[X3D-Public] Scene Authoring Hints: meshes
brutzman at nps.navy.mil
Fri May 29 10:20:09 PDT 2009
I've added a new section to Scene Authoring Hints on meshes. Thanks to
Jeff Weekley for review feedback. Comments and improvements welcome.
* 3D modeling tools with 3D interfaces make it easy to create unusual and irregular geometry, but it often remains hard to size them correctly or shape them precisely. Use reference grids and cross-section slices to check that dimensions are accurate when creating new models.
* Polygon meshes can be created in many different tools. Results are easily exported to X3D, VRML, or other compatible formats. The X3D Resources page lists current conversion and translation tools.
* Exporting a model mesh to X3D polygons usually means that the shape(s) can only be transformed and scaled, with no internal animation modifications at run time. Therefore it is a good idea to split up any pieces of geometry that need to be animated separately.
* Scaling in meters is usually preferred. Using other units (feet, inches, millimeters etc.) is OK if that preserves the original data exactly. Simply adding a parent Transform node with uniform scaling can restore overall units to meters, but watch out for additional scale conversions of event streams if any internal components need animation.
* When maintaining models in an archive, keep both the original version and the .x3d version in case future modifications are needed. Saving documentation, README notes about construction and metadata information is also important.
* Exported polygon meshes often have geometric problems: misaligned front/back normals, clockwise vice counterclockwise (ccw) definition, one-sided versus two-sided, non-coplanar quadrilaterals, etc.
* Running output meshes through one or more "cleanup" tools can help identify and correct these otherwise-obscure errors. Useful tools include Chisel, Meshlab, X3D-Edit quality assurance (QA), Xj3D CAD filter, etc.
* Tool names for nodes are often quite obscure. Assign clear names for DEF/USE labels of top-most parts in order to make animation and inlining easier.
* Many of these issues are common to the export of Computer Aided Design (CAD) models from various formats into X3D. The X3D CAD Working Group is a good source of resources, tools and best practices.
* Geometric compression in the .x3db binary encoding can reduce unnecessary significant digits, merge coplanar polygons, quantize colors and normals, etc.
* NURBS and other parametric surfaces are often preferable to polygonal meshes since they provide exact representations that are concise and work at any level of detail. Nevertheless polygon meshes may be preferable for public distribution if complex source needs to be protected. Including both types of representations within Switch or LOD nodes is another possibility. Applying XML Security can support secure distribution of sensitive portions of an X3D model (such as the high-value NURBS representation).
all the best, Don
Don Brutzman Naval Postgraduate School, Code USW/Br brutzman at nps.edu
Watkins 270 MOVES Institute, Monterey CA 93943-5000 USA work +1.831.656.2149
X3D, virtual worlds, underwater robots, XMSF http://web.nps.navy.mil/~brutzman
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