Modeling with Patch Grids

Patch grids are one of the 3D Studio VIZ® tools that many designers manage to avoid using for years. Coming from a world of AutoCAD® commands, the patch grid user interface seems arcane and imprecise at first. But, with a little help and some experimentation, you are likely to find that patch grids become one of your preferred modeling tools. You can use them to create geometry that would be difficult or impossible to generate otherwise, and the modeling process is similar to sculpting.

In this article you become familiar with patch grids. You learn how to create and edit them and have a chance to discover what makes them so different from other modeling tools you may have used. By the time you complete this project, you are certain to have a set of new modeling techniques in your arsenal.

What's a Patch Grid?
A patch grid is a surface composed of rectangular or triangular elements. You control the shape of the surface using control points located at the corners of each patch. You move the handles associated with each control point to shape the surface as it passes through that point (see Figure 1).

Figure 1: Quad Patch and Tri Patch patch grids showing control vertices with handles.

Figure 2: The Edit Patch modifier converted this lofted object into an object composed of patch grids.

To create a patch grid:

Select the Create tab from the command column.
Select the Geometry icon.
From the Geometry drop-down list, choose Patch Grids.
Select Quad Patch or Tri Patch.
Select two points to create a patch.

Tip: You can combine several patch grids into a single patch grid object. Select one of the objects. Select the Modify tab. Choose Attach, and then select the other patch grid entities. Use this technique when you want to join the vertices of one patch grid to the vertices of another grid.

You can turn almost any 3D object composed of faces, ranging from primitives to NURBS surfaces, into a patch grid. Here's how:

Select the object.
Select the Modify tab.
Choose More.
From the list of modifiers, select Edit Patch.
Choose OK to convert the object into a set of patch grids (see Figure 2).

Figure 3: Move and shape vertices to round corners and refine geometry.

Once you have created one or more patch grids using the techniques just specified, you can shape them using patch grid editing tools.

Controlling Patch Grids by Editing Vertices
Once you have created a model using patch grids, you can change its shape by moving its vertices or moving the handles associated with those vertices. Moving vertices and handles is a great way to round corners and shape edges.

To modify vertices on a patch grid object:

Select the object.
Select the Modify tab.
Select the Vertex icon.
Select two points to put a window around the vertices you want to move.
On the Selection/Xform toolbar, choose the Select and Move icon.
Click and drag to move the selected vertices or their associated handles (see Figure 3).

Figure 4: Once you apply the Edit Patch modifier and select the Vertex icon, you can weld, delete, hide and unhide vertices.

If you select a patch grid object, then apply the Edit Patch modifier as described previously. Selecting the Vertex icon enables you to:

Weld two or more vertices together, making it possible to join patch grids together. Note that the Weld spinner specifies how close selected vertices must be to be welded.
Hide and Unhide vertices, making it easier to move or modify them individually.
Delete patch grids by deleting selected vertices (see Figure 4).

Figure 5: Triangular patch grids created by adding Tri Patches to edges.

Working with Patch Grid Edges
After applying the Edit Patch modifier to an object, you can select the Edge icon. Edges appear between any two adjacent vertices on the same patch grid. Moving an edge moves both the vertices associated with the edge.

The Add Quad and Add Tri buttons enable you to add a patch to any edge of a patch grid object. Just select an edge or edges, and then choose the appropriate button. Selecting Add Quad creates a rectangular patch grid. Selecting Add Tri creates a triangular patch grid (see Figure 5).

Figure 6: Move and rotate edges to fold patch grids like paper.

Select an edge and choose Subdivide from the command column when you want to split the patch grid associated with the edge into two grids. If the Propagate check box is selected, all the adjacent patch grids will also be subdivided. Use Subdivide to add more complex geometry to simple objects (see Figure 6).

Figure 7: Selected patches on a cylinder converted to a patch grid object.

Tip: By default, 3D Studio VIZ software only renders the front, or "Normal," side of a patch grid. To see both sides of a patch grid you must apply a two-sided material to it.

Modifying Patch Grid Surfaces
Some of the most flexible patch grid modeling tools are found by selecting the Patch icon after you have applied an Edit Patch modifier to an object.

For example, to extrude one or more patch grids:

1. Select the object.

2. Select the Modify tab.

3. Select the Patch icon.

4. Select one or more surfaces to extrude (see Figure 7).

Figure 8: Extruding and beveling patch grids lets you create very complex geometry very quickly.

5. Select the Extrude button.

6. In the Extrusion spinner, specify the extrusion amount.

7. In the Outlining spinner, specify a value to bevel the extruded patches (see Figure 8).

Other options available once you've selected the Patch icon enable you to:

Assign Material IDs to selected patches. You need to do this if you want to apply multiple materials to a single patch grid object.
Detach patches from a model, making one object into two objects.
Control whether beveled edges are smoothed with fillets or linear chamfers or not smoothed at all.

Of course you can also just move, rotate, or scale selected patch grids if desired. Because patch grids are smoothed by default, and because they can be shaped using the vector handles at vertices, their shapes are often softer and more "organic" than models created using other techniques.

In Conclusion
Many 3D Studio VIZ users elect to create most of their models in AutoCAD under the mistaken impression that the AutoCAD tools are more sophisticated than those found in 3D Studio VIZ.

In fact, 3D Studio VIZ software lets you create geometry that would be difficult or impossible to develop in AutoCAD. Patch grids, in particular, let you quickly create soft sculpted shapes suitable for furniture or molded architectural details. Take time to experiment with patch grids and you may find that they become your favorite 3D modeling tool.