rfc2077.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 732 行 · 第 1/3 页

   c. The formal syntax for the subtypes of the model primary type
      should look like this:

      Media type name:          model
      Media subtype name:       xxxxxxxx
      Required parameters:      none
      Optional parameters:      dimensionality, state
                                (see below)
      Encoding considerations:  base64 encoding is recommended when
                                transmitting model/* documents through
                                MIME electronic mail.
      Security considerations:  see section 5 below
      Published specification:  This document.
                                See Appendix B for references to some of
                                the expected subtypes.
      Person and email address to contact for further information:
                                Scott D. Nelson <[e-mail]>
                                7000 East Ave.
                                Lawrence Livermore National Laboratory
                                Livermore, CA  94550

   The optional parameters consist of starting conditions and variable
   values used as part of the subtypes.  A base set is listed here for
   illustration purposes only and will be covered in detail as part of
   the respective subtypes:

  dimension := string ; a number indicating the number of dimensions.
                        This is used as a "hint" in selecting
                        applicable viewer programs.








Nelson, et. al.             Standards Track                     [Page 5]

RFC 2077                Model Primary MIME Types            January 1997


  state     := string ; "static" or "dynamic".  In "static", the
                        observer may move about, thus effecting
                        translations, rotations, pans, zooms, etc.
                        but the data does not change.  In "dynamic",
                        the data itself is manipulated via
                        skews, elongations, scales, etc.  Note that
                        time evolution is still a static operation
                        since it is just a translation along one of
                        the principal dimensions while the elongation
                        of a cube or object deformation are dynamic
                        operations.

      Note that this optional parameter list does not limit those
      specified by the various subtypes.

   d. The specific issues relating to the various subtypes are covered
      as part of the description of those specific subtypes.  The
      following is an example of a typical MIME header used for mail
      transport purposes:

         To:   [e-mail]
         From: [e-mail]
         Date: Fri, 30 Aug 96 13:33:19 -0700
         Content-Type: model/mesh; dimension="4"; state="static"
         Content-Transfer-Encoding: base64
         MIME-Version: 1.0
         Subject: model data file

         I1ZSTUwgVjEuMCBhc2NpaQojIFRoaXMgZmlsZSB3YXMgIGdlbmVyY...
         byBDb21tdW5pY2F0aW9ucwojIGh0dHA6Ly93d3cuY2hhY28uY29tC...
         IyB1c2VkIGluIHJvb20gMTkyICh0ZXN0IHJvb20pCiAgIAojIFRvc...
         .
         .
         .

5.  Security Considerations Section

   Note that the data files are "read-only" and do not contain file
   system modifiers or batch/macro commands.  The transported data is
   not self-modifying but may contain interrelationships.  The data
   files may however contain a "default view" which is added by the
   author at file creation time.  This "default view" may manipulate
   viewer variables, default look angle, lighting, visualization
   options, etc.  This visualization may also involve the computation of
   variables or values for display based on the given raw data.  For
   motorized equipment, this may change the position from the hardware's
   rest state to the object's starting orientation.




Nelson, et. al.             Standards Track                     [Page 6]

RFC 2077                Model Primary MIME Types            January 1997


   The internal structure of the data files may direct agents to access
   additional data from the network (i.e. inclusions); the security
   limits of whom are not pre-supposed.  Actions based on these
   inclusions are left to the security definitions of the inclusions.
   Further comments about the security considerations for the subtypes
   will be contained in each subtype's registration.

