Adding Polygons to a Mesh

The Polygon->Polygon command has the same functionality as the Attach mode of the Draw->Polygon command, and it can be used by itself to either fill holes left in polygons or add new polygons that share edges with the current mesh. Polygons can only be added to open edges, so that no more than two polygons can ever share an edge. You can show the open edges of a mesh with the Show->Edge flags command (see Figure 7.14).

After executing the Polygon->Polygon command, pick at least two vertices to attach a polygon. If you are adding a new polygon, then click somewhere off the mesh to place a new point in space, and continue to place new points until you have defined the polygon you want. To complete the polygon, click the middle mouse button.

Figure 7.14 A polygonal mesh with holes showing the edge flags.

If you are patching a hole in a polygon, continue to select only existing vertices until you have plugged the hole. Then use the middle mouse button to complete the polygon.

Take a Shortcut: Working from a Primitive

Now that you know how to start from scratch, making your own objects out of nothing but ambition and the Polygon menu cell, I should tell you that it is almost always easier to start out with a simple geometric polygon primitive and use the polygon duplication and transform tools to create the object you need. From one polygon, you can make many by duplicating the original and transforming it in a number of interesting ways. Many types of angular, precise looking objects can be easily created this way, such as buildings, machines, ovens, and all manner of other manufactured devices. But before we dive in, first a word about the polygon transformations and the Polygon Local Coordinate system.

Polygon Transformations and Local Coordinates

When you are in the POL (polygon) mode, remember that only the selected polygons are acted upon by the Transformation menus’s cells and a number of other commands, including Duplicate and Delete. This means that the object stays as it is, and only the pink polygons colored pink are translated, scaled, rotated, and so on (see Figure 7.15). Don't forget to return to OBJ mode when you want to work on the entire object and not just the individual polygons.

Figure 7.15 A polygonal object with selected polys translating on their own axes.

When you transform a polygon, it needs to have its own local axis to which to transform relative (unless you are in GBL mode). By default, if you select a single polygon and show centers, you will observe that the center is in the geographic middle of the polygon, with the X and Z objects oriented towards the edges of the polygon, and the Y axis pointed outwards along the normal. This is very useful, because you can now use the Y axis to move the polygon in and out along the normal, and you can scale X and Z to make the polygon smaller or larger.

However, if you select more than one polygon and they share an edge, the center is located in the middle of the group, and the orientation is the average of all the polygons in the group (see Figure 7.16). Although this does give the group a center for transformations, it might not be a very meaningful one. (You could create your own center with Cluster Centers.)

If none of the selected polygons in the group shares an edge with another selected polygon, each polygon has its very own center, aligned with just that polygon. This factmakes it possible to transform polygons in large groups instead of individually. Pay attention to the centers on the groups of polygons you transform, or their behavior will make very little sense to you.

Figure 7.16 A polygonal object with grouped polys translating on a common axis.

When you are in POL mode, the Trans menu cells operate only on the selected group of polygons, causing the polygons to move in space relative to the object to which they belong. If you want to animate this change of transformation, use the SaveKey->Object->Shape command, not the SaveKey->Object->Translation command, because you are really adjusting the location of the points that make up the surface. The Rot menu cells rotate the polygons around their local axes (see Figure 7.17).

Figure 7.17 A polygonal object with polys rotated about Y (the normal).

The Scale menu cells make the polygons larger or smaller in X or Y (see Figure 7.18).

Figure 7.18 A polygonal object with selected polygons scaled in X and Z.

You can make more elaborate transformations to a polygon by simply moving the vertices around with the M supra key.

Duplicating Polygons

When you duplicate a selected polygon by using the Duplicate->Selection command while in POL mode, a bunch of special things happen. You might expect that another polygon would be made on top of the last, but that's not what happens. The original polygon is cut into a number of new polygons, one for each edge, and connected to the new polygon that takes over the shape and location of the original (see Figure 7.19). In other words, a border of new polygons is made, all connected by shared edges to a new central polygon.

Figure 7.19 The selected polygon is duplicated and then scaled slightly smaller. Note the bordering polygons.

If this new central polygon is translated away from the object, the new border polygons connect it to the other polygons in the object. If the new center polygon is scaled, the border polygons create a bevel. You should also remain in the Model module while using polygon duplication.

Figure 7.20 A simple polygonal window with frame and sill.

Deleting Polygons

Of course, you sometimes want to remove a polygon while keeping the vertices and edges that it shares with neighbors.

When in POL mode, choosing the Delete->Selection command permanently removes the selected polygon from the object (see Figure 7.21). The edges it used to share now become open, and you can build new polygons to share them. This polygon delete function is a great way to knock holes in an object .

Figure 7.21 The Delete->Selection menu cell.

Extruding Polygons

Individual polygon shapes can also be extruded, with often interesting results. To accomplish this task, you must be working in POL mode, have some polygons selected on an object, and then use the standard Surface]Extrusion command in the Model module. When in POL mode, all the extrusion details in teh Extrusion dialog apply only to the selected polygons on the selected onject! You can extrude polygons along a path, and you can apply a transformation to the polygon at each step of the extrusion, making the resulting polygons smaller, revolved slightly, or offset in space (see Figure 7.22).

Figure 7.22 An object with some polygons selected and extruded.

The polygon Extrude Along a Path option in the Extrusion dialog gives you the power to draw a path, and then have the Y direction of the polygon (the normal) oriented along this path during the extrusion.