Geometry Approximation Property Editor

Hardware Display

These settings do not affect the geometry or rendering of objects; they are used for display purposes only. In particular, objects on paths will follow the curve as mathematically defined, not as displayed by the path line.

In all cases, the step value determines how many straight segments to use to approximate the exact shape of each NURBS segment. The higher the step value, the closer the approximation and you get smoother results. However, the trade-off is a longer processing time.

Setting the number of steps between knots on a surface — Steps = 3 (A) versus Steps = 1 (B).

Parametric - Steps

Surface U, V	Defines the number of steps to draw between knots in U and V for surfaces. This type of approximation uses non-adaptive sampling between parametric intervals.
Curve U	Defines the number of steps between knots in U for drawing curves.
Surf. Curve	Defines the number of steps between knots in U for drawing surface curves like isolines, boundaries, trims, and projected surface curves.

Surface, Surface Trim, Displacement

The settings on the Surface, Surface Trim, and Displacement tabs are similar.

Surface controls how NURBS surfaces are rendered.
Surface Trim controls how the trimmed edges of NURBS surfaces are rendered.
Displacement controls how displacement maps are rendered, if you are using them. It affects both NURBS surfaces and polygon meshes. For information about displacement maps, see Displacement Maps [Texturing]. Displacement approximation is also influenced by the Surface approximation values on NURBS surfaces.

You first choose an approximation method, and then adjust the Refinement Settings as necessary. The Refinement Settings are different for each appromixation method. The Length/Distance/Angle and Fine methods also let you set Subdivision Limit - Recursive Steps. Additionally, the Displacement tab lets you set the maximum displacement.

[approximation method]

Parametric: Useful if you have a largely flat-based surface as well as some displacement. This method operates in UV space. See Surface, Surface Trim, Displacement.

Length/Distance/Angle (LDA): Combines three methods. It operates in camera space or raster space.

Length approximation — Especially useful when combined with view dependency because it lets you control the approximate number of triangles per pixel.
Distance approximation — Used on objects that have a gentle curve, since they don't usually have two neighboring triangles with a large normal distance.
Angle approximation — Used on objects that have localized areas with high curvature, such as a cube with rounded edges.

See LDA (Length/Distance/Angle) Approximation.

Fine approximation — Used on objects that require very detailed approximation. This type of approximation is well-suited to displacement mapping. It operates in camera space or raster space. See Fine Approximation.

Max Displ.

Specifies the maximum displacement value. This value must be set correctly for displacement maps to work properly. If the displacement value is higher than the Max Displacement value, displacement will be clipped. For example, if you have applied a displacement map to an object using a displacement value of 3, but Max Displ. Is set to 2, the displacement will be clipped to 2.

This parameter is available on the Displacement tab only.

Parametric Approximation

The Parametric method of surface approximation defines the U and V steps of the selected object as seen by the geometry. This type of approximation uses non–adaptive sampling between parametric intervals.

The surface on the left uses a Step Value of 1, while the surface on the right uses a Step Value of 3. Notice how using more steps determines how smooth an object's surface looks.

Changing these values changes the number of steps in the object's surface. These parameters remain the same throughout the animation, and if the camera gets very close to the object the steps might be seen.

Refinement Settings for Parametric

U, V Step and Step

Changing these values changes the number of steps in the object's surface or surface trim. These parameters remain the same throughout the animation, and if the camera gets very close to the object the steps might be seen.

NoteThe Displacement tab uses a single Step parameter that is used for both the U and V directions.

LDA (Length/Distance/Angle) Approximation

The LDA approximation method is a curvature-dependent approximation based on subdivisions by length, distance, and angle. This type of approximation uses adaptive sampling between parametric intervals.

Refinement Settings for Length/Distance/Angle

Length	Subdivides the surface or curve so that no edge length of the tessellation (object coordinates) exceeds the Length parameter. Length is given as a distance in camera space or as a fraction of a pixel diagonal in raster space (View Dependent). Small values such as 1 are recommended.
Distance	Specifies the maximum distance between the tessellation and the actual curve or surface. The value is a distance in camera space, or a fraction of a pixel diagonal in raster space (View Dependent). As a starting point, a small distance such as 0.1 is recommended.
Angle	Specifies the maximum angle in degrees between: Normals of adjacent tiles of a polygon or its displacement. Normals of the tessellation of a surface or its displacement. Tangents of adjacent segments of the curve approximation. Large angles such as 45 degrees are recommended.
Stopping Condition	Length, Distance and Angle: Tessellation continues until the Length, Distance and Angle requirements have all been met. Length, Distance or Angle: Tessellation stops when any of the Length, Distance, or Angle requirement has been met.
View Dependent	Switches measurements from object space (off) to raster space (on). For example, an ocean object with displacement-mapped waves would benefit from View Dependent since the object is finely tessellated near the camera but not far from it.

Fine Approximation

The Fine approximation method is a curvature-dependent approximation based on subdivisions by length. It uses a subset of the other approximation types' parameters. Rather than using the minimum possible number of triangles to achieve the desired quality of approximation, Fine approximation can use a large number of triangles to create extremely detailed approximation, without consuming excessive amounts of memory.

It does so by splitting approximated objects in to smaller "sets" that are tessellated individually, and thus do not have the same memory requirements as the entire object.

Refinement Settings for Fine

Length

Subdivides the surface or curve so that no edge length of the a tessellated triangle exceeds this value.

