学习WorldWind有很长时间了，理论学习算是基本完成了。我体会是WW的学习主要分为两大步：WW框架体系学习和WW插件学习。学习WW插件逐步深入后，必然要首先学习Direct3D编程，这也算是我的经验之谈吧。今天Virtual Earth插件学习完成，也标志着我可以从WW理论转向WW实践啦。虽然我总结介绍的是Virtual Earth插件，但是希望网友阅读下面的内容前，最好能够先深入学习Direct3D编程、BMNG和Globe Icon插件的底层渲染，这些都是学习Virtual Earth的基础。
Virtual Earth插件包括以下几个类：
VirtualEarthForm 窗体类
VirtualEarthPlugin插件类，继承自Plugin（重要）
VeReprojectTilesLayer 渲染对象类，继承自Renderable Object（重要）
VeTile 瓦片对象类（真正实现大部分功能的）（重要）
Projection投影变换类（重要）
Search类
PushPin类
我们先看看VirtualEarthPlugin，所有的插件类必须继承自Plugin.cs，必须重写Load()和Unload()方法。这两个方法分别实现插件的加载和卸载。
Load()方法一般是实现添加菜单和添加工具按钮，跟前面介绍插件都很类似的，自己开发插件时模仿着这套路写就行。
public override void Load() { try { //判断当前World为Earth(注：其他星体可通过名字判断) if (ParentApplication.WorldWindow.CurrentWorld.IsEarth) { //初始化VE插件控制窗体 m_Form = new VirtualEarthForm(ParentApplication); m_Form.Owner = ParentApplication; //添加VE插件菜单 m_MenuItem = new MenuItem( " MicroSoft VirtualEarth " ); m_MenuItem.Click += new EventHandler(menuItemClicked); ParentApplication.PluginsMenu.MenuItems.Add(m_MenuItem); // #if DEBUG string imgPath = Path.GetDirectoryName(System.Windows.Forms.Application.ExecutablePath) + " \\Plugins\\VirtualEarth\\VirtualEarthPlugin.png " ; // #else // _pluginDir = this.PluginDirectory; // string imgPath = this.PluginDirectory + @"\VirtualEarthPlugin.png"; // #endif if (File.Exists(imgPath) == false ) { Utility.Log.Write( new Exception( " imgPath not found " + imgPath)); } //添加工具按钮，会出现工具栏里 m_ToolbarItem = new WorldWind.WindowsControlMenuButton( " MicroSoft VirtualEarth " , imgPath, m_Form); ParentApplication.WorldWindow.MenuBar.AddToolsMenuButton(m_ToolbarItem); base .Load(); } } catch (Exception ex) { Utility.Log.Write(ex); throw ; } }Unload()方法就是释放插件窗体，并移除插件菜单项和插件工具按钮。
我们再来看看VeReprojectTilesLayer 类，像其他插件类一样，要重载Initialize()、Update()、Render()方法，但是，VE插件重点在Upadate()方法,当然他还有其他自己特色的方法。
Initialize()方法主要是一些该类全局对象的实例化，如：投影、VE控制窗体及VeTile初始化等。
Render()方法主要是实现插件的三维渲染功能的，但是VE的Render真正实现是调用VeTile类中的Render()方法。
VE插件渲染代码 /// <summary> /// Draws the layer /// </summary> public override void Render(DrawArgs drawArgs) { try { if ( this .isOn == false ) { return ; } if ( this .isInitialized == false ) { return ; } if (drawArgs.device == null ) return ; if (veTiles != null && veTiles.Count > 0 ) { // render mesh and tile(s) bool disableZBuffer = false ; // TODO where do i get this setting // foreach(VeTile veTile in veTiles) // { // veTile.Render(drawArgs, disableZBuffer); // } // camera jitter fix drawArgs.device.Transform.World = Matrix.Translation( ( float ) - drawArgs.WorldCamera.ReferenceCenter.X, ( float ) - drawArgs.WorldCamera.ReferenceCenter.Y, ( float ) - drawArgs.WorldCamera.ReferenceCenter.Z ); // Clear ZBuffer between layers (as in WW) drawArgs.device.Clear(ClearFlags.ZBuffer, 0 , 1.0f , 0 ); // Render tiles（ 这里的GetZoomLevelByTrueViewRange方法是个知识点，该方法是根据WorldCamera视角范围，求取当前球体的缩放层次） int zoomLevel = GetZoomLevelByTrueViewRange (drawArgs.WorldCamera.TrueViewRange.Degrees); //真正的渲染处理是由VeTile类的Render()方法实现的。 