Creating Polygons from NURBS

Create polygons convertsNURBS surfaces, trimmed NURBS surfaces, and faces to polysets. It adjusts the subdivisions of a polyset surface created with construction history.

How to Use

1

Select Pick > Object to pick the spline surface(s) from which you want to create the polysets or the polysets with construction history you want to change.

2

Select Create polygons from the Polygons menu, or click the icon.

The Spline Surface to Polygons control window is displayed.

Spline Surface to Polygons

From this window, you can set parameters for the tessellation process.

Once you have picked valid geometry, the Build button at the bottom of the Spline Surface to Polygons window is highlighted.

• NURBS surfaces, trimmed NURBS surfaces, faces, or polysets created in Alias with Surface tools that keep construction history can be converted to polygons.

• Polygonal data imported from other packages, polysets built using Polygons tools, and polysets created with Surface tools that do not have construction history cannot be converted to polygons. When these types of geometry are picked, the Build button is not highlighted.

3

Click the Build button to create polygonal versions of any non-polygonal data you have selected using the options you have specified in the window. Any polygonal data (with construction history) that you have selected is re-subdivided using the specified settings.

• The side example shows what happens when you use the default settings on a primitive cube.

Modifying a completed surface

After you click the Build button and the system tessellates the geometry, you can do one of the following:

• Modify any options in the Spline Surface to Polygons window to automatically reconvert or subdivide the objects using the new options.

• Pick geometry in another window to unpick any created geometry. The Build button becomes available again, letting you repeat the process on other geometry.

• Select another function or close the Spline Surface to Polygons window to select another tool.

Spline Surface to Polygons Options

Subdivision Type

Choose the subdivision method used for tessellation.

When the type of subdivision is selected, the Spline Surface to Polygons window is automatically updated to reflect the applicable parameters for that type of subdivision.

Adaptive Subdivision

If the Subdivision Type is ADAPTIVE, the Spline Surface to Polygons window expands to display the Adaptive Subdivision parameters.

The ADAPTIVE Subdivision Type subdivides the surface into polygons according to the amount of curvature the surface has. Each surface is converted into the minimum allowable number of polygons that satisfy the curvature parameter. If the curvature parameter cannot be satisfied by subdividing within the minimum subdivision parameter, the minimum subdivisions value is used.

Min

Specifies the minimum number of times each patch on the surface must be subdivided in either the U or the V parameter direction.

Max

Specifies the maximum number of times each patch on the surface may be subdivided in either the U or the V parameter direction.

Curve Threshold

Specifies how close an approximation the resulting polygons must be to the original surface. A higher value gives a better approximation of the surface, within the constraints of the Min/Max subdivision sliders.

Trim Density

Specifies how many triangles should be created along the edges of trimmed regions. Click to the right of the Trim Density heading to display the associated menu.

• LOW - c reates a minimal number of triangles along the trim.

• MEDIUM / HIGH - progressively creates more triangles.

• USE CRV PRECISION - uses the surface's curve precision value (set in Curve Edit > Curve precision -) to control the number of triangles along the trim curve.

Uniform Subdivision

If the Subdivision Type is UNIFORM, the Spline Surface to Polygons window expands to display the Uniform Subdivision parameters.

The UNIFORM Subdivision Type sub-divides each surface into polygons in a uniform manner (each patch is converted into a fixed number of polygons).

Along U

Specify how many times to subdivide each patch on the surface in the U parameter direction

Along V

Specify how many times to subdivide each patch on the surface in the V parameter direction

Renderer Settings

When you use this setting, the Render Globals window should have its Quality Type set to PER OBJECT. This lets you use different uniform or adaptive settings for individual surfaces during the tessellation process. Having the Quality Type set to GLOBAL and changing the Global Quality Parameters produces the same effect as using the UNIFORM or ADAPTIVE subdivision options.

If the Subdivision Type is RENDERER SETTINGS, Alias uses the subdivision control settings that appear in the Render Globals window (see Render > Globals).

