Removing Geometry

In this chapter we'll describe some techniques for removing detail from an object.

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In this Chapter

• Dissolve
• Merge Region
• Collapse
• Derez

You may want to remove geometry if:

• you are exporting to a 3D game platform where polygon count is important
• you want to reduce the amount of detail in a particular region of a model

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Dissolve

Try this:

1. Create a cube.

2. (CLICK-L) on bodies on the element sensitivity menu.

3. (SHIFT-L) on the cube, then (CLICK-R) on Select Elements.

This collects all the faces on the cube.

5. With the faces still selected, Edge Subdivide again.

Your cube should look like this:

6. (SHIFT-L) on one of the segments near the center of the cube.

Figure 12.2 Select a segment to dissolve

7. (CLICK-L) on Dissolve.

The segment is dissolved:

The two faces previously separated by the segment are merged into a single face:

Figure 12.4 The resulting face

Dissolving Segments between Non-Flat Faces

Typically, you'll have some minimum tolerance of acceptability regarding maximum non-flat face angle; this may depend on your renderer.

Note. N-Geometry will let you dissolve any segment.

Try this:

8. (SHIFT-L) on a segment on the edge of the cube.

Figure 12.5 Select a segment on the edge of the cube

9. (CLICK-L) on Dissolve.

The resulting face, although non-flat, can still be selected and modified like any other face on the polyhedra:

10. (SHIFT-L) on the face, then (CLICK-L) on Extrude.

N-Geometry extrudes the face, adding any geometry required to keep the surface of the model intact:

Of course, you can use the Cut operation on the non-flat face if you decide that you actually do need flat faces.

Figure 12.8 Cutting the non-flat face

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Merge Region

Try this:

1. Create a cube.

2. (CLICK-L) on bodies on the element sensitivity menu.

3. (SHIFT-L) on the cube, then (CLICK-R) on Select Elements.

This collects all the faces on the cube.

5. With the faces still selected, Edge Subdivide again.

Your cube should look like this:

6. Collect several contiguous faces.

Figure 12.10 Select several contiguous faces

7. (SHIFT-L) on the collection.

8. (CLICK-L) on Merge Region.

The faces are merged together into a single new face:

The resulting face can be manipulated just like any other face on the polyhedra.

Note. You cannot merge non-contiguous faces. Although N-Geometry does not complain if you try to merge non-contiguous faces, using the command this way has no effect on an object.

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Collapse

• A segment or face is reduced to a vertex.
• In the case of a vertex, the vertex and all its edges are eliminated and the gap is filled with a face.

Essentially, the geometry on the object around the selected element "collapses" to fill the resulting void and maintain surface integrity. Trying a few examples will give you a good feel for how the command works.

Try this:

1. Create a cube.

2. (CLICK-L) on bodies on the element sensitivity menu.

3. (SHIFT-L) on the cube, then (CLICK-R) on Select Elements.

This collects all the faces on the cube.

5. With the faces still selected, Edge Subdivide again.

Your cube should look like this:

Try collapsing a vertex, segment, or face:

Figure 12.13 Collapsing a vertex, segment, or face

Note that collapsing an element can change the shape of your model:

Collapsing Vertices

Figure 12.14 Left, selected points; right, resulting collapse

Collapsing Segments

Figure 12.15 Left, selected segment; right, resulting collapse

Collapsing Faces

Figure 12.16 Left, selected face; right, resulting collapse

Note. The Collapse command can easily generate non-planar (but completely legal for N-Geometry) faces. If you are exporting an object to a game platform that does not allow non-planar faces, use the Illegal Faces command on the body to detect them.

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Derez

To Derez an object:

1. (SHIFT-L) on the object or group of faces.

The following menu appears:

Each of the options attempts to derez the object using different methods:

• Hardness removes faces by merging adjacent faces whose normals are less than the specified angle threshold apart. For example, two faces on a grid that have parallel normals would be merged using this method.


• Angle Threshold specifies the threshold angle for "non-flatness." If adjacent faces interesect at an angle less than the specified value, they are merged into one face.

• Faces lets you derez an object by specifying the maximum number of faces the object can have. N-Geometry tries to keep the shape by combining adjacent flat faces first to meet the maximum number of polygons.
• Triangles is used for objects that have previously been triangulated. This works the same way as for Faces (above), but reduces the triangle count for an object.
• Edges works like Faces above, but counts edges.
• Points works like Faces above, but counts points.
• Number of (elements) lets you specify, for each of the last four methods, the maximum number of elements of the selected type that you want the resulting object to have.

For each of the last four methods, the model is only changed destructively when the number of elements specified cannot maintain the original's shape; N-Geometry always makes its best attempt to maintain the shape of the original model.

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Congratulations!

As with all of the techniques described in this guide, they are best understood when practiced!

Copyright © 1996, Nichimen Graphics Corporation. All rights reserved.