Curve on Database segments are constrained to lie on an underlying database curve, surface, or quilt and are cubic polynomials between successive points. Their slope is defined either by a Catmull-Rom, Akima, or Bezier control algorithm described in the Curve section.
Tip: Projecting a connector onto a database surface creates a new control point for each grid point projected. Since having many control points can make editing a connector time-consuming, consider creating the database-constrained connector by hand, instead of projection, by using the Line on Database or Curve on Database commands.
The On Database frame contains the Begin, Cancel, and Default commands which allow the specification of the underlying database entities. Begin allows you to select the target projection entities from all those currently visible. After this button is pressed, you can select database entities from either the List panel or the Display window. Note that the current set of target entities (if any) is already selected when the button is first pressed. The button remains depressed with the text End now displayed. Pressing End, stops the selection of projection database entities. The selection operation can be canceled at any time by pressing the Cancel button.
When checked on, the Force Drawing in Parametric Space option will allow you to draw curves on surfaces containing multiple parametric (u,v) spaces. Examples of such surfaces are quilts assembled from multiple component quilts and shells. When this option is unchecked (default state), the curve being drawn will halt at the edge of each parametric space.
The selection of database curves can be enabled or disabled using the Allow Drawing on Database Curves option.
The Project Shape To DB command becomes available once a second control point is chosen on a database surface. This command projects the shape of a curve onto a database in two different ways depending on the type of surface. For a B-Spline surface, the slopes of the curve are modified. For a quilt or trimmed surface, control points are added. In the second case, you will most often use this command when a curve passes over a hole on a quilt or trimmed surface. Control points are then added so that the curve goes around the hole. Note that since the point insertion is conservative, the command may need to be applied multiple times to add more control points.
Use Project Shape To DB in the On Database Options frame to modify your line/curve such that it conforms more to the underlying database.
Tip: The Line on Database and Curve on Database commands are particularly useful for creating connectors which do not lie on the boundaries of a database entity. Boundary connectors can be easily created using the Connectors on Database Entities command (refer to the On Database Entities section for further details).