視域調度（視域體裁剪）

　　在WW中用戶改變自己的的視角，紋理影像和高程會動態加載，在視野范圍內的影像和DEM顯示，超出視域范圍的瓦片則不顯示。不僅是瓦片，太陽、大氣網格、三維模型ModelFeature等都會相應的進行剔除。

　　看了ROAM相關的的視域體裁剪論文，有好多采用的是將視域體投影的簡化算法。WW是否也是這樣？我原來以為是，后來在群里和別人聊，說到了Frustum，才知道這個東西的作用。

　　這里涉及的類主要是World_Wind_1.4.0_Source\PluginSDK\ViewFrustum.cs文件中的Frustum類。

　　查看Frustum類，包含公有字段public Plane[] planes = new Plane[6];可以知道WW的視域裁剪采用的是六面體來實現的。

1 using System; 2 using Microsoft.DirectX; 3 using Microsoft.DirectX.Direct3D; 4 5 namespace WorldWind 6 { 7 /// <summary> 8 /// The region of space in the modeled world that may appear on the screen; it is the field of view of the notional camera. 9 /// Used to perform culling of invisible object (prior to rendering) to increase speed. 10 /// See: http://en.wikipedia.org/wiki/Viewing_frustum 11 /// </summary> 12 public class Frustum 13 { 14 public Plane[] planes = new Plane[6]; 15 16 public void Update(Matrix m) 17 { 18 //bottom (down) plane 19 this.planes[0] = new Plane( 20 m.M14 + m.M12, //a 21 m.M24 + m.M22, //b 22 m.M34 + m.M32, //c 23 m.M44 + m.M42 //d 24 ); 25 26 //far plane 27 this.planes[1] = new Plane( 28 m.M14 - m.M13, 29 m.M24 - m.M23, 30 m.M34 - m.M33, 31 m.M44 - m.M43 32 ); 33 34 //right side plane 35 this.planes[2] = new Plane( 36 m.M14 - m.M11, //a 37 m.M24 - m.M21, //b 38 m.M34 - m.M31, //c 39 m.M44 - m.M41 //d 40 ); 41 42 //left side plane 43 this.planes[3] = new Plane( 44 m.M14 + m.M11, //a 45 m.M24 + m.M21, //b 46 m.M34 + m.M31, //c 47 m.M44 + m.M41 //d 48 ); 49 50 //near plane 51 this.planes[4] = new Plane( 52 m.M13, 53 m.M23, 54 m.M33, 55 m.M43); 56 57 //top (up) plane 58 this.planes[5] = new Plane( 59 m.M14 - m.M12, //a 60 m.M24 - m.M22, //b 61 m.M34 - m.M32, //c 62 m.M44 - m.M42 //d 63 ); 64 65 foreach(Plane p in this.planes) 66 p.Normalize(); 67 } 68 69 /// <summary> 70 /// Test if a sphere intersects or is completely inside the frustum. 71 /// </summary> 72 /// <returns>true when the sphere intersects.</returns> 73 public bool Intersects(BoundingSphere c) 74 { 75 foreach(Plane p in this.planes) 76 { 77 float distancePlaneToPoint = p.A * c.Center.X + p.B * c.Center.Y + p.C * c.Center.Z + p.D; 78 if(distancePlaneToPoint < -c.Radius) 79 // More than 1 radius outside the plane = outside 80 return false; 81 } 82 83 //else it's in view 84 return true; 85 } 86 87 /// <summary> 88 /// Test if a point is inside the frustum. 89 /// </summary> 90 /// <returns>true when the point is inside.</returns> 91 /// <param name="v">XYZ in world coordinates of the point to test.</param> 92 public bool ContainsPoint(Vector3 v) 93 { 94 foreach(Plane p in this.planes) 95 if(Vector3.Dot(new Vector3(p.A, p.B, p.C), v) + p.D < 0) 96 return false; 97 98 return true; 99 } 100 101 /// <summary> 102 /// Tests if the view frustum fully contains the bounding box. 103 /// </summary> 104 /// <returns>true when the box is complete enclosed by the frustum.</returns> 105 public bool Contains(BoundingBox bb) 106 { 107 //Code taken from Flip Code Article: 108 // http://www.flipcode.com/articles/article_frustumculling.shtml 109 int iTotalIn = 0; 110 foreach(Plane p in this.planes) 111 { 112 int iInCount = 8; 113 int iPtIn = 1; 114 // TODO: Modify bounding box and only check 2 corners. 115 for(int i = 0; i < 8; i++) 116 { 117 if(Vector3.Dot(new Vector3(p.A,p.B,p.C), bb.corners[i]) + p.D < 0) 118 { 119 iPtIn = 0; 120 --iInCount; 121 } 122 } 123 124 if(iInCount == 0) 125 return false; 126 127 iTotalIn += iPtIn; 128 } 129 130 if(iTotalIn == 6) 131 return true; 132 133 return false; 134 } 135 136 /// <summary> 137 /// Tests if the bounding box specified intersects with or is fully contained in the frustum. 138 /// </summary> 139 /// <returns>true when the box intersects with the frustum.</returns> 140 public bool Intersects(BoundingBox bb) 141 { 142 foreach(Plane p in this.planes) 143 { 144 Vector3 v = new Vector3(p.A,p.B,p.C); 145 bool isInside = false; 146 // TODO: Modify bounding box and only check 2 corners. 147 for(int i = 0; i < 8; i++) 148 { 149 if(Vector3.Dot(v, bb.corners[i]) + p.D >= 0) 150 { 151 isInside = true; 152 break; 153 } 154 } 155 156 if(!isInside) 157 return false; 158 } 159 160 return true; 161 } 162 163 public override string ToString() 164 { 165 string res = string.Format("Near:\n{0}Far:\n{1}", planes[4], planes[1] ); 166 return res; 167 } 168 } 169 }

該類實現的方法主要包括下面5個，后面四個主要是進行相交檢測和包含檢測：

public void Update(Matrix m);

public bool Intersects(BoundingSphere c);

public bool ContainsPoint(Vector3 v);

public bool Contains(BoundingBox bb);

public bool Intersects(BoundingBox bb);

調用了Frustum對象的類其實不多，不過Frustum的重要程度和CameraBase捆綁在一起。

　　QuadTileSet類中的調用方式主要是通過調用camera.ViewFrustum來獲取并進行相關的判斷。在QuadTileSet中總共有3處地方可以查見camera.ViewFrustum，一個是在Update方法中，一個是在GetClosestDownloadRequest，還有一個是在ResetCacheForCurrentView方法中。

　　QuadTile類中調用方式通過調用camera.ViewFrustum實現，主要在在Update方法中和Render方法中。

轉載于:https://www.cnblogs.com/yhlx125/archive/2013/05/29/3103333.html

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