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MGF PACKAGE DESCRIPTION |
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SCCSid "$SunId$ LBL" |
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|
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This package includes a description and parser for a new scene |
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description standard, called for the lack of a better name, MGF |
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for Materials and Geometry Format. It was developed by Greg |
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Ward of the Lawrence Berkeley Laboratory <[email protected]> with |
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help and advice from Rob Shakespeare of Indiana University |
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<[email protected]>, Ian Ashdown of Ledalite Corporation |
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<[email protected]> and Holly Rushmeier of the National |
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Institute for Standards and Technology <[email protected]>. |
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|
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The language itself is described in the file "spec.txt", and |
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the included Makefile should make building the parser library |
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fairly straightforward on most systems. What's left then, is |
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explaining the why and how of using this package. |
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|
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The initial purpose of developing a scene description standard |
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was for inclusion in the Illumination Engineering Society's (IES) |
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standard data representation for luminaires. It occurred to us |
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early on that such a standard might have broader applications, |
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so an effort was made to create a fairly general description |
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language, while keeping it as simple as possible for the people |
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who have to create descriptions with it as well as the programmers |
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who have to support it. |
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|
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Why create a new standard rather than exploiting an existing one? |
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Some of the rationale for our decision is explained at the end of |
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the specification document, but it mostly boils down to materials. |
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As easy as it is to describe physically valid materials, most |
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scene description languages cannot do it. The material specification |
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included in the MGF standard may not be perfect, but at least it's |
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physically plausible. Furthermore, we are committed to making any |
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future modifications to the standard backwards-compatible -- a rather |
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tricky proposition. |
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|
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This takes us to the how of supporting this new standard. The basic |
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approach is to use the standard parser, which does a lot of the work |
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in supporting the language itself. The programmer tells the parser |
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which entities it will support, and the parser does the rest. |
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That way, it isn't necessary to modify the program when a new version |
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of the standard comes out; all one has to do is link to the new |
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standard's parser. (The include file will change as well, so it's |
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not QUITE that simple, but close.) |
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|
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There are two ways to support the language, by linking the parser to |
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the program itself, or by linking the parser to a translator program |
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that expresses MGF entities in the native scene description format. |
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The differences in the two approaches are slight, and we will mention |
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them following a general explanation of the parser and support library. |
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|
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The Parser |
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========== |
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The MGF parser is written in ANSI-C (though the -DNOPROTO flag may be |
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used to get back K&R compatibility). All of the declarations and |
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definitions needed are in the single include file "parser.h". This |
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file is a good place to look for details on using the various support |
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routines as well. The parser itself is parser.c, though it relies for |
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some translations on other C modules. These same support routines will |
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no doubt be useful for applications programmers, and we will explain |
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some of them in the following sections. |
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|
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Initializing the parser is the most important part of writing an MGF |
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program, and it is done through the mg_ehand array and a call to mg_init. |
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The global mg_ehand variable is an array of pointers to entity handler |
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functions. The arguments to these functions is always the same, an |
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argument count and an array of argument pointers (ala main). The return |
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value for these integer functions is one of the error codes defined in |
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parser.h, or MG_OK if the entity was handled correctly. You must |
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set the appropriate entries for the entities you can support, then call |
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mg_init to fill in the rest. Most of the entities you cannot support |
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will be translated into (approximately) equivalent ones you can. |
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Entities that have no equivalent (such as color), will be safely |
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ignored on the input. If you have specified support for some entities |
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without offering support to their prerequisites, mg_init will report an |
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error and exit. |
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|
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Once the parser has been properly initialized, MGF input files may be |
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loaded at will with the mg_load call. This function takes a single |
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argument, which is the name of the MGF file. (The NULL pointer may be |
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used to specify standard input.) The behavior of the parser in part |
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depends on input history, so the mg_clear call should be used after |
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each file if starting fresh is important. This also frees any data |
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structures used by the parser, which may be desirable if the program |
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is going to do something after loading besides exit. |
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|
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Support Functions |
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================= |
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In translating unsupported entities, the parser makes use of a number |
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of support functions, contained in associated C modules. The most |
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important of these are in context.c, which includes three handler |
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functions that can support all color, material and vertex entities. |
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To understand what these functions do, it is necessary to know a |
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little about the MGF language itself, so please familiarize yourself |
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with it now if you haven't already. (See the file "spec.txt".) |
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|
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Context Support |
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=============== |
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The MGF language defines three named contexts, the current vertex, |
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the current color and the current material. (The current color is |
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used mostly for setting parameters in the current material.) There |
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are three handler routines defined in context.c, and they can handle |
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all entities related to these three contexts. The simplest way to |
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support materials, for example, is to initialize the mg_ehand array |
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such that the MG_E_MATERIAL, MG_E_RD, MG_E_RS, etc. entries all point |
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to c_hmaterial. Then, whenever a material is needed, the global |
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c_cmaterial variable will be pointing to a structure with all the |
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current settings. (Note that you would have to also set the color |
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mg_ehand entries to c_hcolor if you intended to support color |
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materials.) |
