c# - 如何处理 WPF 3D 中的漫反射 Material 透明度？

Question

我有一个场景，其中多个模型具有平面和具有透明度的漫反射 Material 。

这些模型有时会由于摄像机角度的变化而重叠，然后这些模型无法正确渲染，如下图所示，蓝色透明面隐藏了红色立方体的一部分:

经过一些研究，我发现在所有情况下都需要深度排序才能正确渲染模型。

是否有任何图书馆已经处理过这个问题，例如通过实现 Newell 算法还是有人知道其他解决方案？

Answer 1

我发现了一种基于模型边界框的排序算法，该算法来自一个非常古老的 Microsoft 博客，该博客已无法访问。

下面是对代码进行一些更改以直接传递 ModelVisual3D 实例的类:

// original class taken from
// https://web.archive.org/web/20071201135137/http://blogs.msdn.com/pantal/archive/2007/07/23/sorting-for-wpf-3d-transparency.aspx
public static class SceneSortingHelper
{
    /// <summary>
    /// Sort Modelgroups in farthest to closest order, to enable transparency
    /// Should be applied whenever the scene is significantly re-oriented
    /// </summary>
    public static void AlphaSort(Point3D CameraPosition, 
        System.Collections.Generic.List<System.Windows.Media.Media3D.ModelVisual3D> ModelVisual3Ds, Transform3D WorldTransform)
    {
        ArrayList list = new ArrayList();
        foreach (var m in ModelVisual3Ds)
        {
            var location =
                WorldTransform.TransformBounds(
                    m.Transform.TransformBounds(
                        m.Content.Transform.TransformBounds(
                            m.Content.Bounds
                        )
                    )
                ).Location;

            double distance = Point3D.Subtract(CameraPosition, location).Length;
            list.Add(new ModelVisual3DDistance(distance, m));
        }
        list.Sort(new DistanceComparer(SortDirection.FarToNear));
        ModelVisual3Ds.Clear();
        foreach (ModelVisual3DDistance d in list)
        {
            ModelVisual3Ds.Add(d.modelVisual3D);
        }
    }

    private class ModelVisual3DDistance
    {
        public ModelVisual3DDistance(double distance, ModelVisual3D modelVisual3D)
        {
            this.distance = distance;
            this.modelVisual3D = modelVisual3D;
        }

        public double distance;
        public ModelVisual3D modelVisual3D;
    }

    private enum SortDirection
    {
        NearToFar,
        FarToNear
    }

    private class DistanceComparer : IComparer
    {
        public DistanceComparer(SortDirection sortDirection)
        {
            _sortDirection = sortDirection;
        }

        int IComparer.Compare(Object o1, Object o2)
        {
            double x1 = ((ModelVisual3DDistance)o1).distance;
            double x2 = ((ModelVisual3DDistance)o2).distance;

            if (_sortDirection == SortDirection.FarToNear)
            {
                if (x1 > x2)
                    return -1;
                else
                if (x1 < x2)
                    return 1;
                else
                    return 0;
            }
            else
            {
                if (x1 > x2)
                    return 1;
                else
                if (x1 < x2)
                    return -1;
                else
                    return 0;
            }
        }

        private SortDirection _sortDirection;
    }
}

我将上述类与以下 Viewport3D 扩展方法结合使用 ...

namespace Viewport3DExtensions
{
    public static class Implementation
    {
        public static void AlphaSortModels(this System.Windows.Controls.Viewport3D viewport3D)
        {
            var projectionCamera = viewport3D.Camera as System.Windows.Media.Media3D.ProjectionCamera;

            if (projectionCamera != null)
            {
                var modelVisual3Ds = new System.Collections.Generic.List<System.Windows.Media.Media3D.ModelVisual3D>();

                foreach (var c in viewport3D.Children)
                {
                    var m = c as System.Windows.Media.Media3D.ModelVisual3D;

                    if (m != null)
                        modelVisual3Ds.Add(m);
                }

                // note:
                //  the following method works well most of the time but sometimes the sort is wrong as the sort is simply based on model bounds,
                //  to get rid of artifacts we would need something like binary space partitioning
                SceneHelper.SceneSortingHelper.AlphaSort(
                    projectionCamera.Position,
                    modelVisual3Ds,
                    new System.Windows.Media.Media3D.Transform3DGroup()
                );

                viewport3D.Children.Clear();

                foreach (var c in modelVisual3Ds)
                    viewport3D.Children.Add(c);
            }
        }
    }
}

...像这样

System.Windows.Controls.Viewport3D scene;  
...
scene.AlphaSortModels();

场景现在看起来不错: