSet<int> ComputeVisibility(int cellIdx)
    {
        if (cellIdx >= _portalGraph.Length)
        {
            return [];
        }

        // A cell can always see itself, so we'll add that now
        var visible = new HashSet<int>();
        visible.Add(cellIdx);

        // Additionally a cell can always see it's direct neighbours (obviously)
        foreach (var edgeIndex in _portalGraph[cellIdx])
        {
            var edge = _edges[edgeIndex];
            var neighbourIdx = edge.Destination;
            visible.Add(neighbourIdx);
            
            // Neighbours of our direct neighbour are always visible, unless they're coplanar
            foreach (var innerEdgeIndex in _portalGraph[neighbourIdx])
            {
                var innerEdge = _edges[innerEdgeIndex];
                if (innerEdge.Destination == cellIdx || edge.Poly.IsCoplanar(innerEdge.Poly))
                {
                    continue;
                }

                ExplorePortalRecursive(visible, edge.Poly, new Poly(innerEdge.Poly), neighbourIdx, innerEdge.Destination, 0);
            }
        }
        
        return visible;
    }
    
    private void ExplorePortalRecursive(
        HashSet<int> visible,
        Poly sourcePoly,
        Poly previousPoly,
        int previousCellIdx,
        int currentCellIdx,
        int depth)
    {
        // TODO: Might need to lose this
        if (depth > 1024)
        {
            return;
        }
        
        visible.Add(currentCellIdx);
        
        // Only one edge out of the cell means we'd be going back on ourselves
        if (_portalGraph[currentCellIdx].Count <= 1)
        {
            return;
        }
        
        // TODO: If all neighbours are already in `visible` skip exploring?
        
        var separators = new List<Plane>();
        GetSeparatingPlanes(separators, sourcePoly, previousPoly, false);
        GetSeparatingPlanes(separators, previousPoly, sourcePoly, true);

        // The case for this occuring is... interesting ( idk )
        if (separators.Count == 0)
        {
            return;
        }
        
        // Clip all new polys and recurse
        foreach (var edgeIndex in _portalGraph[currentCellIdx])
        {
            var edge = _edges[edgeIndex];
            if (edge.Destination == previousCellIdx || previousPoly.IsCoplanar(edge.Poly) || sourcePoly.IsCoplanar(edge.Poly))
            {
                continue;
            }

            var poly = new Poly(edge.Poly);
            foreach (var separator in separators)
            {
                ClipPolygonByPlane(ref poly, separator);
            }
            
            if (poly.Vertices.Count == 0)
            {
                continue;
            }
            
            ExplorePortalRecursive(visible, sourcePoly, poly, currentCellIdx, edge.Destination, depth + 1);
        }
    }

    // TODO: We're getting multiple separating planes that are the same, let's not somehow?
    private static void GetSeparatingPlanes(List<Plane> separators, Poly p0, Poly p1, bool flip)
    {
        for (var i = 0; i < p0.Vertices.Count; i++)
        {
            // brute force all combinations
            // there's probably some analytical way to choose the "correct" v2 but I couldn't find anything online
            var v0 = p0.Vertices[i];
            var v1 = p0.Vertices[(i + 1) % p0.Vertices.Count];
            for (var j = 0; j < p1.Vertices.Count; j++)
            {
                var v2 = p1.Vertices[j];
                
                var normal = Vector3.Cross(v1 - v0, v2 - v0);
                if (normal.LengthSquared() < Epsilon)
                {
                    // colinear (or near colinear) points will produce an invalid plane
                    continue;
                }
                
                normal = Vector3.Normalize(normal);
                var d = -Vector3.Dot(v2, normal);
                
                // Depending on how the edges were built, the resulting plane might be facing the wrong way
                var distanceToSource = MathUtils.DistanceFromPlane(p0.Plane, v2);
                if (distanceToSource > Epsilon)
                {
                    normal = -normal;
                    d = -d;
                }
                
                var plane = new Plane(normal, d);
                
                if (MathUtils.IsCoplanar(plane, flip ? p0.Plane : p1.Plane))
                {
                    continue;
                }
                
                // All points should be in front of the plane (except for the point used to create it)
                var invalid = false;
                var count = 0;
                for (var k = 0; k < p1.Vertices.Count; k++)
                {
                    if (k == j)
                    {
                        continue;
                    }
                    
                    var dist = MathUtils.DistanceFromPlane(plane, p1.Vertices[k]);
                    if (dist > Epsilon)
                    {
                        count++;
                    }
                    else if (dist < -Epsilon)
                    {
                        invalid = true;
                        break;
                    }
                }

                if (invalid || count == 0)
                {
                    continue;
                }

                if (flip)
                {
                    plane.Normal = -normal;
                    plane.D = -d;
                }
                
                separators.Add(plane);
            }
        }
    }
    
    private enum Side 
    {
        Front,
        On,
        Back
    }
    
    // TODO: is this reference type poly going to fuck me?
    // TODO: Should this and Poly be in MathUtils?
    private void ClipPolygonByPlane(ref Poly poly, Plane plane)
    {
        var vertexCount = poly.Vertices.Count;
        if (vertexCount == 0)
        {
            return;
        }
        
        // Firstly we want to tally up what side of the plane each point of the poly is on
        // This is used both to early out if nothing/everything is clipped, and to aid the clipping
        // var distances = new float[vertexCount];
        // var sides = new Side[vertexCount];
        // var counts = new int[3];
        _clipDistances.Clear();
        _clipSides.Clear();
        _clipCounts[0] = 0;
        _clipCounts[1] = 0;
        _clipCounts[2] = 0;
        for (var i = 0; i < vertexCount; i++)
        {
            var distance = MathUtils.DistanceFromPlane(plane, poly.Vertices[i]);
            _clipDistances.Add(distance);
            _clipSides.Add(distance switch {
                > Epsilon => Side.Front,
                <-Epsilon => Side.Back,
                _ => Side.On,
            });
            _clipCounts[(int)_clipSides[i]]++;
        }

        // Everything is within the half-space, so we don't need to clip anything
        if (_clipCounts[(int)Side.Back] == 0 && _clipCounts[(int)Side.On] != vertexCount)
        {
            return;
        }
        
        // Everything is outside the half-space, so we clip everything
        if (_clipCounts[(int)Side.Front] == 0)
        {
            poly.Vertices.Clear();
            return;
        }
        
        var vertices = new List<Vector3>();
        for (var i = 0; i < vertexCount; i++)
        {
            var i1 = (i + 1) % vertexCount;
            var v0 = poly.Vertices[i];
            var v1 = poly.Vertices[i1];
            var side = _clipSides[i];
            var nextSide = _clipSides[i1];
            
            // Vertices that are inside/on the half-space don't get clipped
            if (_clipSides[i] != Side.Back)
            {
                vertices.Add(v0);
            }
            
            // We only need to do any clipping if we've swapped from front-to-back or vice versa
            // If either the current or next side is On then that's where we would have clipped to
            // anyway so we also don't need to do anything
            if (side == Side.On || nextSide == Side.On || side == nextSide)
            {
                continue;
            }

            // This is how far along the vector v0 -> v1 the front/back crossover occurs
            var frac = _clipDistances[i] / (_clipDistances[i] - _clipDistances[i1]);
            var splitVertex = v0 + frac * (v1 - v0);
            vertices.Add(splitVertex);
        }

        poly.Vertices = vertices;
    }
}