higher order surfaces in opengl with nv_evaluators cass everitt [email protected]
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Higher Order Surfaces in OpenGL with NV_evaluators
Higher Order Surfaces in OpenGL with NV_evaluators
Cass Everitt
Why surfaces?Higher order primitives
Animation
Level of Detail
Bandwidth
Filtering
Overview
• What are the “general evaluators” defined NV_evaluators?
• How do they differ from standard OpenGL evaluators?
• Important new features• Per-edge tessellation control• Fractional tessellation• Support for all per-vertex attributes• Direct support for triangular patches
What are the “general evaluators” defined by NV_evaluators?
• Like OpenGL evaluators, general evaluators can be used evaluate polynomial and rational bivariate functions defined relative to the Bezier basis
Why New Evaluators?
• OpenGL evaluators have not seen widespread use due to a variety of problems• Difficult to optimize
• spec too strict on order of evaluation
• Incomplete support for all vertex attributes (particularly multiple texture coordinates)
• Difficult to use with adaptive LOD (tessellation scheme too rigid)
Why New Evaluators? (2)
• General evaluators address these issues• Ease of optimization
• Support only 4-component maps
• Evaluate full range of all enabled maps at once
• Support all vertex attributes• Provide more flexible tessellation schemes
• Eases varying LOD on adjacent patches
• Provides mechanism for smooth LOD
• Direct support for triangular patches
Per-Edge Tessellation Specification
• Easy to vary the tessellation across a patch
• Easy to render adjacent patches without cracks• glMapParameterfvNV(GL_EVAL_2D_NV,
GL_MAP_TESSELLATION_NV, edge_tess_ptr);
Four Independent Tessellation Factors
Generalized Tessellation isImportant
Fractional Tessellation
• Not only is each edge’s tessellation independent, the tessellation does not have to be integral values!
• Enable smooth changes in LOD
Fractional Number of Segments
12.0
12.5
13.0
13.5
14.0
• Because of symmetry, edges always match if their control points and tessellation number are the same
Support for All Vertex Attributes• A map can be applied to any numeric vertex
attribute (0-15) with values as described below
attribute number
per-vertex Parameter
per-vertex Command
4 secondary color glSecondaryColorEXT5 Fog coordinate glFogCoordEXT8 Texture coord 0 glMultiTexCoord9 Texture coord 1 glMultiTexCoord10 Texture coord 2 glMultiTexCoord11 Texture coord 3 glMultiTexCoord
0 vertex position glVertex1 vertex weights glVertexWeightEXT2 normal glNormal3 Primary color glColor
14 Texture coord 6 glMultiTexCoord15 Texture coord 7 glMultiTexCoord
12 Texture coord 4 glMultiTexCoord13 Texture coord 5 glMultiTexCoord
Triangular Patches are Necessary
3DStudioMax
Patch outlines
Hardware emulation
Direct Support for Triangular Patches
• Triangular patches can be converted to regular patches, but NV_evaluators provides support directly
