Stitching NURBS Curves

Stitch curves creates a triangle mesh between two NURBS curves, including curves, curves on surface, or surface isoparms. Once the two curves have been selected, a triangular stitch is generated and the density, (or the number of polygons) of the stitch can be interactively adjusted using the mouse or the keyboard.

How to Use

If you make a mistake, you can deselect the first curve by clicking on it again. This lets you select another curve to be the first curve in your stitch.

1

Select Stitch curves from the Polygons menu, or click this icon.

The system prompts:

Select first curve for curve stitching.

If you make a mistake when you select the second curve, select Del active from the Delete menu to delete the active polygonal stitch and start again.

2

Once selected, the system prompts:

Select second curve for curve stitching OR select last curve to undo selection.

3

Click to select the second curve. When both curves have been selected, a triangular stitch of polygons is generated between them and this new polyset is displayed as active.

Changing the Density of the Stitch

The prompt line displays Stitch curves statistics which are updated when you adjust the density. For example, the following is displayed for the above illustration.

(4 polygons) Enter new stitch criteria (prec1, 
prec2, offset): 

You can adjust the density of the stitch using one of the following methods.

Example

The side example shows what happens when you click and drag with the left mouse button along the first curve.

Using the Keyboard

You can also use the keyboard to assign curve precisions to each of the selected curves to specify the polygonal density of the entire stitch. Curve precisions for Stitch curves are integer values from 1 to 129 -a value of 1 generating the smallest number of vertices along a curve and a value of 129 generating the largest. A curve precision value of zero means "do not change".

Example

The side example shows what happens when you type the number 12 at the prompt:

Stitch offset and curve Direction

Click and drag the right mouse button or type a third value at the prompt line to adjust the stitch offset. This offset is the

Changing the stitch offset is useful on closed curves where you want to make some adjustments so that the stitch aligns a little better along the curves. If only one of the curves is closed, it is helpful to always select the closed curve as the second curve for the stitch.

number of vertices along the second selected curve to start stitching to, from the first vertex generated along the first selected curve.

An offset of zero will start stitching to the first vertex of the second curve. A zero offset is the default, and if keyboard input is used and a third value (for offset) is not entered, a zero offset is assumed.

Stitch vertices are generated in the direction of each curve, so if the two selected curves are in opposing directions the stitch will be `twisted'. If this occurs, use Edit > Undo to eliminate the twisted stitch, select Object Edit > Reverse > Reverse direction, select one of the curves to change its direction, select the Stitch curves tool, then try stitching the two curves again.

Vertex texture coordinates in a curve stitch

• Texture T values are constant for vertices generated along each curve with a value of 0.0 for T along the first curve and a value of 1.0 for T along the second curve.

• Vertices along the curve stitch are assigned texture S values from 0.0, for the first vertex generated on each curve, to 1.0, for the last vertex generated on each curve.

In some cases an appropriate offset can compensate for curves with opposing directions to create a non- twisted stitch.