先上代码

using System.IO;
using System.Collections;
using UnityEngine;
using UnityEngine.UI;
using System;

public class Test3 : MonoBehaviour
{
    
       

    public RawImage image;
    public Texture2D te;
    public Texture2D texture;
    void Update()
    {
    
    
        if (Input.GetKeyDown(KeyCode.A))
        {
    
    
            image.texture = AmendAtlas(new UnityEngine.Vector2(0,1536),new UnityEngine.Vector2(512,512),te,texture);
        }
    }
    /// <summary>
    /// 修改图集
    /// </summary>
    public Texture2D AmendAtlas(UnityEngine.Vector2 _修改图片的起始点, UnityEngine.Vector2 _修改图集的范围,Texture2D _图集,Texture2D _要添加的图片)
    {
    
    
        Color[] colors = _图集.GetPixels();
        Texture2D texture2D = new Texture2D(_图集.width, _图集.height);
        texture2D.SetPixels(colors);
        texture2D.SetPixels((int)_修改图片的起始点.x, (int)_修改图片的起始点.y, (int)_修改图集的范围.x, (int)_修改图集的范围.y, _要添加的图片.GetPixels());
        texture2D.Apply();

        return texture2D;
    }
   
    /// <summary>
    /// 图片压缩
    /// </summary>
    /// <param name="tex"></param>
    /// <param name="width"></param>
    /// <param name="height"></param>
    /// <returns></returns>
    public Texture2D ReSetTextureSize(Texture2D tex, int width, int height)
    {
    
    
        var rendTex = new RenderTexture(width, height, 24, RenderTextureFormat.ARGB32);
        rendTex.Create();
        Graphics.SetRenderTarget(rendTex);
        GL.PushMatrix();
        GL.Clear(true, true, Color.clear);
        GL.PopMatrix();
        var mat = new Material(Shader.Find("Unlit/Transparent"));
        mat.mainTexture = tex;
        Graphics.SetRenderTarget(rendTex);
        GL.PushMatrix();
        GL.LoadOrtho();
        mat.SetPass(0);
        GL.Begin(GL.QUADS);
        GL.TexCoord2(0, 0);
        GL.Vertex3(0, 0, 0);
        GL.TexCoord2(0, 1);
        GL.Vertex3(0, 1, 0);
        GL.TexCoord2(1, 1);
        GL.Vertex3(1, 1, 0);
        GL.TexCoord2(1, 0);
        GL.Vertex3(1, 0, 0);
        GL.End();
        GL.PopMatrix();
        var finalTex = new Texture2D(rendTex.width, rendTex.height, TextureFormat.ARGB32, false);
        RenderTexture.active = rendTex;
        finalTex.ReadPixels(new Rect(0, 0, finalTex.width, finalTex.height), 0, 0);
        finalTex.Apply();
        return finalTex;
    }
   

    public void writeCaptureDataToFile(Texture2D texture, string dataPath, string filename)
    {
    
    
        string path_full =Path.Combine(dataPath,filename+".jpg");
 
        // 存入jpg文件
        StartCoroutine(saveTexture2DtoFile(texture, path_full));
    }
 
    // 保存图片到指定目录
    private IEnumerator saveTexture2DtoFile(Texture2D texture, string path, Action<object> callback=null)
    {
    
    
        //等待渲染线程结束  
        yield return new WaitForEndOfFrame();
 
        byte[] textureData = texture.EncodeToJPG();
        System.IO.File.WriteAllBytes(path, textureData);
 
        callback?.Invoke(null);
        Debug.Log("图片文件写入完毕：" + path);
    }
}

这图集大小为 1024 * 2048
修改红色框住的部分 并填补
该区域大小为 512 * 512
起始点为该区域的左下角 位置为 （0，1536）

填入数据
然后运行代码
得到下面的结果

Over