Viewing and Rendering Polygon Meshes
You can control the smoothness and other viewing properties of polygon meshes using geometry approximation. Geometry approximation also allows you to optimize playback and interaction of polygon meshes.
Applying and Editing Geometry Approximation
By default, the scene root node has a Geometry Approximation property which is inherited by every object in the scene. You can apply a local Geometry Approximation property to selected objects, hierarchies (in branch mode), groups, layers, and partitions. This allows you to control the display and rendering of different objects in different ways.
To apply a local Geometry Approximation property
1. Select an element: object, hierarchy (in branch mode), group, layer, or partition.
2. Choose Get > Property > Geometry Approximation from any toolbar.
To edit an existing Geometry Approximation property
1. Select an element.
2. Display its viewing or rendering properties, then click the Geometry Approximation tab.
Geometry Approximation on Polygon Meshes
Geometry Approximation is a multipurpose property that controls the display and rendering of polygon meshes, NURBS curves and surfaces, hair, and objects with displacement maps.
Only the settings on the Polygon Mesh property page are described in detail in this section—for more information see Geometry Approximation [Rendering].
Discontinuity — Faceted and Smooth Polygons
The Discontinuity parameters on the Polygon Mesh page of the Geometry Approximation property editor control whether the objects are faceted or smooth at the edges.
The illusion of smoothness is created by averaging the normals of adjacent polygons. When normals are averaged in this way, the shading is a smooth gradient along the surface of a polygon. When normals are not averaged, there is an abrupt change of shading at the polygon edges.
Automatic discontinuity lets you turn off the averaging of normals for sharper edges and the discontinuity Angle lets you specify how sharp edges must be before they appear faceted. If the dihedral angle (angle between normals) of two adjacent polygons is less than the Discontinuity Angle, the normals are averaged; otherwise, they are not averaged.
You can achieve different effects by adjusting these two parameters:
• If Automatic is on, then the Angle determines the threshold for faceted polygons.
• If Automatic is on and Angle is 0, the object is completely faceted.
• If Automatic is off, the object is completely smooth.
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For faster interaction, turn off Automatic discontinuity. This also avoids shading artifacts that can result if an edge crosses the discontinuity Angle, thus becoming smooth or faceted as the object deforms during animation. Instead of relying on Automatic discontinuity for faceted edges, mark the specific edges as hard — see Discontinuity on Selected Edges. |
Discontinuity and Displacement
Discontinuity between polygons also affects displacement at render time. If there is a discontinuity across an edge, the displacement occurs in different directions on either side of the edge — the result is “exploded” polygons!
If you are using displacement on polygon meshes, make sure that automatic discontinuity is off or that the angle threshold is high enough so that your meshes don’t come apart at the seams.
Discontinuity on Selected Edges
In addition to setting the geometry approximation for an entire object, you can make selected edges discontinuous by marking them as “hard.”
To mark edges as hard
1. Select some edges.
2. Choose Modify > Component > Mark Hard Edge/Vertex from the Model toolbar.
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• The Mark Hard Edge/Vertex command is also used to create sharp creases in subdivision surfaces. • You can select hard edges using the Crease Edge selection filter available from the Filter menu button of the Select panel. |
To toggle the display of hard edges
When you display hard edges, they are highlighted in dark blue. This may be useful if your display mode is set to Wireframe and you can’t see the effect of hard edges on the shading.
Do one of the following:
• To toggle the display of hard edges in a single 3D view, click the eye icon (Show menu) and choose Boundaries and Hard Edges.
or
• To toggle the display of hard edges in all open 3D views, choose Display > Attributes > Boundaries and Hard Edges from the main menu.
Polygon mesh edges that have been marked as hard are highlighted in dark blue, while boundary edges are light blue. You may need to turn off Display > Attributes > Clusters (except Sample) to see the effect on hard edges.
Subdivisions
The OGL Level, and Render Level parameters are used with subdivision surfaces. For details, see Subdivision Surfaces.
The OGL Triangulation Update parameter on the Polygon Mesh page of the Geometry Approximation property editor controls the balance between speed and accuracy when interacting with polygon mesh objects as well as when playing back animation in the OpenGL display modes such as Hidden Line, Constant, Shaded, and Textured. There are four settings:
• Recreate at each refresh is the most accurate and always recalculates triangulation. However, 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.
The Angle-based normals option controls the method used to calculate the normal at each vertex when rendering:
• 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 many situations.
• 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.
Displacement Mapping
The options on the Displacement page of the Geometry Approximation property editor are used when you apply displacement maps to polygon meshes and other objects. For more information, see Creating a Displacement Map [Texturing].
Autodesk Softimage v.7.5