Unity UI Polygon

UIPolygon.cs

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

[RequireComponent(typeof(CanvasRenderer))]
public class UIPolygon : Graphic {
    [Header("UI Polygon")]
    [SerializeField] private Vector2[] offsets = new Vector2[] {
        new Vector2(-1f, 1f),
        new Vector2(1f, 1f),
        new Vector2(1f, -1f),
        new Vector2(-1f, -1f)
    };
    private List<Vector2> points = new List<Vector2>();
    [SerializeField] private Vector2 padding = new Vector2(0f, 0f);
    [SerializeField] private bool fillCenter = true;

    [Header("Border")]
    [SerializeField] private bool hasBorder = true;
    [SerializeField] private float borderWidth = 5f;
    [SerializeField] private Color borderColor = Color.black;

    [Header("Shadow")]
    [SerializeField] private bool hasShadow = true;
    [SerializeField] private Color shadowColor = new Color(0f, 0f, 0f, 0.5f);
    [SerializeField] private Vector2 shadowOffset = new Vector2(10f, -10f);

    private void Start() {
        UpdatePoints();
    }

    private Rect lastRect;
    private Vector2 lastPadding;
    private void Update() {
        Rect currentRect = rectTransform.rect;
        if (currentRect != lastRect || padding != lastPadding) {
            UpdatePoints();
            lastRect = currentRect;
            lastPadding = padding;
        }
    }

    #if UNITY_EDITOR
    private void OnValidate() {
        UpdatePoints();
    }
    #endif

    private void UpdatePoints() {
        points.Clear();
        Rect rect = rectTransform.rect;
        for (int i = 0; i < offsets.Length; i++) {
            points.Add(((rect.size * 0.5f) + padding) * offsets[i]);
        }
        SetAllDirty();
    }

    protected override void OnPopulateMesh(VertexHelper vh) {
        if (points == null || points.Count < 3) {
            return;
        }
        vh.Clear();

        int vertexCount = 0;
        if (hasShadow) {
            vh.AddVert(shadowOffset, shadowColor, Vector2.zero);
            vertexCount++;

            for (int i = 0; i < points.Count; i++) {
                vh.AddVert(points[i] + shadowOffset, shadowColor, Vector2.zero);
                vertexCount++;
            }

            for (int i = 0; i < points.Count; i++) {
                vh.AddTriangle(0, i + 1, ((i + 1) % points.Count) + 1);
            }
        }

        if (fillCenter) {
            vh.AddVert(Vector2.zero, color, Vector2.zero);
            vertexCount++;

            for (int i = 0; i < points.Count; i++) {
                vh.AddVert(points[i], color, Vector2.zero);
            }
            
            for (int i = 0; i < points.Count; i++) {
                int idx1 = i + vertexCount;
                int idx2 = ((i + 1) % points.Count) + vertexCount;
                vh.AddTriangle(vertexCount, idx1, idx2);
            }

            vertexCount += points.Count;
        }

        if (hasBorder) {
            for (int i = 0; i < points.Count; i++) {
                Vector2 p1 = points[i];
                Vector2 p2 = points[(i + 1) % points.Count];

                Vector2 edge = p2 - p1;
                Vector2 edgePerp = Vector2.Perpendicular(edge.normalized);

                Vector2 offset = edgePerp * borderWidth;

                UIVertex v1 = new UIVertex();
                v1.position = p1 + offset;
                v1.color = borderColor;
                v1.uv0 = Vector2.zero;
                
                UIVertex v2 = new UIVertex();
                v2.position = p2 + offset;
                v2.color = borderColor;
                v2.uv0 = Vector2.zero;
                
                UIVertex v3 = new UIVertex();
                v3.position = p2 - offset;
                v3.color = borderColor;
                v3.uv0 = Vector2.zero;
                
                UIVertex v4 = new UIVertex();
                v4.position = p1 - offset;
                v4.color = borderColor;
                v4.uv0 = Vector2.zero;

                vh.AddVert(v1);
                vh.AddVert(v2);
                vh.AddVert(v3);
                vh.AddVert(v4);

                int idx = vertexCount;
                vh.AddTriangle(idx, idx + 1, idx + 2);
                vh.AddTriangle(idx, idx + 2, idx + 3);
                
                vertexCount += 4;
            }

            for (int i = 0; i < points.Count; i++) {
                Vector2 current = points[i];
                Vector2 prev = points[(i - 1 + points.Count) % points.Count];
                Vector2 next = points[(i + 1) % points.Count];

                Vector2 toPrev = (prev - current).normalized;
                Vector2 toNext = (next - current).normalized;

                Vector2 prevPerp = Vector2.Perpendicular(toPrev);
                Vector2 nextPerp = Vector2.Perpendicular(toNext);

                // Check if this is an inner corner (concave) or outer corner (convex)
                float crossProduct = (toPrev.x * toNext.y) - (toPrev.y * toNext.x);

                Vector2 prevOffset = prevPerp * borderWidth * Mathf.Sign(crossProduct);
                Vector2 prevCorner = current - prevOffset;
                
                Vector2 nextOffset = nextPerp * borderWidth * Mathf.Sign(crossProduct);
                Vector2 nextCorner = current + nextOffset;
                
                float segmentAngle = Vector2.Angle(toPrev, toNext) * Mathf.Deg2Rad;
                float miterLength = borderWidth / Mathf.Sin(segmentAngle / 2);

                Vector2 miterDir = (nextPerp - prevPerp).normalized * Mathf.Sign(crossProduct);
                Vector2 miterOffset = (miterDir * miterLength);
                Vector2 miterCorner = current + miterOffset;

                UIVertex v1 = new UIVertex();
                v1.position = current;
                v1.color = borderColor;
                v1.uv0 = Vector2.zero;
                
                UIVertex v2 = new UIVertex();
                v2.position = miterCorner;
                v2.color = borderColor;
                v2.uv0 = Vector2.zero;
                
                UIVertex v3 = new UIVertex();
                v3.position = prevCorner;
                v3.color = borderColor;
                v3.uv0 = Vector2.zero;
                
                UIVertex v4 = new UIVertex();
                v4.position = nextCorner;
                v4.color = borderColor;
                v4.uv0 = Vector2.zero;
                
                vh.AddVert(v1);
                vh.AddVert(v2);
                vh.AddVert(v3);
                vh.AddVert(v4);
                
                int idx = vertexCount;
                vh.AddTriangle(idx, idx + 1, idx + 2);
                vh.AddTriangle(idx, idx + 1, idx + 3);
                
                vertexCount += 4;
            }
        }
    }
}