SpriteMeshRaycastFilter: easy Unity UI non-rectangular click detector without reading textures

SpriteMeshRaycastFilter.cs

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

// Restrict raycasting to a sprite mesh shape
// Could use Image.alphaHitTestMinimumThreshold to mask but that requires read/write images which can't be packed
[RequireComponent(typeof(Image))]
public class SpriteMeshRaycastFilter : MonoBehaviour, ICanvasRaycastFilter {

    private RectTransform rectTransform;
    private Image image;
    
    private class CachedSpriteMesh {
        public readonly ushort[] Triangles;
        public readonly Vector2[] Positions;

        public CachedSpriteMesh(ushort[] tris, Vector2[] posns) {
            Triangles = tris;
            Positions = posns;
        }
    }
    // instance ID -> cached tris/positions since we can only get copies
    private static Dictionary<int, CachedSpriteMesh> cachedSpriteMeshes;
    private CachedSpriteMesh spriteMesh;


    private void GetReferences() {
        if (rectTransform == null)
            rectTransform = GetComponent<RectTransform>();
        if (image == null)
            image = GetComponent<Image>();
        if (cachedSpriteMeshes == null)
            cachedSpriteMeshes = new Dictionary<int, CachedSpriteMesh>();
    }

    private CachedSpriteMesh GetSpriteMesh() {
        if (spriteMesh == null) {
            // This is not threadsafe, obvs - OK to use in Unity thread though
            var sprite = image.sprite;
            int id = image.sprite.GetInstanceID();
            if (!cachedSpriteMeshes.TryGetValue(id, out spriteMesh)) {
                spriteMesh = new CachedSpriteMesh(sprite.triangles, sprite.vertices);
                cachedSpriteMeshes.Add(id, spriteMesh);
            }
        }

        return spriteMesh;
    }

    public bool IsRaycastLocationValid(Vector2 screenPoint, Camera eventCamera) {
        GetReferences();

        if (!RectTransformUtility.ScreenPointToLocalPointInRectangle(
            rectTransform, screenPoint, eventCamera, out var local))
            return false;

        Vector2 uiSz = image.GetPixelAdjustedRect().size;
        Vector2 spriteSz = image.sprite.rect.size;
        Vector2 sz = uiSz;
        if (image.preserveAspect) {
            float spriteAspect = spriteSz.x / spriteSz.y;
            float uiAspect = uiSz.x / uiSz.y;
            if (uiAspect > spriteAspect) {
                // UI is wider, width is limited
                sz.x = sz.y * spriteAspect;
            } else {
                // UI is taller, height is limited
                sz.y = sz.x / spriteAspect;
            }
        }

        // Adjust the local pos into normalised space
        local /= sz;
        // Adjust via pivot so that position is relative to centre
        local += rectTransform.pivot - new Vector2(0.5f, 0.5f);

        var sm = GetSpriteMesh();

        // Debug
        // for (int i = 0; i < sm.Triangles.Length; i += 3) {
        //     Debug.DrawLine(sm.Positions[sm.Triangles[i]], sm.Positions[sm.Triangles[i+1]]);
        //     Debug.DrawLine(sm.Positions[sm.Triangles[i+1]], sm.Positions[sm.Triangles[i+2]]);
        //     Debug.DrawLine(sm.Positions[sm.Triangles[i+2]], sm.Positions[sm.Triangles[i+0]]);
        // }
        // Vector2 xoffset = new Vector2(0.1f, 0);
        // Vector2 yoffset = new Vector2(0, 0.1f);
        // Debug.DrawLine(local - xoffset, local + xoffset, Color.cyan);
        // Debug.DrawLine(local - yoffset, local + yoffset, Color.cyan);

        
        for (int i = 0; i < sm.Triangles.Length; i += 3) {
            if (local.PointInTriangle2D(
                sm.Positions[sm.Triangles[i]],
                sm.Positions[sm.Triangles[i + 1]],
                sm.Positions[sm.Triangles[i + 2]])) {
                return true;
            }
        }

        return false;
    }
    
    
}
/*
This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org/>
*/

Vector2Extensions.cs

using UnityEngine;

public static class Vector2Extensions {
    /// <summary>
    /// Return whether a point is inside a triangle in 2D
    /// </summary>
    /// <param name="p">Point to test</param>
    /// <param name="t0">Vertex of the triangle</param>
    /// <param name="t1">Vertex of the triangle</param>
    /// <param name="t3">Vertex of the triangle</param>
    /// <returns></returns>
    public static bool PointInTriangle2D(this Vector2 p, Vector2 t0, Vector2 t1, Vector2 t3) {
        var s = t0.y * t3.x - t0.x * t3.y + (t3.y - t0.y) * p.x + (t0.x - t3.x) * p.y;
        var t = t0.x * t1.y - t0.y * t1.x + (t0.y - t1.y) * p.x + (t1.x - t0.x) * p.y;

        if (s < 0 != t < 0)
            return false;

        var A = -t1.y * t3.x + t0.y * (t3.x - t1.x) + t0.x * (t1.y - t3.y) + t1.x * t3.y;

        return A < 0 ?
            s <= 0 && s + t >= A :
            s >= 0 && s + t <= A;
    }    
}