[WorldWind学习]17.视域调度（视域体裁剪）

视域调度（视域体裁剪）

　　在WW中用户改变自己的的视角，纹理影像和高程会动态加载，在视野范围内的影像和DEM显示，超出视域范围的瓦片则不显示。不仅是瓦片，太阳、大气网格、三维模型ModelFeature等都会相应的进行剔除。

　　看了ROAM相关的的视域体裁剪论文，有好多采用的是将视域体投影的简化算法。WW是否也是这样？我原来以为是，后来在群里和别人聊，说到了Frustum，才知道这个东西的作用。

　　这里涉及的类主要是World_Wind_1.4.0_Source\PluginSDK\ViewFrustum.cs文件中的Frustum类。

　　查看Frustum类，包含公有字段public Plane[] planes = new Plane[6];可以知道WW的视域裁剪采用的是六面体来实现的。

using System;
using Microsoft.DirectX;
using Microsoft.DirectX.Direct3D;

namespace WorldWind
{
    /// <summary>
    /// The region of space in the modeled world that may appear on the screen; it is the field of view of the notional camera.
    /// Used to perform culling of invisible object (prior to rendering) to increase speed.
    /// See: http://en.wikipedia.org/wiki/Viewing_frustum
    /// </summary>
    public class Frustum
    {
        public Plane[] planes = new Plane[6];

        public void Update(Matrix m)
        {
            //bottom (down) plane
            this.planes[0] = new Plane(
                m.M14 + m.M12, //a
                m.M24 + m.M22, //b
                m.M34 + m.M32, //c
                m.M44 + m.M42 //d
                );
            
            //far plane
            this.planes[1] = new Plane(
                m.M14 - m.M13,
                m.M24 - m.M23,
                m.M34 - m.M33,
                m.M44 - m.M43
                );

            //right side plane
            this.planes[2] = new Plane(
                m.M14 - m.M11, //a
                m.M24 - m.M21, //b
                m.M34 - m.M31, //c
                m.M44 - m.M41 //d
                );

            //left side plane
            this.planes[3] = new Plane(
                m.M14 + m.M11,    //a
                m.M24 + m.M21,    //b
                m.M34 + m.M31,    //c
                m.M44 + m.M41    //d
                );

            //near plane
            this.planes[4] = new Plane(
                m.M13,
                m.M23,
                m.M33,
                m.M43);

            //top (up) plane
            this.planes[5] = new Plane(
                m.M14 - m.M12, //a
                m.M24 - m.M22, //b
                m.M34 - m.M32, //c
                m.M44 - m.M42 //d
                );

            foreach(Plane p in this.planes)
                p.Normalize();
        }

        /// <summary>
        /// Test if a sphere intersects or is completely inside the frustum.
        /// </summary>
        /// <returns>true when the sphere intersects.</returns>
        public bool Intersects(BoundingSphere c)
        {
            foreach(Plane p in this.planes)
            {
                float distancePlaneToPoint = p.A * c.Center.X + p.B * c.Center.Y + p.C * c.Center.Z + p.D;
                if(distancePlaneToPoint < -c.Radius)
                    // More than 1 radius outside the plane = outside
                    return false;
            }

            //else it's in view
            return true;
        }

        /// <summary>
        /// Test if a point is inside the frustum.
        /// </summary>
        /// <returns>true when the point is inside.</returns>
        /// <param name="v">XYZ in world coordinates of the point to test.</param>
        public bool ContainsPoint(Vector3 v)
        {
            foreach(Plane p in this.planes)
                if(Vector3.Dot(new Vector3(p.A, p.B, p.C), v) + p.D < 0)
                    return false;

            return true;
        }

        /// <summary>
        /// Tests if the view frustum fully contains the bounding box.
        /// </summary>
        /// <returns>true when the box is complete enclosed by the frustum.</returns>
        public bool Contains(BoundingBox bb)
        {
            //Code taken from Flip Code Article:
            // http://www.flipcode.com/articles/article_frustumculling.shtml
            int iTotalIn = 0;
            foreach(Plane p in this.planes)
            {
                int iInCount = 8;
                int iPtIn = 1;
                // TODO: Modify bounding box and only check 2 corners.
                for(int i = 0; i < 8; i++)
                {
                    if(Vector3.Dot(new Vector3(p.A,p.B,p.C), bb.corners[i]) + p.D < 0)
                    {
                        iPtIn = 0;
                        --iInCount;
                    }
                }

                if(iInCount == 0)
                    return false;

                iTotalIn += iPtIn;
            }

            if(iTotalIn == 6)
                return true;

            return false;
        }

        /// <summary>
        /// Tests if the bounding box specified intersects with or is fully contained in the frustum.
        /// </summary>
        /// <returns>true when the box intersects with the frustum.</returns>
        public bool Intersects(BoundingBox bb)
        {
            foreach(Plane p in this.planes)
            {
                Vector3 v = new Vector3(p.A,p.B,p.C);
                bool isInside = false;
                // TODO: Modify bounding box and only check 2 corners.
                for(int i = 0; i < 8; i++)
                {
                    if(Vector3.Dot(v, bb.corners[i]) + p.D >= 0)
                    {
                        isInside = true;
                        break;
                    }
                }

                if(!isInside)
                    return false;
            }

            return true;
        }
    
        public override string ToString()
        {
            string res = string.Format("Near:\n{0}Far:\n{1}", planes[4], planes[1] );
            return res;
        }
    }
}

该类实现的方法主要包括下面5个，后面四个主要是进行相交检测和包含检测：

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);

调用了Frustum对象的类其实不多，不过Frustum的重要程度和CameraBase捆绑在一起。

　　QuadTileSet类中的调用方式主要是通过调用camera.ViewFrustum来获取并进行相关的判断。在QuadTileSet中总共有3处地方可以查见camera.ViewFrustum，一个是在Update方法中，一个是在GetClosestDownloadRequest，还有一个是在ResetCacheForCurrentView方法中。

　　QuadTile类中调用方式通过调用camera.ViewFrustum实现，主要在在Update方法中和Render方法中。