Function available in CINEMA 4D Prime, Visualize, Broadcast, Studio & BodyPaint 3D

Polygon Reduction Object

Basic Coord. ObjectPolygon Reduction Object

Polygon Reduction Object

The Polygon Reduction deformer enables you to reduce the number of polygons of any Cinema 4D geometric object and operates in a flexible and intuitive way. Polygon Reduction is available from the

Why use polygon reduction?

Many 3D tasks demand models that use as few polygons as possible. Unfortunately, the various techniques used to create models often means that more polygons are created than are necessary. Worse still, without polygon reduction tools, these models must be reduced by hand.

Here are a few examples to show the importance of polygon reduction:

- Modern 3D games need characters with low polygon counts for smooth animation in real-time.

- Models for the Internet, in formats such as VRML, need a low polygon count for quick download.

- Models created by 3D scanners often have an extremely high number of polygons; reducing the number of polygons makes these objects easier to handle.

To summarize: reduce the number of polygons for faster renders and lower memory requirements.

What does polygon reduction do?

The Polygon Reduction deformer works hard to reduce the number of polygons in the chosen object quickly and accurately - this is its mission in life. It always tries to reduce the polygon count according to your wishes and provides many options for user control.

Polygon Reduction works with triangles (not quads) and, if necessary, an automatic triangulation of the object will be performed before reduction.

Polygon Reduction is great for simplifying an object mesh, taking care of any resulting mesh inconsistencies (e.g., fold-overs, boundary cracks) but it does not create a dynamic map of this process (for multiple LODs — levels of detail).

Polygon Reduction manages and diminishes the number of polygons in an object but it cannot take care of the exact number of points in the object.

Using Polygon Reduction

To apply Cinema 4D’s polygon reduction to an existing object, proceed as follows.

- Choose Create / Deformer / Polygon Reduction to create a new object in the root of the Object Manager, named Polygon Reduction.
- Now move this object into the hierarchy of the object that you wish to affect; you can place the Polygon Reduction object anywhere within an object’s hierarchy - polygon reduction will take place on this object and all its children. It works as a deformer.
- The strength can be reduced and the reduction settings edited in the Attributes Manager.

While the polygon reduction is taking place, information on the process (including the original and the expected final polygon count) will appear in Cinema 4D’s status bar (bottom left).

Further usage advice

Remember that Polygon Reduction acts as a deformer i.e., you place it within your object structure in the Object Manager - it will reduce the polygons of its parent object and all its children. You can use Polygon Reduction on any geometric object (Generators, Object Primitive, Array etc.).

If you apply the deformer to a Subdivision Surface object, the deformer can be placed either at the same hierarchy level as the Subdivision Surface object, or at the same hierarchy level as the Subdivision Surface child object. Since the Subdivision Surface operates on the first Child object in its hierarchy, the deformer should not be the first child object of the Subdivision Surface object.

Converting the final object

While you are working with Polygon Reduction to obtain a reduced polygon count that suits your purposes, the deformer maintains the original mesh and re-calculates as you experiment. When you are fully satisfied with your new object you should select it in the Object Manager and use Mesh / Conversion / Current State To Object to produce a new object without the original mesh - then you can delete the original object.

Hints and tips

Quality

Using Boundary Curve Preservation always gives good results if you want to preserve the original mesh boundaries of non-manifold objects i.e., objects that are not entirely closed. For a manifold object (sphere, capsule, cube etc.) it makes no difference if this check is on or off since the algorithm detects manifold objects automatically and deactivates boundary checks if they are on.

Using the Polygon Quality Preservation check is very helpful with objects that have co-planar faces (faces with polygons in the same plane), such as cubes or planes. The result of using this option is that the generated mesh is very well distributed and this leads to a substantially faster reduction process, as long as no clustering-points (points that have a large number of neighboring edges) appear in the reduction process.

Tests show that using sliver checks (Polygon Quality Preservation) gives a better mesh when applied to objects with no co-planar faces and a major improvement in the mesh when working with co-planar faces.

With some objects like a cone, a sphere, a circle etc., which have one or more central points in the mesh, around which there are many edges, it is normal for the algorithm to keep the clustering as high as possible at this point, because the point is not generated during the process of reduction, but exists in the base mesh.

Here is an example of sliver triangles generated as a result of the reduction (if the Polygon Quality Preservation option is not used):

… and sliver triangles preserved when this option is on, due to their existence in the original object mesh:

Such preservation results from the fact that the algorithm detects that collapses in the area of the slivers will lead to other slivers and therefore postpones the reduction.

As you can see, adjusting these quality factors can influence very strongly the way in which the reduction process proceeds.

In some rare cases the accumulative nature of these options can lead to the prevalence of some checks over others, with the result that no perfect decision for the reduction can be made.

This can be observed in some special cases when the sliver check and/or the boundary check prevails over the fold-over check and so the final mesh shows some fold-overs.

What happens is that the algorithm reaches a point at which it has to decide either to generate a fold-over, to break the object boundary, or to generate a sliver. So it takes the user-defined quality factor into account and calculates the better decision. In such a case, in order to avoid the fold-over, a higher Mesh Quality Factor should be used.

Speed

Generally, all checks will slow down the speed of simplification to a greater or lesser extent. This is because, for every potential edge collapse, each check has to take into account the face areas, vector angles, orthogonal plane distances etc. for every one of the neighboring polygons.

General advice

Here is some universal advice about the use of Polygon Reduction’s options:

- On an object with co-planar faces use Co-Planar Optimization. This will speed up the simplification significantly.
- On co-planar objects, if not using Co-Planar Optimization, enable Polygon Quality Preservation. This will generate a really well distributed mesh and will work more quickly than with the option disabled.
- Leave the Boundary Curve Preservation option enabled. It will leave the boundaries of non-manifold objects alone and will not interfere at all when the object is manifold.

Some results generated with Polygon Reduction: