官方论坛上面老外讨论的解答

生成CubeMap使用的是Camera内置的接口 Camera.RenderToCubemap

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using UnityEditor;
using UnityEngine;

public class CubeMapMaker : ScriptableWizard
{
    public Transform renderFromPosition=null;
    public Cubemap cubemap=null;

    void OnWizardUpdate()
    {
        //选择一个gameObject用于确定渲染起始位置，同时选择一个cubemap用于渲染
        helpString = "Select transform to render from and cubemap to render into";
        bool isValid = (renderFromPosition != null) && (cubemap != null);
    }
    void OnWizardCreate()
    {
        //创建用于渲染的临时相机
        GameObject gameObject = new GameObject("CubemapCamera");
        gameObject.AddComponent<Camera>();
        //将临时相机绑到目标物体上
        gameObject.transform.position = renderFromPosition.position;
        gameObject.transform.rotation = Quaternion.identity;
        //渲染为cubemap
        gameObject.GetComponent<Camera>().RenderToCubemap(cubemap);
        //删除临时相机
        DestroyImmediate(gameObject);
    }

    [MenuItem("GameObject/Render2Cubemap")]
    static void Render2Cubemap()
    {
        ScriptableWizard.DisplayWizard<CubeMapMaker>(
            "Render Cubemap", "Render"
        );
    }
}

导出Cuebmap是通过读取CubemapFace然后用Texture2D的功能导出为图片，但是由于一些问题，导出图片之前要先将Texture2D垂直和水平翻转

using UnityEngine;
using UnityEditor;
using System.Linq;
using System.IO;

public class SaveCubeMap2Png : ScriptableWizard
{
    public Cubemap cubemap = null;

    [MenuItem("GameObject/SaveCubeMap2Png")]
    private static void MenuEntryCall()
    {
        ScriptableWizard.DisplayWizard<SaveCubeMap2Png>("SaveCubeMap2Png", "Save");
    }

    public static void FlipPixels(Texture2D texture, bool flipX, bool flipY)
    {
        Color32[] originalPixels = texture.GetPixels32();

        var flippedPixels = Enumerable.Range(0, texture.width * texture.height).Select(index =>
         {
             int x = index % texture.width;
             int y = index / texture.width;
             if (flipX)
                 x = texture.width - 1 - x;

             if (flipY)
                 y = texture.height - 1 - y;

             return originalPixels[y * texture.width + x];
         }
        );

        texture.SetPixels32(flippedPixels.ToArray());
        texture.Apply();
    }

    private void OnWizardCreate()
    {
        int width = cubemap.width;
        int height = cubemap.height;
        Texture2D texture2D = new Texture2D(width, height, TextureFormat.RGB24, false);

        for (int i = 0; i < 6; i++)
        {
            texture2D.SetPixels(cubemap.GetPixels((CubemapFace)i));

            //翻转像素，由于某种原因，导出的图片需要进行翻转
            FlipPixels(texture2D, true, true);
            //此处导出为png
            File.WriteAllBytes(Application.dataPath + "/" + cubemap.name + "_" + ((CubemapFace)i).ToString() + ".png", texture2D.EncodeToPNG());
        }
        DestroyImmediate(texture2D);
    }
    private void OnWizardUpdate()
    {
        helpString = "Select cubemap to save to individual .png";
        if (Selection.activeObject is Cubemap && cubemap == null)
            cubemap = Selection.activeObject as Cubemap;
        isValid = (cubemap != null);
    }
}