wwdpublic/Content.Client/DeltaV/Lamiae/SnakeOverlay.cs

using Content.Shared.SegmentedEntity;
using Content.Shared.Humanoid;
using Content.Shared.Humanoid.Markings;
using Content.Client.Resources;
using Robust.Client.ResourceManagement;
using Robust.Client.Graphics;
using Robust.Shared.Enums;
using System.Numerics;
using System.Linq;


namespace Content.Client.Lamiae;

/// <summary>
/// This draws lamia segments directly from polygons instead of sprites. This is a very novel approach as of the time this is being written (August 2024) but it wouldn't surprise me
/// if there's a better way to do this at some point. Currently we have a very heavy restriction on the tools we can make, forcing me to make several helpers that may be redundant later.
/// This will be overcommented because I know you haven't seen code like this before and you might want to copy it.
/// This is an expansion on some techniques I discovered in (https://github.com/Elijahrane/Delta-v/blob/49d76c437740eab79fc622ab50d628b926e6ddcb/Content.Client/DeltaV/Arcade/S3D/Renderer/S3DRenderer.cs)
/// </summary>
public sealed class SnakeOverlay : Overlay
{
    private readonly IResourceCache _resourceCache;
    private readonly IEntityManager _entManager;
    private readonly SharedTransformSystem _transform;
    private readonly SharedHumanoidAppearanceSystem _humanoid = default!;

    // Look through these carefully. WorldSpace is useful for debugging. Note that this defaults to "screen space" which breaks when you try and get the world handle.
    public override OverlaySpace Space => OverlaySpace.WorldSpaceEntities;

    // Overlays are strange and you need this pattern where you define readonly deps above, and then make a constructor with this pattern. Anything that creates this overlay will then
    // have to provide all the deps.
    public SnakeOverlay(IEntityManager entManager, IResourceCache resourceCache)
    {
        _resourceCache = resourceCache;
        // we get ent manager from SnakeOverlaySystem turning this on and passing it
        _entManager = entManager;
        // with ent manager we can fetch our other entity systems
        _transform = _entManager.EntitySysManager.GetEntitySystem<SharedTransformSystem>();
        _humanoid = _entManager.EntitySysManager.GetEntitySystem<SharedHumanoidAppearanceSystem>();

        // draw at drawdepth 3
        ZIndex = 3;
    }

    // This step occurs each frame. For some overlays you may want to conisder limiting how often they update, but for player entities that move around fast we'll just do it every frame.
    protected override void Draw(in OverlayDrawArgs args)
    {
        // load the handle, the "pen" we draw with
        var handle = args.WorldHandle;

        // Get all lamiae the client knows of and their transform in a way we can enumerate over
        var enumerator = _entManager.AllEntityQueryEnumerator<SegmentedEntityComponent, TransformComponent>();

        // I go over the collection above, pulling out an EntityUid and the two components I need for each.
        while (enumerator.MoveNext(out var uid, out var lamia, out var xform))
        {
            // Skip ones that are off-map. "Map" in this context means interconnected stuff you can travel between by moving, rather than needing e.g. FTL to load a new map.
            if (xform.MapID != args.MapId)
                continue;

            // Skip ones where they are not loaded properly, uninitialized, or w/e
            if (lamia.Segments.Count < lamia.NumberOfSegments)
            {
                _entManager.Dirty(uid, lamia); // pls give me an update...
                continue;
            }

            // By the way, there's a hack to mitigate overdraw somewhat. Check out whatever is going on with the variable called "bounds" in DoAfterOverlay.
            // I won't do it here because (1) it's ugly and (2) theoretically these entities can be fucking huge and you'll see the tail end of them when they are way off screen.
            // On a PVS level I think segmented entities should be all-or-nothing when it comes to PVS range, that is you either load all of their segments or none.

