|Applicable Blender version: 2.67.|
A simple, non-extruded curved line on its own will not be visible when rendered in Blender. Things are different, however, when you bevel the curve—that is, use some two-dimensional shape as a cross-section, and the line curve becomes a guide for extruding the shape into the third dimension.
Start with a curve object—any curve will do. Look in the Curve Context in the Properties window, for the Geometry panel. There you should see two editable fields with the title “Bevel:” above them, one labelled “Depth:” and the other “Resolution:”. Try setting the depth to something like 0.1.
Now your curve is no longer a simple line. It should have a V-shaped cross section, perhaps like a piece of bent angle iron.
Now try increasing the “Resolution:” value, and you should see the V cross section start to smooth out, until at about a resolution of 2 or 3, it looks like a curved half pipe.
Now look further up, in the Shape panel. There should be a popup menu with the title “Fill:” above it; for a 3D curve, by default the item selected is “Half”. Try the “Front” and “Back” items, and you should see that these give you just halves of your half pipe. Now try “Full”, and your half pipe should now be a complete pipe. You can also join the ends together by checking the “Cyclic: U” box.
Start and End Bevel Factors: These reduce the extent of the bevel shape, so that instead of extending the full length of the curve, they go from and to the specified fractions of the length. These become more useful when the cross section of the shape is no longer uniform, when you apply a custom taper (below).
Now change the curve to 2D. The Fill options now become “Both”, “Front”, “Back” and “None”, where “Both” is like the “Half” setting for 3D curves. Note there is no equivalent of “Full”. But there is a new feature: if you check the “Cyclic: U” box, it will fill in the entire interior of the curve!
Try this with a Bézier circle or a NURBS circle, and you should get a pancake-like object. For added flavour, give it a nonzero Extrude value, and this will make the shape even thicker.
Try adding another curve, say a circle. Now go back to your first curve, and find the field titled “Bevel Object:” in the Geometry panel in the Curve Context properties. Click on it, and a popup menu should appear, and you should see the name of the second curve (e.g. ”BezierCircle” or “NurbsCircle”). Select that name, and you should see your bevelled curve immediately take on the cross-section of the new curve.
Only a line curve can be used as a bevel. Surfaces won’t work, and non-curve/surface objects won’t work. They just won’t show up for you to pick from.
Select the second curve again, go into Edit mode, and try messing around with the control points: you should your changes immediately get reflected in the shape of the bevelled curve. You may find that the orientation of the second curve doesn’t coincide with the orientation of the bevel cross-section. This is easy to fix, because any change you make to the rotation and position of the second curve in object mode will have no effect on its use as a bevel, so you can freely reorient and reposition it to make it easier to match its shape up with the bevel cross section. (Scaling does, however, have an effect.)
Tilt and Radius: Go back to the first curve, and try the radius-scaling+ and tilting + functions you previously learned about. You will see that they affect the scaling and orientation of the bevel cross-section along the curve.
Now try adding a third curve object. (As with custom bevels, only line curve objects will work.) Go back to your first curve, find the “Taper Object:” field (it should be next to the “Bevel Object:” field you’ve already used), and select the name of your newly-added curve. The bevelled shape will now most likely squish down in a most peculiar way. Select your third curve, that you are using as a taper, and go into Edit mode. Now try adjust control points, and observe the effect on the bevelled shape: what you should find is that the object-local Y-coordinate of each control point governs the thickness of the bevelled shape at the corresponding position along the shape given by its X-coordinate, while the object-local Z-coordinate doesn’t have any effect.
As with the bevel shape, you can freely rotate and reposition (and this time, even rescale) the taper shape in object mode, and it will have no effect on its taper function: only alterations of the control points in edit mode have an effect.
Tilt still works. Try+ and + on the first curve as before; you should find that the former is now ignored (the size of the cross section being completely governed by the taper shape), but the latter still works.
The taper object should not have its own nonzero bevel or extrude settings. This only confuses the issue.