Controlling Tangency

The choice of different data point types provides the user with a convenient and intuitive means of specifying tangency conditions at points along a curve. By using tangent controls it may be possible to reduce the number of data points defining a curve, or to ensure that specific requirements (such as flatness) are fulfilled. The following example is the result of thinning data points from the aft section considered in Removing Data Points, but also specifying tangents at certain points. The result is a further reduction in the number of data points:

Figure 3:55. Using tangent controls to reduce the number of data points.

The ends of tangent controls are selected and dragged or nudged in exactly the same way as data points.

The specific type of a data point can be changed using the dropdown boxes in the cells of the Point Type column in the Data Points grid:

Figure 3:56. The Point Type column’s drop down list.

Alternatively, select a data point graphically and click the <Tab> key to cycle through the seven types. The order of this cycling reflects an attempt to minimize the changes to the curve geometry between consecutive changes:

Figure 3:57. Cyclic sequence for the seven different data point types.

When changing from a type without specified tangent information to one where it is required, the information is calculated from the curve itself, prior to effecting the change. Note that curve end points must be of type Ordinary or Tangent.

Tangents can be specified numerically in the Data Points grid. Columns pre , pre and pre specify the 'pre' tangent direction (that means, of the curve running into the data point) as angles of rotation about the x, y and z axes. The angles are measured as follows:

Figure 3:58. Tangent angle definition.

Columns post , post and post specify the 'post' tangent direction (that means, of the curve running out of the data point) in the same way.