【Unity】刮刮乐效果（擦除图片像素值）

实现类似刮刮乐效果，擦除图片指定像素值（修改图片Alfa通道） 

参考Unity 刮刮乐工程源码的实现原理，对实现方式有一些调整

这里RawImage需要保持原图大小，不能缩放，不然坐标计算会有偏差

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

[RequireComponent(typeof(RawImage))]
public class EraseHandler : MonoBehaviour, IPointerDownHandler, IDragHandler, IPointerUpHandler
{
    //擦除完成调用事件
    public Action eraseFinishEvent;

    //笔刷半径
    [SerializeField] int brushRadius = 50;

    //擦除比例，擦除比例高于该值，是为擦除完成，自动擦除剩余部分
    [SerializeField] float finishPercent = 0.9f;

    //擦除点偏移量,距离上个擦除点>=该值时开始新的擦除点
    [SerializeField] float drawOffset = 10f;

    //是否以擦除完成
    bool isFinish;

    //要擦除的图片
    RawImage eraseImage;
    Texture2D eraseTexture;
    //图片长宽
    int textureWidth;
    int textureHeight;

    //图片大小
    float textureLength;
    //擦除部分图片大小
    float eraseLength;

    Camera mainCamera;

    void Awake()
    {
        eraseImage = GetComponent();
        mainCamera = Camera.main;
    }

    void Start()
    {
        Init();
    }

    void Init()
    {
        isFinish = false;
        eraseLength = 0;

        //原擦除图片
        //Texture2D originalTexture = (Texture2D)eraseImage.mainTexture;
        Texture2D originalTexture = (Texture2D)eraseImage.texture;

        //被擦除的图片，展示擦除过程
        eraseTexture = new Texture2D(originalTexture.width, originalTexture.height, TextureFormat.ARGB32, false);
        textureWidth = eraseTexture.width;
        textureHeight = eraseTexture.height;
        eraseTexture.SetPixels(originalTexture.GetPixels());
        eraseTexture.Apply();

        Debug.Log(textureWidth + " - " + textureHeight);

        eraseImage.texture = eraseTexture;

        textureLength = eraseTexture.GetPixels().Length;
    }

    #region Pointer Event

    public void OnPointerDown(PointerEventData eventData)
    {
        if (isFinish)
            return;

        tempLastPoint = eventData.position;
        ErasePoint(eventData.position);
    }

    Vector2 tempEventPoint;
    Vector2 tempLastPoint;
    public void OnDrag(PointerEventData eventData)
    {
        if (isFinish)
            return;

        tempEventPoint = eventData.position;

        //距离上个擦除点 >= 该值时开始新的擦除点
        if ((tempEventPoint - tempLastPoint).sqrMagnitude < drawOffset * drawOffset)
            return;

        //擦除点
        ErasePoint(tempEventPoint);
        //记录点
        tempLastPoint = tempEventPoint;
    }

    public void OnPointerUp(PointerEventData eventData)
    {
        if (isFinish)
            return;

        ErasePoint(eventData.position);
    }

    #endregion

    Vector3 tempWorldPoint;
    Vector3 tempLocalPoint;
    Vector2Int pixelPos;
    void ErasePoint(Vector2 screenPos)
    {
        //点击位置坐标转换，正交相机无效，试着改成平行相机试一下
        tempWorldPoint = mainCamera.ScreenToWorldPoint(screenPos);
        tempLocalPoint = transform.InverseTransformPoint(tempWorldPoint);

        //相对图片像素点坐标
        pixelPos.x = (int)tempLocalPoint.x + textureWidth / 2;
        pixelPos.y = (int)tempLocalPoint.y + textureHeight / 2;

        //点击位置是否在图片范围内
        if (pixelPos.x < 0 || pixelPos.x >= textureWidth || pixelPos.y < 0 || pixelPos.y >= textureHeight)
            return;

        //遍历笔刷长宽范围内像素点
        for (int i = -brushRadius; i <= brushRadius; i++)
        {
            //超左/右边界
            if (pixelPos.x + i < 0 || pixelPos.x + i >= textureWidth)
                continue;

            for (int j = -brushRadius; j <= brushRadius; j++)
            {
                //超上/下边界
                if (pixelPos.y + j < 0 || pixelPos.y + j >= textureHeight)
                    continue;

                //是否在圆形范围内
                if (Mathf.Pow(i, 2) + Mathf.Pow(j, 2) > Mathf.Pow(brushRadius, 2))
                    continue;

                //像素点色值
                Color color = eraseTexture.GetPixel(pixelPos.x + i, pixelPos.y + j);

                //判断透明度,是否已擦除
                if (Mathf.Approximately(color.a, 0))
                    continue;

                //修改像素点透明度
                color.a = 0;
                eraseTexture.SetPixel(pixelPos.x + i, pixelPos.y + j, color);

                //擦除数量统计
                eraseLength++;
            }
        }
        eraseTexture.Apply();

        //判断擦除进度
        RefreshErasePercent();
    }

    float tempPercent;
    void RefreshErasePercent()
    {
        if (isFinish)
            return;

        tempPercent = eraseLength / textureLength;

        tempPercent = (float)Math.Round(tempPercent, 2);

        Debug.Log("擦除百分比 : " + tempPercent);

        if (tempPercent >= finishPercent)
        {
            isFinish = true;

            eraseImage.enabled = false;

            //触发结束事件
            if (eraseFinishEvent != null)
                eraseFinishEvent.Invoke();
        }
    }

}