int tileDrawn = VeTile.Render(drawArgs, disableZBuffer, veTiles, zoomLevel); // Try current level first if (tileDrawn == 0 ) VeTile.Render(drawArgs, disableZBuffer, veTiles, prevLvl); // If nothing drawn, render previous level // camera jitter fix drawArgs.device.Transform.World = drawArgs.WorldCamera.WorldMatrix; // Render logo RenderDownloadProgress(drawArgs, null , 0 ); } // else pushpins only // render PushPins if (pushPins != null && pushPins.Count > 0 ) { RenderPushPins(drawArgs); } } catch (Exception ex) { Utility.Log.Write(ex); } }
VE中获取缩放级别的方法是很巧妙的，根据原作者文章知：VE的缩放级别是1-19级甚至更低，是以2的阶乘递减的。方法我们自己开发时可以重用，但是它的思想还是要好好体会的。
注：缩放级别为 180 、90、45、22.5、……
根据视角范围获取缩放级别 public int GetZoomLevelByTrueViewRange( double trueViewRange) { int maxLevel = 3 ; //视角范围为45度 int minLevel = 19 ; int numLevels = minLevel - maxLevel + 1 ; int retLevel = maxLevel; for ( int i = 0 ; i < numLevels; i ++ ) { retLevel = i + maxLevel; double viewAngle = 180 ; for ( int j = 0 ; j < i; j ++ ) { viewAngle = viewAngle / 2.0 ; } if (trueViewRange >= viewAngle) { break ; } } return retLevel; }
VE插件最最关键的方法为Update(),因为VE实质上就是不断地根据缩放级别更新影像数据，其实就是构建要渲染绘制的对象集合，最后由Render()方法完成渲染绘制。所以该方法是VE中最复杂的，当然也就是VE的处理精华所在，也就是我们研究学习的重点啦。（说这些，主要是告诉大家这里是重点，要好好研究和关注）
Update()代码 /// <summary> /// Update layer (called from worker thread) /// </summary> public override void Update(DrawArgs drawArgs) { try { if ( this .isOn == false ) { return ; } // NOTE for some reason Initialize is not getting called from the Plugin Menu Load/Unload // it does get called when the plugin loads from Startup // not sure what is going on, so i'll just call it manually if ( this .isInitialized == false ) { this .Initialize(drawArgs); return ; } // get lat, lon 获取经纬度、倾斜角度、高度等 double lat = drawArgs.WorldCamera.Latitude.Degrees; double lon = drawArgs.WorldCamera.Longitude.Degrees; double tilt = drawArgs.WorldCamera.Tilt.Degrees; // determine zoom level double alt = drawArgs.WorldCamera.Altitude; // could go off distance, but this changes when view angle changes // Angle fov = drawArgs.WorldCamera.Fov; // stays at 45 degress // Angle viewRange = drawArgs.WorldCamera.ViewRange; // off of distance, same as TVR but changes when view angle changes // 获取当前视角范围 Angle tvr = drawArgs.WorldCamera.TrueViewRange; // off of altitude // smallest altitude = 100m // tvr = .00179663198575926 // start altitude = 12756273m // tvr = 180 // WW _levelZeroTileSizeDegrees 获取缩放级别，上面已经介绍啦 // 180 90 45 22.5 11.25 5.625 2.8125 1.40625 .703125 .3515625 .17578125 .087890625 0.0439453125 0.02197265625 0.010986328125 0.0054931640625 int zoomLevel = GetZoomLevelByTrueViewRange(tvr.Degrees); //只要到一定缩放级别时，才启用VE（这里我们可以学习，来控制一些图层的显示） // dont start VE tiles until a certain zoom level if (zoomLevel < veForm.StartZoomLevel) { this .RemoveAllTiles(); this .ForceRefresh(); return ; } // WW tiles // double tileDegrees = GetLevelDegrees(zoomLevel); // int row = MathEngine.GetRowFromLatitude(lat, tileDegrees); // int col = MathEngine.GetColFromLongitude(lon, tileDegrees); // VE tiles double metersY; double yMeters; int yMetersPerPixel; int row; /* //WRONG - doesn't stay centered away from equator //int yMeters = LatitudeToYAtZoom(lat, zoomLevel); //1024 double sinLat = Math.Sin(DegToRad(lat)); metersY = earthRadius / 2 * Math.Log((1 + sinLat) / (1 - sinLat)); //0 yMeters = earthHalfCirc - metersY; //20037508.342789244 yMetersPerPixel = (int) Math.Round(yMeters / MetersPerPixel(zoomLevel)); row = yMetersPerPixel / pixelsPerTile; */ // CORRECT // int xMeters = LongitudeToXAtZoom(lon, zoomLevel); // 1024 //计算弧长，（earthRadius 地球半径，DegToRad(lon)经度的度值转弧度值） //DegToRad()为角度转弧度的方法，很简单。弧长=半径*弧度值。（这里球体的弧度值认为有负值） double metersX = earthRadius * DegToRad(lon); // 0 //因为从西到东算列数，所以要从-180度算列所在的弧度的总长度的 double xMeters = earthHalfCirc + metersX; // 20037508.342789244 //获取总的像素数。MetersPerPixel(zoomLevel))是计算每像素代表的米数。（知识点） int xMetersPerPixel = ( int )Math.Round(xMeters / MetersPerPixel(zoomLevel)); //获取列数，从西-》东（-180-》180）来算列数 int col = xMetersPerPixel / pixelsPerTile; // reproject - overrides row above // this correctly keeps me on the current tile that is being viewed //使用UV结构体来存放经纬度，然后调用proj.Forward(uvCurrent)实现投影变换 UV uvCurrent = new UV(DegToRad(lon), DegToRad(lat)); uvCurrent = proj.Forward(uvCurrent); metersY = uvCurrent.V; //这里为啥是“—”？？看过该插件原作者的文章才会知道：原来VE的行数是从北向南算的，而WW是从南向北计算行数的。 yMeters = earthHalfCirc - metersY; //获取总像素数 yMetersPerPixel = ( int )Math.Round(yMeters / MetersPerPixel(zoomLevel)); //获取行数 row = yMetersPerPixel / pixelsPerTile; 说明：计算行列数，是为了后面获取切片后图片，并将正确的图片作为纹理渲染到正确的位置上，当然这过程还有很多处理，我会一一分析的。 // update mesh if VertEx changes if (prevVe != World.Settings.VerticalExaggeration) { lock (veTiles.SyncRoot) { VeTile veTile; for ( int i = 0 ; i < veTiles.Count; i ++ ) { veTile = (VeTile)veTiles[i]; if (veTile.VertEx != World.Settings.VerticalExaggeration) { //创建网格MESH(稍后重点分析) veTile.CreateMesh( this .Opacity, World.Settings.VerticalExaggeration); } } } } prevVe = World.Settings.VerticalExaggeration; // if within previous bounds and same zoom level, then exit if (row == prevRow && col == prevCol && zoomLevel == prevLvl && tilt == preTilt) { return ; } // System.Diagnostics.Debug.WriteLine("CHANGE"); lock (veTiles.SyncRoot) { VeTile