CV curves are NURBS curves controlled by control vertices (CVs). The CVs don’t lie on the curve. They define a control lattice that encloses the curve. Each CV has a weight that you can adjust to change the curve.
While you’re creating a CV Curve you can click to create more than one CV at the same location (or close to it), increasing the influence of the CVs in that region of the curve. Creating two coincident CVs sharpens the curvature. Creating three coincident CVs creates an angular corner in the curve. This technique can help you shape the curvehowever, if you later move the CVs individually, you lose this effect. (You can also obtain the influence of multiple CVs by fusing CVs.)
Tip: Object-level NURBS curves can’t contain surface sub-objects, so you can’t use them as the basis for a full NURBS model. But you can convert the curve object to a NURBS surface object using the Edit Stack button in the Modify panel.
The creation parameters are the same for both Point Curves and CV Curves.
The controls in this area change the accuracy and kind of curve approximation used to generate and display the curve.
You can make curves renderable by assigning them a thickness with these controls. The curve renders as a cylindrical object. Renderable NURBS curves appear when you render a scene using Render; they are not rendered in shaded viewports.
Renderable: Turn on to make the curve renderable.
Thickness: Sets the renderable thickness of the curveits circular diameterusing the current 3DS MAX units.
Generate Mapping Coordinates: Generates mapping coordinates so you can apply mapped materials to the renderable curve.
The Keyboard Entry rollout lets you create a NURBS curve by typing. Use the TAB key to move between the controls in this rollout. To press a button, use ENTER while the button is active.
X, Y, and Z: Enter the coordinates of the next CV to add.
Weight: Enter the weight of the CV.
Add Point: Adds the CV to the curve.
Close: Ends creation of the curve and creates a segment between the last CV and the initial CV, to make the curve a closed curve.
Finish: Ends creation of the curve, leaving it open ended.
Note: If you begin the curve by clicking without dragging, this also creates the curve’s first point or CV. However, if you release the mouse more than five pixels away from where you initially pressed it, this creates an additional point or CV.
While you are creating a NURBS curve of either type, you can press BACKSPACE to remove the last point or CV you created, and then previous points or CVs in reverse order.
As with splines, if you click over the curve’s initial point or CV, a dialog appears asking whether you want the curve to be closed. Click No to leave the curve open, Yes to close the curve. (You can also close a curve when you edit it at the Curve sub-object level.) When a closed curve is displayed at the Curve sub-object level, the initial point or CV is displayed as a green circle, and a green tick mark indicates the curve’s direction.