How Do You Create a Dynamic Surface Trimming Curve?


A dynamic surface trimming curve is just a simple variation of a curve that is attached to a surface. It is one in which the ends of the curve are either looped back around to itself in a closed loop or one in which the ends are dragged to touch the sides of the surface.


To trim a surface using an attached surface curve:

1.   Attach a curve to the surface you wish to trim.

2.   Move (drag) the ends of the curve to the boundary of the surface or loop the ends around to form a closed loop. Be careful to make sure that the ends meet or are right on the boundary edge. (see Note 6)

3.   Select the Surf-Trim Surf-Trim Surf command.

4.   First, pick the curve to use as the trimming curve.

5.   Second, pick an area on the surface that you wish to trim off.

6.   Repeat steps 4 and 5 as many times as you want. The Trim Surf command will stay active or current until you select another command.


Note 1: Actually, the surface is really not trimmed off – it is just turned off. You can still turn on the “trimmed-off” section of the surface and edit the surface edit points. Right click on the surface and select the box to display the trimmed off points.


Note 2: You can still drag any point on the trimming curve, but now the program will dynamically update the display of the trimmed surface.


Note 3: You can also move any visible edit point on the surface and watch the trimmed surface update. To edit the “trimmed off” surface edit points, you first need to “turn on” the display of the trimmed off points.


Note 4: If the ends of the trim curve are attached to the boundary of the surface, the program will convert the attached curve to a closed loop that includes part of the boundary of the surface. This won’t be noticeable unless you move one of the original boundary end points of the curve.


Note 5: You are allowed to create self-intersecting trimming curves, but the results will be “un-defined”. This means that the program won’t “crash”, but the results may not be what you expect. This is not allowed, however, by the OpenGL system used to display the rendered view of the model. If you have trim curves that intersect or overlap, then the OpenGL routines will not use them!


Note 6: You don’t have to close the trim curve or move it to an edge exactly. It just has to be within the “Parametric Gap Distance”, as defined in the Options-Other Options dialog box. Each surface is parametrically defined over a region between (0,0) to (1,1). The parametric gap distance is defined with these coordinates. A distance of .01 means that the gap distance is 1/100th of the size of the length or width of the surface.