TransparencySorter Class

WPF 3D can show transparent 3D objects. But to show them correctly, the transparent objects need to be defined after all the objects that are visible through them. This does not mean their position in 3D space but their index in the object's hierarchy (index in the parent's Children collection). The objects that are defined after the transparent objects cannot be seen through the transparent objects.

The following image shows the transparency problem in a simple 3D scene (because the red boxes are defined after the semi-transparent GlassPlane they are not visible throug the GlassPlane. The order of objects is visible in the right):

To solve the problem, the GlassPlane needs to be moved after the non-transparent objects. The following image is showing the correctly rendered scene.

For cases when the number of transparent objects is not big and if the transparent objects are not positioned in 3D space one after another, the simple transparency sorting is usually enough to produce correct rendering in WPF 3D.

But simply moving transparent objects after non-transparent objects is not always enough. For example if transparent object A is visible through transparent object B, than B must be defined after A. The problem in this case occurs when the camera is rotated so that B is visible through A. Now B must be defined before A. To solve this problem correctly, firstly the transparent objects must be moved after non-transparent objects and than transparent objects must be sorted by their distance to the camera. The sorting must be done after the camera is changed.

TransparencySorter class can be used to perform a simple transparency sorting with just moving all transparent objects after non-transparent objects, or to perform a ByCameraDistance transparency sorting where the transparent objects are sorted by their distance to the camera.

The sorting by camera distance can be also automated so the sorting is automatically done when the camera is changed. To improve performance sorting is not done on each camera change, but is done only if the angle of the camera is changed for more than CameraAngleChange. By default the CameraAngleChange is 5. This means that the transparent object will be resorted after the camera has changed for more than 5 degrees.

If it is not known if the scene contains any transparent objects (for example if it created from a external file), it is possible to check for transparent objects with the GetTransparentObjectsCount(Model3DGroup) method. If the objects are read with Ab3d.Reader3ds, it is possible to get the number of transparent objects with TransparentObjectsCount property.

The following line of code is enough to perform a simple transparency sorting:

Ab3d.Utilities.TransparencySorter.SimpleSort(myRootObjectsGroup);

The following line of code is enough to perform a transparency sorting by camera distance (sorting is performed only once):

Ab3d.Utilities.TransparencySorter.SortByCameraDistance(myRootObjectsGroup, myCamera);

Instead of using Ab3d.Camera (for example TargetPositionCamera), it is also possible to use camera's position (Point3D):

Ab3d.Utilities.TransparencySorter.SortByCameraDistance(myRootObjectsGroup, cameraPosition);

To automatically sort the objects when the camera is changed the following code can be used:

TransparencySorter transparencySorter;

transparencySorter = new TransparencySorter(myRootObjectsGroup, myCamera);
transparencySorter.StartSortingOnCameraChanged();

// transparencySorter.StopSortingOnCameraChanged(); // Stops the sorting on camera changed

When creating an instance of TransparencySorter class, it is possible to monitor the sorting with subscribing to TransparentModelIndexChanged and SortingCompleted events. To implement custom sorting or use custom values to sort the objects, it is possible to derive a custom class from TransparencySorter and override some of the metods that are used to perform the sorting.

Reference