nurbs to polygon conversion

3deeder · 02-12-2006

Seems to me the nurbs to polygon conversion is used quite commonly in the tutorials. I cannot figure out the different options that are available in the option box. When do I use general vs. standard vs. count vs. control points. What is chord height ratio mean or fractional tolerance or 3D delta or minimal edge tolerance. The tutorials tell me to always use control points but I am not sure why. Any forthcoming explanation would be much appreciated since I can't find the information anywhere.

That's okay 3deeder. Not a problem.

I also get funny messages with maya help sometimes, since it's using some funky implementation of a web-server to display the pages. Whenever I get these errors, 9 times out of 10 I'm offline. When I hit help, the (dial-up) "connect to internet" dialog appears. My maya help also says it's unavailable, UNTIL I hit cancel or ok for connect to the internet. Not that that'll probably help you much.

But anyway, to go into the meanings of the options a little deeper..

"
Polygonal Modeling

Modify > Convert > NURBS to Polygons

Converts the selected NURBS objects to polygons.
Related topics

Convert NURBS surfaces to a polygon mesh
Modify > Convert > NURBS to Polygons >
Attaching multiple output meshes

Attaching multiple output meshes

Attach Multiple Output Meshes

Produces a single mesh, even if the original NURBS object consists of separate surfaces.

For example, if you convert a NURBS cube (which consists of six separate faces) with Attach Multiple Output Meshes on, the resulting polygon cube will consist of a single mesh. If Attach Multiple Output Meshes is off, the polygon cube will consist of six separate meshes.

Merge Tolerance

Controls how close vertices must be in order to be merged into a single mesh (when Attach Multiple Output Meshes is on).

Match Render Tessellation

When Match Render Tessellation is turned on it uses the existing render tessellation values that are set for the NURBS surface when converting it to a polygon mesh. As a result, the converted polygon mesh will match the rendered version of the NURBS surface. When History is turned on the NURBS to Polygon conversion will update whenever the NURBS render tessellation values are modified.

Outputting to triangles or quads

Type

Select the type of polygons to use when you convert NURBS geometry to polygonal data.

If you select Triangle (the default), 3-sided polygons are created. If you select Quads, 4-sided polygons are created.

Note

When you tessellate a trimmed NURBS surface, some 3-sided (triangle) polygons may be created along the trim edge even when the option is set to Quads.

Choosing a tessellation method

Tessellation means that you create a set of polygons from NURBS geometry. Each tessellation method provides you with options that let you control the resulting polygonal surface.

There are four tessellation methods: Standard fit, General, Count, and Control Points.
Standard fit

Standard Fit is the default tessellation method. It is "adaptive" tessellation, meaning that the following options are used to determine when to stop the tessellation.

For example, the tessellation stops at the Fractional Tolerance value you set. If there is an edge shorter than the Minimal Edge Length, the tessellation stops on that edge. If the surface is flat enough within the edge (the specified chord/height ratio is small enough), the tessellation stops there.

Chord Height Ratio

The Chord Height Ratio is the ratio between the maximum distance of the curve from the polygon edge used to approximate it and the chord length. The chord length is the linear distance between two polygon vertices.

Valid values range between 0 and 1, where larger values result in fewer polygon vertices.

For example, the default value, 0.1, means that the height must be larger than 1/10 of the chord length before additional edit points are created.

Fractional Tolerance

The Fractional Tolerance value determines the degree of accuracy maintained between the original surface and the interpolated polygonal surfaces. The default is to be accurate to within 0.01 units, where a unit refers to the current unit of linear measure (the default unit of measure is centimeters). Therefore, at no point will the polygonal surface be more than the tolerance distance away from the original NURBS surface.

In this next example, notice how you can enhance the polygonal surface's accuracy when you change the Fractional Tolerance value from 1 to 0.01.

Minimal Edge Length

Enter a value or use the Minimal Edge Length slider to set the minimum length of the edges of the triangles or quads that are created.

3D Delta

The 3D Delta value determines the 3D spacing for U and V isoparms on a surface that makes up the initial grid for the tessellation. In the following example, the 3D Delta value is changed from the default 0.1 to 1.0.

General

Set the Tessellation Method to General to display the following options.
Setting the initial tessellation controls

Unless Use Chord Height or Use Chord Height Ratio is turned on, a uniform tessellation is performed. Each span/surface is split into a number of polygons depending on the Number U and V values you set.

U Type/V Type

The U Type and V Type pop-up menu items let you specify whether you want to split the surface based on where the spans are (then split each span), or based on the parameterization of the whole surface.

Number U/Number V

Each span or surface is split into the number of polygons you specify here.

Specifying the secondary tessellation controls

If Use Chord Height or Use Chord Height Ratio is turned on, you can set a specific value for both the Chord Height and the Chord Height Ratio. A value greater than 0 results in fewer polygon vertices if the ratio on the curve is greater than the current value. For example, the default value, 0.1, means that the height must be larger than 1/10 of the chord length before additional edit points are created.

Turn Edge Swap on to produce triangles with the opposite orientation for the final quadrilateral.
Count

Set the Tessellation Method to Count to display the following slider.

Count slider

Use the Count slider to determine how many polygons the surface can be tessellated into. See the following examples.

Control Points

This tessellation method converts the NURBS model to polygons while matching the CVs of the original NURBS surface. There are no other options for this operation.

Notes

When you use the Control Points Tessellation Method:

* The Type option you set is ignored and the resulting polygon is in Quads by default.
* If you convert trimmed NURBS surfaces the surfaces convert as though they were not trimmed.

When you use the Attach Multiple Output Meshes option, the operation may fail to attach surfaces for a variety of reasons:

* When attaching a number of surfaces, and some of the surfaces are extremely small, the tolerance must be set to an appropriately small value or the attach operation will fail.
* When attaching multiple surfaces, all of the surfaces must be contiguous to within the user-defined tolerance. Separated regions cannot be attached in a single operation. Use the attach operation multiple times for this case; once for each contiguous region.
* The surfaces to be attached must have their normals facing in the same direction or the attach will fail."

- taken from Maya online help.

Btw, if you installed to the default directory, you can find the help-files in
C:\Program Files\Alias\Maya7.0\docs\Documents\Maya7.0\en_US

Good luck getting them to fire up correctly without the aforementioned web-server though. :p
Just open the buggers one by one.

If you open the index file in the above directory, the help system won't work properly, but it WILL tell you where to go looking for the file that's got the help in it that you need. - Least it does in here with firefox 1.5.08

Hope this is of some use to ya.
Simon

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