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• Bounding Box: Snaps to one of the corners of a bounding box

• Perpendicular: Snaps to a spline's next perpendicular point

• Tangent: Snaps to a spline's next tangent point

• Vertex: Snaps to polygon vertices

• Endpoint: Snaps to a spline's end point or the end of a polygon edge

• Edge/Segment: Snaps to positions only on an edge

• Midpoint: Snaps to a spline's midpoint or the middle of a polygon edge

• Face: Snaps to any point on the surface of a face

• Center Face: Snaps to the center of a face

Several snap points specific to NURBS objects, such as NURBS points and curves, are also shown in Figure 7.18. These points include:

• CV: Snaps to any NURBS Control Vertex subobject

• Point: Snaps to a NURBS point

• Curve Center: Snaps to the center of the NURBS curve

• Curve Normal: Snaps to a point that is normal to a NURBS curve

• Curve Tangent: Snaps to a point that is tangent to a NURBS curve

• Curve Edge: Snaps to the edge of a NURBS curve

• Curve End: Snaps to the end of a NURBS curve

• Surf Center: Snaps to the center of a NURBS surface

• Surf Normal: Snaps to a point that is normal to a NURBS surface

• Surf Edge: Snaps to the edge of a NURBS surface

Setting snap options

The Grid and Snap Settings dialog box holds a panel of Options, shown in Figure 7.20, in which you can set the marker size and whether they display. The Snap Preview Radius defines the radial distance from the snap point required before the object that is being moved is displayed at the target snap point as a preview. This value can be larger than the actual Snap Radius and is meant to provide visual feedback on the snap operation. The Snap Radius setting determines how close the cursor must be to a snap point before it snaps to it.

The Angle and Percent values are the strengths for any Rotate and Scale transformations, respectively. The Snap to Frozen Objects lets you control whether frozen items can be snapped to. You also can cause translations to be affected by the designated axis constraints with the Use Axis Constraints option. The Display Rubber Band option draws a line from the object's starting location to its snapping location.

Within any viewpoint, holding down the Shift key and right-clicking in the viewport can access a pop-up menu of grid points and options. This pop-up quadmenu lets you quickly add or reset all the current snap points and change snap options, such as Transformed Constraints and Snap to Frozen.

FIGURE 7.20

The Options panel includes settings for marker size and color and the Snap Strength value.

Using the Snaps toolbar

As a shortcut to enabling the various snapping categories, you can access the Snaps toolbar by right-clicking the main toolbar away from the buttons and selecting Snaps from the popup menu. The Snaps toolbar, shown in Figure 7.21, can have several toggle buttons enabled at a time. Each enabled button is highlighted in yellow.

FIGURE 7.21

The Snaps toolbar provides a quick way to access several snap settings.

Tutorial: Creating a lattice for a methane molecule

Many molecules are represented by a lattice of spheres. Trying to line up the exact positions of the spheres by hand could be extremely frustrating, but using the Snap feature makes this challenge . . . well . . . a snap.

One of the simpler molecules is methane, which is composed of one carbon atom surrounded by four smaller hydrogen atoms. To reproduce this molecule as a lattice, you first need to create a tetrahedron primitive and snap spheres to each of its corners.

To create a lattice of the methane molecule, follow these steps:

1. Right-click the Snap toggle button in the main toolbar to open the Grid and Snap Settings, and enable the Grid Points and Vertex options. Then click the Snap toggle button (or press the S key) to enable 3D Snap mode.

2. Select the Create⇒Extended Primitives⇒Hedra menu command, set the P Family Parameter to 1.0, and drag in the Top viewport from the center of the Home Grid to the first grid point to the right to