Engine如何获取sde中镶嵌数据集的概视图overviews作为图片显示？

​            ILayer layer = axMapControl1.get_Layer(0);

            IMosaicLayer mLayer = layer as IMosaicLayer;

            IMosaicDataset mosaicDataset = mLayer.MosaicDataset;

            IFeatureClass raCatalog = mosaicDataset.Catalog;

            IFeature fea = raCatalog.GetFeature(4);

            IRasterValue rasValue = fea.get_Value(2) as IRasterValue;

            IRaster raster = rasValue.Raster;



            IRasterExporter raExporter = new RasterExporterClass();

            byte[] bytesArr;

            bytesArr = raExporter.ExportToBytes(raster, png);

            MemoryStream stream = new MemoryStream(bytesArr);



            BitmapImage bi = new BitmapImage();

            bi.BeginInit();

            bi.StreamSource = stream;

            bi.EndInit();

            pictureBox1.Image = Image.FromStream(stream);