veTile; //使之前存放的veTile，标记为“暂时不需要”，这里没有删除 for ( int i = 0 ; i < veTiles.Count; i ++ ) { veTile = (VeTile)veTiles[i]; veTile.IsNeeded = false ; } } // metadata ArrayList alMetadata = null ; if (veForm.IsDebug == true ) { alMetadata = new ArrayList(); alMetadata.Add( " yMeters " + yMeters.ToString()); alMetadata.Add( " metersY " + metersY.ToString()); alMetadata.Add( " yMeters2 " + yMeters.ToString()); alMetadata.Add( " vLat " + uvCurrent.V.ToString()); // alMetadata.Add("xMeters " + xMeters.ToString()); // alMetadata.Add("metersX " + metersX.ToString()); // alMetadata.Add("uLon " + uvCurrent.U.ToString()); } //添加目前VeTile（这是重点，稍后分析） // add current tiles first AddVeTile(drawArgs, row, col, zoomLevel, alMetadata); // then add other tiles outwards in surrounding circles //添加周边的VeTile AddNeighborTiles(drawArgs, row, col, zoomLevel, null , 1 ); AddNeighborTiles(drawArgs, row, col, zoomLevel, null , 2 ); AddNeighborTiles(drawArgs, row, col, zoomLevel, null , 3 ); // Extend tile grid if camera tilt above some values //根据倾斜角度决定是否继续添加相邻Tile单位 if (tilt > 45 ) AddNeighborTiles(drawArgs, row, col, zoomLevel, null , 4 ); if (tilt > 60 ) AddNeighborTiles(drawArgs, row, col, zoomLevel, null , 5 ); // if(prevLvl > zoomLevel) // zooming out // { // } lock (veTiles.SyncRoot) { VeTile veTile; //移除不需要的veTile图片，为啥是现在移除？？（思考一下有啥好处） for ( int i = 0 ; i < veTiles.Count; i ++ ) { veTile = (VeTile)veTiles[i]; if (veTile.IsNeeded == false ) { veTile.Dispose(); veTiles.RemoveAt(i); i -- ; } } } //保存当前基本的行列、缩放级别、倾斜角度 prevRow = row; prevCol = col; prevLvl = zoomLevel; preTilt = tilt; } catch (Exception ex) { Utility.Log.Write(ex); } }
我们来看看AddVeTile(drawArgs, row, col, zoomLevel, alMetadata);方法，因为该方法是AddNeighborTiles(drawArgs, row, col, zoomLevel, null, 1);方法的基础。
AddVeTile()方法代码
private void AddVeTile(DrawArgs drawArgs, int row, int col, int zoomLevel, ArrayList alMetadata) { //TODO handle column wrap-around //haven't had to explicitly handle this yet bool tileFound = false; lock (veTiles.SyncRoot) { foreach (VeTile veTile in veTiles) { if (veTile.IsNeeded == true) { continue; }
//判断当前的veTile对象，是否是当前缩放级别下的行列对应的“瓦片”，这里涉及到VeTile类的IsEqual（）方法 if (veTile.IsEqual(row, col, zoomLevel) == true) { veTile.IsNeeded = true; tileFound = true; break; } } } if (tileFound == false) { //exit if zoom level has changed
//获取当前新的缩放级别 int curZoomLevel = GetZoomLevelByTrueViewRange(drawArgs.WorldCamera.TrueViewRange.Degrees); if (curZoomLevel != zoomLevel) { return; }
//之前没下载过，需要创建新的VeTile对象。CreateVeTile方法很重要 VeTile newVeTile = CreateVeTile(drawArgs, row, col, zoomLevel, alMetadata); newVeTile.IsNeeded = true; lock (veTiles.SyncRoot) { veTiles.Add(newVeTile); } } }
CreateVeTile()方法实现，相当底层啦！我自己研究了很长时间，希望能给大家说明白。即使不明白，知道是干啥知道咋用就行啦，不用深究原理的。
