WPF 3D MeshGeometry3D类的Positions和TriangleIndices属性研究

1 MeshGeometry3D 类

MeshGeometry3D 类，用于生成三维形状的三角形基元；
类的参考在此；
https://learn.microsoft.com/zh-cn/dotnet/api/system.windows.media.media3d.meshgeometry3d?view=windowsdesktop-7.0

写在xaml语法里面是 ，如果写到C#语言里面就要new一个此类的对象；

Positions属性就是来表示组成3D图形的所有三角形的顶点的；

TriangleIndices通过将Positions的顶点集合按一定顺序组合起来，组成所需要的三角形；

就是如果画一个三角形，需要3个顶点；如果画一个面，正方形面，并不是4个顶点；因为基元是三角形，只能由三角形构成其他形状；画一个正方形面，需要六个顶点，指定2个三角形来构成一个正方形面；

先看基本的三角形；按照这个示意的图来画一个基本三角形；顶点的顺序如下图的0、1、2；

三角形的三个顶点是：(0,0,0)，(1,0,0)，(0,1,0)；把摄像机放到呈现区域的中心，屏幕外面10个值的地方，往屏幕里边看，如下图；

    这时只有Positions属性，TriangleIndices将按默认；

    如果增加TriangleIndices="0,1,2"，加和不加都是一样；如果调整TriangleIndices点的顺序，三角形将不可见；

此时点的顺序是如下图标记；

 如果画一个正方形面，四个顶点是0,0,0  1,0,0   1,1,0    0,1,0；此时需要6个顶点，画2个三角形，来组成正方形面；但是2个三角形可以共享一条边，有2个顶点是重合的；

通过给出4个顶点的坐标，然后用TriangleIndices来指定2个三角形的顶点顺序，

     

TriangleIndices，0,1,2 指定一个三角形，2,3,0 指定另一个三角形，这样来绘出此正方形面；

<Page
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Grid>  
  
  <Viewport3D>
        <Viewport3D.Camera>
            <PerspectiveCamera Position="0,0,10" LookDirection="0,0,-2" UpDirection="0,1,0"/>
        Viewport3D.Camera>
        <Viewport3D.Children>                                          
            <ModelVisual3D x:Name="Light">
                <ModelVisual3D.Content>
                    <AmbientLight/>
                ModelVisual3D.Content>
            ModelVisual3D>
 
        <ModelVisual3D> 
          <ModelVisual3D.Content> 
            <GeometryModel3D> 
                <GeometryModel3D.Geometry>
                  <MeshGeometry3D Positions="0,0,0,1,0,0,1,1,0,0,1,0" TriangleIndices="0,1,2 2,3,0"/> 
                GeometryModel3D.Geometry> 
                <GeometryModel3D.Material> 
                  <DiffuseMaterial Brush="Yellow" /> 
                GeometryModel3D.Material> 
              GeometryModel3D>
            ModelVisual3D.Content>
        ModelVisual3D>
       Viewport3D.Children> 
Viewport3D>
 
  Grid>
Page>

2 从C#代码使用MeshGeometry3D类

前面了解了 MeshGeometry3D 类是生成三维形状的基元；前面的代码是在xaml里；
下面看一下从C#代码里生成三维场景和图形，以及使用MeshGeometry3D类；

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Media3D;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
 
namespace jiyuan
{
    /// 
    /// MainWindow.xaml 的交互逻辑
    /// 
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
 
        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            // Declare scene objects.
            Viewport3D myViewport3D = new Viewport3D();
            Model3DGroup myModel3DGroup = new Model3DGroup();
            GeometryModel3D myGeometryModel = new GeometryModel3D();
            ModelVisual3D myModelVisual3D = new ModelVisual3D();
 
            PerspectiveCamera myPCamera = new PerspectiveCamera();
 
            myPCamera.Position = new Point3D(0, 0, 2);
 
            myPCamera.LookDirection = new Vector3D(0, 0, -1);
 
            myPCamera.FieldOfView = 60;
 
            myViewport3D.Camera = myPCamera;
 
            DirectionalLight myDirectionalLight = new DirectionalLight();
            myDirectionalLight.Color = Colors.White;
            myDirectionalLight.Direction = new Vector3D(-0.61, -0.5, -0.61);
 
            myModel3DGroup.Children.Add(myDirectionalLight);
 
            MeshGeometry3D myMeshGeometry3D = new MeshGeometry3D();
 