            // Color.White is drawing without modifying color. For clothed tails, we should use White. For skin, we should use the color of the marking.
            // TODO: Better way to cache this
            Color? col = null;
            if (_entManager.TryGetComponent<HumanoidAppearanceComponent>(uid, out var humanoid))
                if (humanoid.MarkingSet.TryGetCategory(MarkingCategories.Tail, out var tailMarkings))
                    col = tailMarkings.First().MarkingColors.First();

            DrawLamia(handle, lamia, col ?? Color.White);
        }
    }

    // This is where we do the actual drawing.
    private void DrawLamia(DrawingHandleWorld handle, SegmentedEntityComponent lamia, Color color)
    {
        // We're going to store all our verticies in here and then draw them
        List<DrawVertexUV2D> verts = new List<DrawVertexUV2D>();

        // Radius of the initial segment
        float radius = lamia.InitialRadius;

        // We're storing the left and right verticies of the last segment so we can start drawing from there without gaps
        Vector2? lastPtCW = null;
        Vector2? lastPtCCW = null;

        var tex = _resourceCache.GetTexture(lamia.TexturePath);

        int i = 1;
        // do each segment except the last one normally
        while (i < lamia.Segments.Count - 1)
        {
            // get centerpoints of last segment and this one
            var origin = _transform.GetWorldPosition(_entManager.GetEntity(lamia.Segments[i - 1]));
            var destination = _transform.GetWorldPosition(_entManager.GetEntity(lamia.Segments[i]));

            // get direction between the two points and normalize it
            var connectorVec = destination - origin;
            connectorVec = connectorVec.Normalized();

            //get one rotated 90 degrees clockwise
            var offsetVecCW = new Vector2(connectorVec.Y, 0 - connectorVec.X);

            //and counterclockwise
            var offsetVecCCW = new Vector2(0 - connectorVec.Y, connectorVec.X);

            /// tri 1: line across first segment and corner of second
            if (lastPtCW == null)
            {
                verts.Add(new DrawVertexUV2D(origin + offsetVecCW * radius, Vector2.Zero));
            }
            else
            {
                verts.Add(new DrawVertexUV2D((Vector2) lastPtCW, Vector2.Zero));
            }

            if (lastPtCCW == null)
            {
                verts.Add(new DrawVertexUV2D(origin + offsetVecCCW * radius, new Vector2(1, 0)));
            }
            else
            {
                verts.Add(new DrawVertexUV2D((Vector2) lastPtCCW, new Vector2(1, 0)));
            }

            verts.Add(new DrawVertexUV2D(destination + offsetVecCW * radius, new Vector2(0, 1)));

            // tri 2: line across second segment and corner of first
            if (lastPtCCW == null)
            {
                verts.Add(new DrawVertexUV2D(origin + offsetVecCCW * radius, new Vector2(1, 0)));
            }
            else
            {
                verts.Add(new DrawVertexUV2D((Vector2) lastPtCCW, new Vector2(1, 0)));
            }

            lastPtCW = destination + offsetVecCW * radius;
            verts.Add(new DrawVertexUV2D((Vector2) lastPtCW, new Vector2(0, 1)));
            lastPtCCW = destination + offsetVecCCW * radius;
            verts.Add(new DrawVertexUV2D((Vector2) lastPtCCW, new Vector2(1, 1)));

            // slim down a bit for next segment
            radius *= lamia.SlimFactor;

            i++;
        }

        // draw tail (1 tri)
        if (lastPtCW != null && lastPtCCW != null)
        {
            verts.Add(new DrawVertexUV2D((Vector2) lastPtCW, new Vector2(0, 0)));
            verts.Add(new DrawVertexUV2D((Vector2) lastPtCCW, new Vector2(1, 0)));

            var destination = _transform.GetWorldPosition(_entManager.GetEntity(lamia.Segments.Last()));

            verts.Add(new DrawVertexUV2D(destination, new Vector2(0.5f, 1f)));
        }

        // Draw all of the triangles we just pit in at once
        handle.DrawPrimitives(DrawPrimitiveTopology.TriangleList, texture: tex, verts.ToArray().AsSpan(), color);
    }
}