private VeTile CreateVeTile(DrawArgs drawArgs, int row, int col, int zoomLevel, ArrayList alMetadata) {
//初始化VeTile对象 VeTile newVeTile = new VeTile(row, col, zoomLevel); //metadata if (alMetadata != null) { foreach (string metadata in alMetadata) { newVeTile.AddMetaData(metadata); } } //获取纹理图片，即从服务器端下载图片用作纹理（重点，稍后详细分析） //thread to download new tile(s) or just load from cache newVeTile.GetTexture(drawArgs, pixelsPerTile); //handle the diff projection
//每个像素代表的距离 double metersPerPixel = MetersPerPixel(zoomLevel);
//获取当前级别总的网格边数 double totalTilesPerEdge = Math.Pow(2, zoomLevel);
//总长度（我认为：就是地球周长） double totalMeters = totalTilesPerEdge * pixelsPerTile * metersPerPixel;
//（我认为：halfMeters就是地球周长的一半） double halfMeters = totalMeters / 2; //do meters calculation in VE space //the 0,0 origin for VE is in upper left
//在VE空间坐标系下，计算距离，原点在左上角
//首先求出WW坐标系下的，N、W（这里就是上面求行列数的逆运算啦） double N = row * (pixelsPerTile * metersPerPixel); double W = col * (pixelsPerTile * metersPerPixel); //now convert it to +/- meter coordinates for Proj.4 //the 0,0 origin for Proj.4 is 0 lat, 0 lon //-22 to 22 million, -11 to 11 million
//在以经纬度为0 0为原点的坐标系下的新的N W,坐标系方向轴为 N——》S; W——》E N = halfMeters - N; W = W - halfMeters;
//计算出单元格的 E S double E = W + (pixelsPerTile * metersPerPixel); double S = N - (pixelsPerTile * metersPerPixel);
//给新的瓦片单元格的UL UR LL LR赋值，其实就是单元网格的四角的坐标（理解这点很重要，是为了创建Mesh用） newVeTile.UL = new UV(W, N); newVeTile.UR = new UV(E, N); newVeTile.LL = new UV(W, S); newVeTile.LR = new UV(E, S); //create mesh byte opacity = this.Opacity; //from RenderableObject float verticalExaggeration = World.Settings.VerticalExaggeration;
//重点CreateMesh（）方法，里面也涉及数学，不是太容易理解的 newVeTile.CreateMesh(opacity, verticalExaggeration); newVeTile.CreateDownloadRectangle(drawArgs, World.Settings.DownloadProgressColor.ToArgb()); return newVeTile; }
VeTile类，是真正实现渲染方法，也主要用于球体和平面坐标的转换处理、计算，算是VE插件的核心啦，这里面涉及大量的数学知识，主要编程为Direct3D编程。我们现在来看看CreateMesh（）方法。
注：如果你看懂CreateMesh(),就说明你已经理解VE大部分啦
// NOTE this is a mix from Mashi's Reproject and WW for terrain public void CreateMesh( byte opacity, float verticalExaggeration) { this .vertEx = verticalExaggeration; int opacityColor = System.Drawing.Color.FromArgb(opacity, 0 , 0 , 0 ).ToArgb(); meshPointCount = 32 ; // 64; // 96 // How many vertices for each direction in mesh (total: n^2) // vertices = new CustomVertex.PositionColoredTextured[meshPointCount * meshPointCount]; // Build mesh with one extra row and col around the terrain for normal computation and struts vertices = new CustomVertex.PositionNormalTextured[(meshPointCount + 2 ) * (meshPointCount + 2 )]; int upperBound = meshPointCount - 1 ; float scaleFactor = ( float ) 1 / upperBound; // using(Projection proj = new