MapObject地图显示速度优化

MapObjects2.ImageLayer.OpenCatalog方法可以构建一个图片层，然后添加到AxMapObjects2.AxMap地图组件中，以实现采用位图的方式显示地图背景，达到地图更加美观和信息全的效果。

在比例尺相对较大时，如果把当前比例尺所有瓦片地图全部一次性加载到图层中时，速度就成了一个头疼的问题

在这里，采用加载当前窗口大小区域的图片来达到速度的优化

代码如下

 /// <summary>
        /// 添加图片层
        /// </summary>
        /// <param name="tableName"></param>
        /// <param name="layerName"></param>
        public void AddImageLayer(string tableName, string layerName)
        {
            double left = _map.Extent.Left;
            double rigth = _map.Extent.Right;
            double top = _map.Extent.Top;
            double bottom = _map.Extent.Bottom;
            double width = 0;
            double height = 0;

            MapObjects2.Point pt = _map.ToMapPoint(map_center_x, map_center_y);
            switch (level)
            { 
                case 1:
                    width = pic_width * res1;
                    height = pic_height * res1;
                    break;
                case 2:
                    width = pic_width * res2;
                    height = pic_height * res2;
                    break;
                case 3:
                    width = pic_width * res3;
                    height = pic_height * res3;
                    break;
                case 4:
                    width = pic_width * res4;
                    height = pic_height * res4;
                    break;
                case 5:
                    width = pic_width * res5;
                    height = pic_height * res5;
                    break;
            }
            string connStr = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + System.Windows.Forms.Application.StartupPath + "//map.mdb;Persist Security Info=False";
            
            ADODB.Connection conn = new ADODB.ConnectionClass();
            conn.ConnectionString = connStr;
            conn.Open(connStr, "", "", 0);
            ADODB.Command comm = new ADODB.CommandClass();
            comm.ActiveConnection = conn;
            string sql = "";
            if (level != 1 && !isFromResource)
            {
                sql = "select * from " + tableName;
                sql += " where xmin>=" + (left - width);
                sql += " and xmax<=" + (rigth + width);
                sql += " and ymin>=" + (bottom - height);
                sql += " and ymax<=" + (top + height);
            }
            else
            {
                sql = "select * from " + tableName;
            }
            comm.CommandText = sql;
            MapObjects2.Table t = new MapObjects2.TableClass();
            
            t.Command = comm;
            
            MapObjects2.ImageLayer imgLyr = new MapObjects2.ImageLayerClass();
            
            try
            {
                imgLyr.OpenCatalog(t, @"", MapObjects2.ImageCatalogConstants.moOpenImagesInDisplay);
            }
            catch (Exception ex)
            {
            }
            _map.Layers.Add(imgLyr);
        }

代码中res1-res5代表一个像素显示的经度，也就是各个比例尺下经度的不同表示，pic_width和pic_height表示瓦片地图的图像宽度和高度。_map就是AxMapObjects2.AxMap组件。

值得注意的是当第一次加载图片时，_map.Extend因为还没有根据加载的图片进行初始化，所以第一次加载时采用全部加载图片，因为当比例尺小图片的数量比较少，对速度没有影响。

如果有任何问题或者建议，欢迎讨论