6. Authors' Addresses

      S. D. Nelson
      Lawrence Livermore National Laboratory,
      7000 East Ave., L-153,
      Livermore CA 94550, USA.
      E-Mail: [e-mail]

      C. Parks
      National Institute of Standards & Technology
      Bldg 220, Room B-344
      Gaithersburg, MD 20899, USA.
      E-Mail: [e-mail]

      Mitra
      WorldMaker
      1056 Noe
      San Francisco, CA 94114
      E-Mail: [e-mail]

7. Expected subtypes

   Table 1 lists some of the expected model sub-type names.  Suggested 3
   letter extensions are also provided for DOS compatibility but their
   need is hopefully diminished by the use of more robust operating
   systems on PC platforms.  The "silo" extension is provided for
   backwards compatibility.  Mesh has an extensive list of hints since
   the present variability is so great.  In the future, the need for
   these hints will diminish since the files are self describing.  This
   document is not registering these subtypes.  They will be handled
   under separate documents.













Nelson, et. al.             Standards Track                     [Page 7]

RFC 2077                Model Primary MIME Types            January 1997


Table 1.

   Primary/sub-type           Suggested extension(s)    Reference

   model/iges                         igs,iges              [8]
   model/vrml                         wrl                   [9]
   model/mesh                         msh, mesh, silo       [10]

   It is expected that model/mesh will also make use of a number of
   parameters which will help the end user determine the data type
   without examine the data.  However, note that mesh files are self-
   describing.

      regular+static, unstructed+static, unstructured+dynamic,
      conformal+static, conformal+dynamic, isoparametric+static,
      isoparametric+dynamic

   The sub-types listed above are some of the anticipated types that are
   already in use.  Notice that the IGES type is already registered as
   "application/iges" and that RFC states that a more appropriate type
   is desired.  Note that the author of "application/iges" is one of the
   authors of this "model" submission and application/iges will be re-
   registered as model/iges at the appropriate time.

   The VRML type is gaining wide acceptance and has numerous parallel
   development efforts for different platforms.  These efforts are
   fueled by the release of the QvLib library for reading VRML files;
   without which the VRML effort would be less further along.  This has
   allowed for a consistent data type and has by defacto established a
   set of standards. Further VRML efforts include interfaces to other
   kinds of hardware (beyond just visual displays) and it is proposed by
   those involved in the VRML effort to encompass more of the five
   senses.  Unlike other kinds of "reality modeling" schemes, VRML is
   not proprietary to any one vendor and should experience similar
   growth as do other open standards.

   The mesh type is an offshoot of existing computational meshing
   efforts and, like VRML, builds on a freely available library set.
   Also like VRML, there are other proprietary meshing systems but there
   are converters which will convert from those closed systems to the
   mesh type.  Meshes in general have an association feature so that the
   connectivity between nodes is maintained.  It should be noted that
   most modern meshes are derived from CAD solids files.








Nelson, et. al.             Standards Track                     [Page 8]

RFC 2077                Model Primary MIME Types            January 1997


8. Appendices

8.1 Appendix A -- extraneous details about expected subtypes

 VRML Data Types

   The 3D modeling and CAD communities use a number of file formats to
   represent 3D models, these formats are widely used to exchange
   information, and full, or lossy, converters between the formats exist
   both independently and integrated into widely used applications. The
   VRML format is rapidly becoming a standard for the display of 3D
   information on the WWW.

 Mesh Data Types

   For many decades, finite element and finite difference time domain
   codes have generated mesh structures which attempt to use the
   physical geometry of the structures in connection with various
   physics packages to generate real world simulations of events
   including electromagnetic wave propagation, fluid dynamics, motor
   design, etc.  The resulting output data is then post processed to
   examine the results in a variety of forms.  This proposed mesh
   subtype will include both geometry and scalar/vector/tensor results
   data.  An important point to note is that many modern meshes are
   generated from solids constructed using CAD packages.

   Motivation for mesh grew out of discussions with other communities
   about their design requirements.  Many CAD or scene descriptions are
   composed of a small number of complex objects while computational
   meshes are composed of large numbers of simple objects.  A 1,000,000
   element 3D mesh is small.  A 100,000,000 element 3D structured mesh
   is large.  Each object can also have an arbitrary amount of
   associated data and the mesh connectivity information is important in
   optimizing usage of the mesh.  Also, the mesh itself is usually