sneak/Shared/LinesOfSight.cs

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using System;

namespace SemiColinGames {
  public sealed class LinesOfSight : IDisposable {

    const int NUM_EDGE_VERTICES = 60;

    readonly VertexBuffer vertexBuffer;
    readonly IndexBuffer indexBuffer;
    // coneVertices[0] is the eye position; the rest are the edge vertices.
    readonly VertexPositionColor[] coneVertices = new VertexPositionColor[NUM_EDGE_VERTICES + 1];
    // The number of total triangles drawn is one less than the number of edge points.
    readonly int[] indices = new int[(NUM_EDGE_VERTICES - 1) * 3];

    Color color = Color.FromNonPremultiplied(new Vector4(0, 0, 1, 0.6f));

    public LinesOfSight(GraphicsDevice graphics) {
      vertexBuffer = new VertexBuffer(
           graphics, typeof(VertexPositionColor), NUM_EDGE_VERTICES * 3, BufferUsage.WriteOnly);
      indexBuffer = new IndexBuffer(
          graphics, typeof(int), indices.Length, BufferUsage.WriteOnly);
    }

    ~LinesOfSight() {
      Dispose();
    }

    public void Dispose() {
      vertexBuffer.Dispose();
      indexBuffer.Dispose();
      GC.SuppressFinalize(this);
    }

    public void Update(NPC[] npcs, AABB[] collisionTargets) {
      NPC npc = npcs[0];

      Vector2 eyePos = npc.EyePosition;
      float visionRange = npc.VisionRange;
      Vector2 ray = npc.VisionRay;
      float fov = npc.FieldOfView;

      float visionRangeSq = visionRange * visionRange;
      float fovStep = fov / (NUM_EDGE_VERTICES - 1);

      coneVertices[0] = new VertexPositionColor(new Vector3(npc.EyePosition, 0), color);
      for (int i = 0; i < NUM_EDGE_VERTICES; i++) {
        float angle = -fov / 2 + fovStep * i;
        Vector2 rotated = ray.Rotate(angle);
        Vector2 closestHit = Vector2.Add(eyePos, rotated);
        float hitTime = 1f;

        Vector2 halfTileSize = new Vector2(World.TileSize / 2.0f, World.TileSize / 2.0f);
        for (int j = 0; j < collisionTargets.Length; j++) {
          AABB box = collisionTargets[j];
          if (Math.Abs(box.Position.X - npc.Position.X) > visionRange + halfTileSize.X) {
            continue;
          }
          Vector2 delta = Vector2.Add(halfTileSize, Vector2.Subtract(box.Position, eyePos));
          if (delta.LengthSquared() > visionRangeSq) {
            continue;
          }
          Hit? maybeHit = box.IntersectSegment(eyePos, rotated);
          if (maybeHit != null) {
            Hit hit = maybeHit.Value;
            if (hit.Time < hitTime) {
              hitTime = hit.Time;
              closestHit = hit.Position;
            }
          }
        }

        float tint = 0.6f - hitTime / 2;
        Color tinted = Color.FromNonPremultiplied(new Vector4(0, 0, 1, tint));
        coneVertices[i + 1] = new VertexPositionColor(new Vector3(closestHit, 0), tinted);
      }
    }

    public void Draw(GraphicsDevice graphics, BasicEffect lightingEffect) {

      for (int i = 0; i < NUM_EDGE_VERTICES - 1; i++) {
        indices[i * 3] = 0;
        indices[i * 3 + 1] = i + 1;
        indices[i * 3 + 2] = i + 2;
      }

      vertexBuffer.SetData(coneVertices);
      indexBuffer.SetData(indices);
      graphics.SetVertexBuffer(vertexBuffer);
      graphics.Indices = indexBuffer;

      foreach (EffectPass pass in lightingEffect.CurrentTechnique.Passes) {
        pass.Apply();
        graphics.DrawIndexedPrimitives(PrimitiveType.TriangleList, 0, 0, indices.Length / 3);
      }
    }
  }
}