Approximate Total

If the Subdivision Type is APPROXIMATE TOTAL, the Spline Surface to Polygons window expands to display the Approximate Total parameters.

Each selected hierarchy of surfaces is tessellated repeatedly with different adaptive subdivision parameters until Alias finds settings that produce a total polygon count close to the requested number of polygons.

If the requested polygon count cannot be satisfied within the given search tolerance, this function returns with either the closest tessellation that it found below the requested total, or the minimum number of polygons that it can possibly create. You can press Esc during the test tessellations if you want to abandon the search prematurely.

Total

The total number of polygons into which each spline surface is tessellated. Since the number of polygons that a spline surface can be tessellated into is not a continuous function, it may not be possible to achieve this number exactly.

To prevent an infinite search, the next two parameters let you specify tolerances. The search continues until one of the tolerances is satisfied.

Count Tolerance

Specify how close the number of polygons created can be to the value in Count Total. When Alias finds parameter settings that give a polygon count within Total +/- Count Tolerance, it stops searching.

Curve Tolerance

It may not be possible to find a tessellation that satisfies the requested total within the given count tolerance. This parameter lets the search end when the changes to the curvature threshold falls below this value.

The degree to which this function can find a value close to the requested total varies widely, depending on the type of surface being tessellated and its variations in curvature. In general, the function can get closer to the requested total for surfaces whose curvature varies widely and continually.

For example, the surface in the side diagram could be tessellated to within 8 polygons of a requested count fairly easily.

For surfaces whose curvature varies little (such as a sphere whose curvature is constant), the approximate total can be very far off. Using an adaptive minimum value of 1 and an adaptive maximum value of 4, a sphere can be tessellated into either 48, 336, 416, 608, or 960 triangle polygons. If the adaptive maximum value is lowered, it is also possible to produce counts of 144, 244, and 800.

Output Type

Choose the type of polygons created.

• QUADRILATERALS - 4-sided polygons are created.

• TRIANGLES - 3-sided polygons are created. If you select TRIANGLES, you can also choose a CONSTANT or ALTERNATE orientation of the triangles from the Tri Direction menu.

• CONSTANT - this sets the bias direction between triangle strips to be the same throughout the surface. The triangles will all have the same orientation.

• ALTERNATE - this changes the bias direction between triangle strips. If one row of triangles is oriented so their shared side travels from lower left to upper right, the next row of triangles is oriented so their shared side travels from upper left to lower right.

See the side examples:

Rebuild options

The Rebuild options let you set parameters to minimize the number of polygons created during NURBS or face to polygon conversion, or to minimize the number of polygons in a polygonal surface that has construction history.

Alias does this by approximating the existing geometry with another piece of geometry with the specified complexity. Using these options, you can specify exactly how many polygons should make up a surface.

Rebuild Surface

This option specifies whether or not the Rebuild operation should be carried out during conversion or subdivision. Choose YES to cause the surface to be rebuilt.

Rebuild U

This option is available only when using ADAPTIVE or UNIFORM Subdivision Types. It is used to specify how many sections the rebuilt surface should have in the U direction.

Rebuild V

This option is available only when using the ADAPTIVE or UNIFORM Subdivision Types. It is used to specify how many sections the rebuilt surface should have in the V direction.

Rebuild vs. Subdivision

It is important to note that the specified subdivision parameters will be used by the system regardless of whether the Rebuild option has been turned on or not.

For example, a surface that has been rebuilt to have only one span in U and one span in V will still create 4 quadrilaterals if the Uniform Subdivision parameters Along U and V have been set to 2, 2.

Under normal circumstances, if you want a model with a minimal polygon count, set the Adaptive Subdivision Min and Max parameters to 1 and 1, and use the Rebuild U and Rebuild V options to adjust the number of polygons created. In this case, the conversion or re-subdivision process results in:

RebuildU x RebuildV

quadrilaterals being created or...

RebuildU x RebuildV x 2