            Vector3DCollection myNormalCollection = new Vector3DCollection();
            myNormalCollection.Add(new Vector3D(0, 0, 1));
            myNormalCollection.Add(new Vector3D(0, 0, 1));
            myNormalCollection.Add(new Vector3D(0, 0, 1));
            myNormalCollection.Add(new Vector3D(0, 0, 1));
            myNormalCollection.Add(new Vector3D(0, 0, 1));
            myNormalCollection.Add(new Vector3D(0, 0, 1));
            myMeshGeometry3D.Normals = myNormalCollection;
 
            Point3DCollection myPositionCollection = new Point3DCollection();
            myPositionCollection.Add(new Point3D(-0.5, -0.5, 0.5));
            myPositionCollection.Add(new Point3D(0.5, -0.5, 0.5));
            myPositionCollection.Add(new Point3D(0.5, 0.5, 0.5));
            myPositionCollection.Add(new Point3D(0.5, 0.5, 0.5));
            myPositionCollection.Add(new Point3D(-0.5, 0.5, 0.5));
            myPositionCollection.Add(new Point3D(-0.5, -0.5, 0.5));
            myMeshGeometry3D.Positions = myPositionCollection;
 
            PointCollection myTextureCoordinatesCollection = new PointCollection();
            myTextureCoordinatesCollection.Add(new Point(0, 0));
            myTextureCoordinatesCollection.Add(new Point(1, 0));
            myTextureCoordinatesCollection.Add(new Point(1, 1));
            myTextureCoordinatesCollection.Add(new Point(1, 1));
            myTextureCoordinatesCollection.Add(new Point(0, 1));
            myTextureCoordinatesCollection.Add(new Point(0, 0));
            myMeshGeometry3D.TextureCoordinates = myTextureCoordinatesCollection;
 
            Int32Collection myTriangleIndicesCollection = new Int32Collection();
            myTriangleIndicesCollection.Add(0);
            myTriangleIndicesCollection.Add(1);
            myTriangleIndicesCollection.Add(2);
            myTriangleIndicesCollection.Add(3);
            myTriangleIndicesCollection.Add(4);
            myTriangleIndicesCollection.Add(5);
            myMeshGeometry3D.TriangleIndices = myTriangleIndicesCollection;
 
            myGeometryModel.Geometry = myMeshGeometry3D;
   
            LinearGradientBrush myHorizontalGradient = new LinearGradientBrush();
            myHorizontalGradient.StartPoint = new Point(0, 0.5);
            myHorizontalGradient.EndPoint = new Point(1, 0.5);
            myHorizontalGradient.GradientStops.Add(new GradientStop(Colors.Yellow, 0.0));
            myHorizontalGradient.GradientStops.Add(new GradientStop(Colors.Red, 0.25));
            myHorizontalGradient.GradientStops.Add(new GradientStop(Colors.Blue, 0.75));
            myHorizontalGradient.GradientStops.Add(new GradientStop(Colors.LimeGreen, 1.0));
 
            DiffuseMaterial myMaterial = new DiffuseMaterial(myHorizontalGradient);
            myGeometryModel.Material = myMaterial;
 
            RotateTransform3D myRotateTransform3D = new RotateTransform3D();
            AxisAngleRotation3D myAxisAngleRotation3d = new AxisAngleRotation3D();
            myAxisAngleRotation3d.Axis = new Vector3D(0, 3, 0);
            myAxisAngleRotation3d.Angle = 40;
            myRotateTransform3D.Rotation = myAxisAngleRotation3d;
            myGeometryModel.Transform = myRotateTransform3D;
 
            myModel3DGroup.Children.Add(myGeometryModel);
 
            myModelVisual3D.Content = myModel3DGroup;
            myViewport3D.Children.Add(myModelVisual3D);
 
            this.Content = myViewport3D;
        }
    }
}

首先生成几个wpf元素，和xaml里的对应；
然后生成一个摄像机，赋值一些参数；把摄像机加入场景；
然后把光源也加入；

然后生成一个MeshGeometry3D类的对象；
然后生成向量来设置此对象的法线；
然后生成一些Point3D点来赋值给此对象的Positions属性；前面是在xaml里赋值的；
然后设置此对象的纹理坐标；
然后设置此对象的TriangleIndices属性；前面是在xaml里赋值的；
把此对象加入场景；

然后添加画刷和材质，添加一个变换；

运行，