獲取unity prefab的預覽圖像

官方的api可以直接獲取預覽圖像，如下所示：

Tex=AssetPreview.GetAssetPreview(Object m)as Texture;

但是如果prefab是組合體的話（即一個prefab下包含2個或多個子物體），便只返回null。可以算是unity的一個bug吧。

所以需要自己寫函數來實現prefab圖像預覽。

思路是將Prefab實例化到場景，並利用RenderTexture進行攝像機截取圖像作為預覽圖像，代碼如下：

        /// <summary>
        /// 獲取預覽圖象
        /// </summary>
        /// <param name="obj"></param>
        /// <returns></returns>
        private Texture GetAssetPreview(GameObject obj)
        {
            GameObject clone = GameObject.Instantiate(obj);
            Transform cloneTransform = clone.transform;
            cloneTransform.position = new Vector3(-1000, -1000, -1000);
            //cloneTransform.localRotation = new Quaternion(0, 0, 0, 1);

            Transform[] all = clone.GetComponentsInChildren<Transform>();
            foreach (Transform trans in all)
            {
                trans.gameObject.layer = 21;
            }

            Bounds bounds = GetBounds(clone);
            Vector3 Min = bounds.min;
            Vector3 Max = bounds.max;
            GameObject cameraObj = new GameObject("render camera");
            cameraObj.transform.position = new Vector3(cloneTransform.position.x, (Max.y + Min.y) / 2f, Max.z + (Max.z - Min.z));

            Vector3 center = new Vector3(cloneTransform.position.x, (Max.y + Min.y) / 2f, cloneTransform.position.z);

            cameraObj.transform.LookAt(center);

            Camera renderCamera = cameraObj.AddComponent<Camera>();
            renderCamera.backgroundColor = new Color(0.8f, 0.8f, 0.8f, 1f);
            renderCamera.clearFlags = CameraClearFlags.Color;
            renderCamera.cameraType = CameraType.Preview;
            renderCamera.cullingMask = 1 << 21;
            int angle = (int)(Mathf.Atan2((Max.y - Min.y) / 2, (Max.z - Min.z)) * 180 / 3.1415f * 2);
            renderCamera.fieldOfView = angle;

            RenderTexture texture = new RenderTexture(64, 64, 0, RenderTextureFormat.Default);
            renderCamera.targetTexture = texture;

            renderCamera.RenderDontRestore();

            RenderTexture tex = new RenderTexture(64, 64, 0, RenderTextureFormat.Default);
            Graphics.Blit(texture, tex);

            Object.DestroyImmediate(clone);
            Object.DestroyImmediate(cameraObj);

            return tex;
        }
        /// <summary>
        /// 獲得某物體的bounds
        /// </summary>
        /// <param name="obj"></param>
        private Bounds GetBounds(GameObject obj)
        {
            Vector3 Min = new Vector3(99999, 99999, 99999);
            Vector3 Max = new Vector3(-99999, -99999, -99999);
            MeshRenderer[] renders = obj.GetComponentsInChildren<MeshRenderer>();
            for (int i = 0; i < renders.Length; i++)
            {
                if (renders[i].bounds.min.x < Min.x)
                    Min.x = renders[i].bounds.min.x;
                if (renders[i].bounds.min.y < Min.y)
                    Min.y = renders[i].bounds.min.y;
                if (renders[i].bounds.min.z < Min.z)
                    Min.z = renders[i].bounds.min.z;

                if (renders[i].bounds.max.x > Max.x)
                    Max.x = renders[i].bounds.max.x;
                if (renders[i].bounds.max.y > Max.y)
                    Max.y = renders[i].bounds.max.y;
                if (renders[i].bounds.max.z > Max.z)
                    Max.z = renders[i].bounds.max.z;
            }

            Vector3 center = (Min + Max) / 2;
            Vector3 size = new Vector3(Max.x - Min.x, Max.y - Min.y, Max.z - Min.z);
            return new Bounds(center, size);
        }

 后補：unity在2017.2版本已經修復了這個問題，可以放心用Tex=AssetPreview.GetAssetPreview(Object m)as Texture了。

感謝博友@跳出定向思維的發現，之前的unity舊版本也可以通過給prefab添加標簽來解決問題，添加了任意標簽后就可以使用自帶的獲取預覽api了。