CG Modeling 2: NURBS - An Introduction to Computer Graphics for Artists - page 266

If your software allows you to display tangents, turn them on and you will see
why. If a tangent is quite long, and then is paired with a short tangent or one moving
in the opposite direction, the surface must be reconciled between the two. To accom-
plish this, it is forced to create an absurdly long or twisted curve connection. The
result is a surface with obvious distortion. The types of distortion you are likely
to see are:

14.4.11.1 Twisting

This is the equivalent of a bow-tie face in polygons, where a pair of misaligned
tangents causes the bow-tie effect.

14.4.11.2 Rippling

This is caused by a full or partial row of control points that have been moved
together from a position between two rows of control vertices. This happens when
an artist doesn't realize that the selected vertices are close enough to others that
moving them will require severe distortion in a small area in order to maintain the
shape of the surface after the move.

14.4.11.3 Shredded

Excessively long tangents near the last row of control points before reaching a
surface boundary cause this. In an effort to reconcile the surface out tangents and
these long tangents, the surfaces maximum display subdivision limit is exceeded
beyond your application's ability to represent the geometry. When this happens,
your patch will appear to be shredded, or torn, around these long tangents. It is
not actually torn, as you will discover if you change the subdivision level to a
higher number.

14.4.11.4 Empty

It is possible to create a surface with such serious tangent problems that it will not
display at all. It is more common to make a legal surface and then modify it into this
state. Either way, the effect is caused for reasons similar to a shredded patch. In this
case however, the tangents are so bad that none of it can be displayed. If you ever
invoke a create patch command and don't seem to get a result, check your scene
node diagram to see if an object was created. Sometimes you will fi nd this is the
case. If so, you should delete the error geometry.