When View Dependent is disabled, Length is calculated in object space, and edges that exceed the Length value in Softimage units are subdivided until all edges are shorter than the Length value, or until the Max subdivision level (specified in the Subdivision Limit parameters) is reached.

When View Dependent is enabled, Length is calculated in raster space, and edges that exceed the Length value, as a fraction of a pixel diagonal, are subdivided until all edges are shorter than the Length value, or until the Max subdivision level (specified in the Subdivision Limit parameters) is reached.

View Dependent

Switches measurements from object space (off) to raster space (on).

For example, an ocean object with displacement-mapped waves would benefit from View Dependent since the object is finely tessellated near the camera but not far from it.

Sharpness (%)

Controls whether triangles generated by fine approximation use interpolated or geometric normals.

When the Sharpness value is set to 0, triangles generated by fine approximation use interpolated normals, producing a smooth, rounded look.

When the Sharpness value is set to 100, triangles generated by fine approximation use geometric normals, producing a crisp, faceted look.

Setting Sharpness to values between 0 and 100 interpolated between the completely smooth look that you get when Sharpness is set to 0 and the completely faceted look you get when it's set to 100. For example, a value of 50 results in a half-smooth/half-faceted look.

Sharpness 0%

Sharpness 50%

Sharpness 100%

Subdivision Limit - Recursive Steps

To prevent massive amounts of subdividing, a limit can be assigned. These parameters are available for the Length/Distance/Angle and Fine methods.

Min	Specifies the minimal triangulation (or recursion levels of adaptive subdivision).
Max	Edges are further subdivided until they satisfy the given criterion or the Max subdivision level is reached.

Polygon Mesh

Controls how polygon mesh objects are displayed and rendered.

Discontinuity

Controls the continuity of the shading between polygons. For more details, see Controlling Discontinuity Using Geometry Approximation [Polygon Modeling].

Automatic	Creates a discontinuity in the shading as if there is a sharp edge between two polygons if their dihedral angle (the angle between their normals) is greater than the Discontinuity Angle. Otherwise, normals are averaged to give an illusion of smoothness.
Angle	Specifies the angle above which edges are sharp when Automatic Discontinuity is on. For a completely faceted polygon mesh object, set this value to 0.

Subdivision

Controls the geometry subdivision level for display and rendering. For more details, see Subdivision Levels [Polygon Modeling].

OGL Level

The number of times to subdivide the surface iteratively for display in the 3D views.

Render Level

The number of times to subdivide the surface iteratively when rendering.

Each iteration increases the number of polygons, which can result in very dense and heavy geometry.

OGL Triangulation Update

Controls the balance between accuracy and speed when drawing polygons in the OpenGL display modes such as Hidden Line, Constant, Shaded and Textured.

Recreate at each refresh is the most accurate and always recalculates triangulation. Playback and interaction are slowest in this mode.
Skip during interaction does not recompute triangulation of polygons while you are working, and waits for you to pause before retriangulating. The display updates faster as you modify objects, but you may see some artifacts in the OpenGL views, for example, when a polygon becomes concave while manipulating points. Triangulation is still recomputed when required during playback.
Skip during interaction and playback does not recompute triangulation of polygons while you are working nor while animation is playing back. Again, this mode is faster but you may see some artifacts in the OpenGL views.

Update only when topology changes never retriangulates unless you modify the topology, for example, by adding or removing points. This mode is the fastest but potentially creates the most artifacts in the OpenGL views.

Lack of retriangulation can produce artifacts while moving points.

Angle-based normals (slower)

Controls how the normals at each point are calculated when rendering:

When on, Softimage uses the average of the adjacent polygon angles, weighted according to the angle of the corresponding polygon at the point. The result is more accurate in some situations but takes longer to calculate.
When off, Softimage uses the unweighted average of the adjacent polygon normals. This option is substantially faster. Although it is less accurate, the result may not be noticeable in most situations.

Hair

These parameters determine the subdivision approximation for viewing or rendering hair with the Hair Renderer shader or other Softimage shaders.

Subdivisions

Curve Type

Generates a general Bezier curve for the hair. You can specify the degree of curve approximation.

Linear creates a linear curve approximation (straight segments). If you select this option, the OGL Level and Render Level options are unavailable because you cannot approximate straight lines with more lines.
Quadratic creates a quadratic curve approximation (smooth). This option doesn't guarantee a continuous curve throughout the entire length of the hair. If the original line segments are at too steep of an angle, there may be a cusp where they meet.

If you select this option, additional points required to construct the curves are generated. This results in one extra control point for each generated hair point, less one.
Cubic creates a cubic curve approximation (smoothest), guaranteeing continuity between hair segments. If you select this option, additional points required to construct the curves are generated. This results in two new control points.

OGL Level

The subdivision level for approximating the curves when viewing hair in the OpenGL views in the viewports.

This parameter generates 2^Level line segments for approximation. For example, if this value is 0, the curves are approximated by 1 line segment; for a value of 1, by 2 segments; for a value of 2, by 4 segments, etc.

The slider displays a range of 0 to 4, but you can enter a value up to 7 in the text box.

Render Level

The subdivision level for approximating the curves when rendering hair (in the render region or to file).

This parameter generates 2^Level line segments for approximation. For example, if this value is 0, the curves are approximated by 1 line segment; for a value of 1, by 2 segments; for a value of 2, by 4 segments, etc.

The slider displays a range of 0 to 4, but you can enter a value up to 7 in the text box.