Projection(m_projectionParameters)) // { double uStep = (UR.U - UL.U) / upperBound; double vStep = (UL.V - LL.V) / upperBound; UV curUnprojected = new UV(UL.U - uStep, UL.V + vStep); // 将平面坐标投影转换为WW球面坐标 // figure out latrange (for terrain detail) UV geoUL = _proj.Inverse(m_ul); UV geoLR = _proj.Inverse(m_lr); double latRange = (geoUL.U - geoLR.U) * 180 / Math.PI; // 将弧度转为角度（这里没再使用下面四个变量，原方法用到啦，不研究！） North = geoUL.V * 180 / Math.PI; South = geoLR.V * 180 / Math.PI; West = geoUL.U * 180 / Math.PI; East = geoLR.U * 180 / Math.PI; // 半径 float meshBaseRadius = ( float )_layerRadius; UV geo; Vector3 pos; double height = 0 ; for ( int i = 0 ; i < meshPointCount + 2 ; i ++ ) { for ( int j = 0 ; j < meshPointCount + 2 ; j ++ ) { // 将平面坐标（proj 4）投影转换为WW球面坐标 geo = _proj.Inverse(curUnprojected); // 将弧度转为角度 // Radians -> Degrees geo.U *= 180 / Math.PI; geo.V *= 180 / Math.PI; if (_terrainAccessor != null ) { if (_veForm.IsTerrainOn == true ) { // height = heightData[i, j] * verticalExaggeration; // original : need to fetch altitude on a per vertex basis (in VE space) to have matching tile borders (note PM) height = verticalExaggeration * _terrainAccessor.GetElevationAt(geo.V, geo.U, Math.Abs(upperBound / latRange)); } else { height = 0 ; } } // 将空间坐标转换为笛卡尔坐标 pos = MathEngine.SphericalToCartesian( geo.V, geo.U, _layerRadius + height); // 构建Mesh顶点集合 int idx = i * (meshPointCount + 2 ) + j; vertices[idx].X = pos.X; vertices[idx].Y = pos.Y; vertices[idx].Z = pos.Z; // double sinLat = Math.Sin(geo.V); // vertices[idx].Z = (float) (pos.Z * sinLat); vertices[idx].Tu = (j - 1 ) * scaleFactor; vertices[idx].Tv = (i - 1 ) * scaleFactor; // vertices[idx].Color = opacityColor; curUnprojected.U += uStep; } curUnprojected.U = UL.U - uStep; curUnprojected.V -= vStep; } // } // 构建索引集合，分组存放Mesh点集合中的点。学Direct3D编程，看懂下面的很容易的 int slices = meshPointCount + 1 ; indices = new short [ 2 * slices * slices * 3 ]; for ( int i = 0 ; i < slices; i ++ ) { for ( int j = 0 ; j < slices; j ++ ) { indices[( 2 * 3 * i * slices) + 6 * j] = ( short )(i * (meshPointCount + 2 ) + j); indices[( 2 * 3 * i * slices) + 6 * j + 1 ] = ( short )((i + 1 ) * (meshPointCount + 2 ) + j); indices[( 2 * 3 * i * slices) + 6 * j + 2 ] = ( short )(i * (meshPointCount + 2 ) + j + 1 ); indices[( 2 * 3 * i * slices) + 6 * j + 3 ] = ( short )(i * (meshPointCount + 2 ) + j + 1 ); indices[( 2 * 3 * i * slices) + 6 * j + 4 ] = ( short )((i + 1 ) * (meshPointCount + 2 ) + j); indices[( 2 * 3 * i * slices) + 6 * j + 5 ] = ( short )((i + 1 ) * (meshPointCount + 2 ) + j + 1 ); } } // Compute normals and fold struts calculate_normals(); fold_struts( false , meshBaseRadius); }
（未完待续……）
转载于:https://www.cnblogs.com/wuhenke/archive/2009/12/24/1631739.html
相关资源：数据结构—成绩单生成器