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|
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In addition to the three handler functions, context.c contains a |
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few support routines that make life simpler. For vertices, there |
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is the c_getvertex call, which returns a pointer to a named vertex |
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structure (or NULL if there is no corresponding definition for the |
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given name). This function is needed for support of most surface |
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entities. For color support, there is the analogous c_getcolor call, |
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and the c_ccvt routine, which is used to convert from one color |
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representation to another (e.g. spectral color to xy chromaticity |
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coordinates). Also, there is a function called c_isgrey, which |
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simply returns 1 or 0 based on whether the passed color structure |
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is close to grey or not. Finally, there is the c_clearall routine, |
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which clears and frees all context data structures, and is the |
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principle action of the parser's mg_clear function. |
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|
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Transform Support |
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================= |
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If your program is supporting any geometry at all (and what would be |
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the point if it wasn't?) you will need to support the transform |
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entity (MG_E_XF). This would be tricky, if it weren't for the support |
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routines provided, which make the task fairly painless. First, there |
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is the transform handler itself, xf_handler. Just set the MG_E_XF |
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entry of the mg_ehand array to this function. Then, anytime you want |
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to transform something, call one of the associated functions, xf_xfmpoint, |
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xf_xfmvect, xf_rotvect or xf_scale. These functions transform a 3-D |
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point, 3-D vector (without translation), rotate a 3-D vector (without |
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scaling) and scale a floating-point value, respectively. |
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|
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Object Support |
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============== |
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The MGF language includes a single entity for naming objects, MG_E_OBJECT. |
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It is mostly provided as a convenience for the user, so that individual |
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geometric parts may be easily identified. Although supporting this entity |
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directly is possible, it's hierarchical nature requires maintaining a stack |
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of object names. The object handler in object.c provides this functionality. |
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Simply set the MG_E_OBJECT entry of the mg_ehand array to obj_handler, |
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and the current object name list will be kept in the global array obj_name. |
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The number of names is stored in the global obj_nnames variable. To clear |
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this array (freeing any memory used in the process), call obj_clear. |
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|
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Loading vs. Translating |
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======================= |
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As mentioned in the introduction, the parser may be used either to load |
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data into a rendering program directly, or to get MGF input for translation |
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to another file format. In either case, the procedure is nearly identical. |
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The only important difference is what you do with the parser data structures |
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after loading. For a translator, this is not an issue, but rendering |
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programs usually need all the memory they can get. Therefore, once the |
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input process is complete, you should call the mg_clear function to free |
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the parser data structures and return to an initialized state (i.e. it |
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is never necessary to recall the mg_init routine). |
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|
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Also, if you use some of the support functions, you should call their |
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specific clearing functions. For the transform module, the call is |
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xf_clear. For the object support module, the call is obj_clear. The |
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context routines use the c_clearall function, but this is actually |
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called by mg_clear, so calling it again is unnecessary. |
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|
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Linking Vertices |
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================ |
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Although the MGF language was designed with linking vertices in mind, |
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there are certain aspects which make this goal more challenging. |
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Specifically, the ability to redefine values for a previously named |
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vertex is troublesome for the programmer, since the same vertex can |
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have different values at different points in the input. Likewise, the |
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effect of the transform entity on surfaces rather than vertices means |
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that the same named vertex can appear in many positions. |
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|
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It is not possible to use the parser data structures directly for |
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linking vertices, but we've taken a couple of steps in the support |
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routines to make the task of organizing your own data structures a |
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little easier. First, there is a clock member in the C_VERTEX |
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structure that is incremented on each change. (The same member is |
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contained in the C_COLOR and C_MATERIAL structures.) Second, the |
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current transform (pointed to by xf_context) contains a unique |
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identifier, xf_context->xid. This is a long integer that will be |
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different for each unique transform. (It is actually a hash key on the |
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transformation matrix, and there is about 1 chance in 2 billion that |
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two different matrices will hash to the same value. Is this a bug? |
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I guess it depends on how long the programmer lives -- or vice versa.) |
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|
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There are two ways to use of this additional information. One |
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is to record the vertex clock value along with it's id and the |
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current xf_context->xid value. If another vertex comes along with |
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the same name, but one of these two additional values fails to match, |
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then it (probably) is a different vertex. Alternatively, one can reset |
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the clock member every time a new vertex is stored. That way, it is |
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only necessary to check the clock against zero rather than storing this |
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value along with the vertex name and transform id. If the name and |
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transform are the same and the clock is zero, then it's the same vertex |
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as last time. |
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|
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Yet another approach is to ignore the parser structures entirely and |
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focus on the actual vertex values. After all, the user is not compelled |
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to reuse the same vertex names for the same points. It is just as likely |
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that the same vertices will appear under different names, so that none |
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of the above would help to merge them. The most sure-fire approach to |
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linking identical vertices is therefore to hash the point and normal |
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values directly and use the functions in lookup.c to associate them. |
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You will have to write your own hash function, and we recommend making |
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one that allows a little slop so that nearly identical points hash to |
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the same value. |
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|
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Examples |
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======== |
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Two example translator programs are included with this package. |
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|
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The simplest is a translator from MGF to MGF called mgfilt.c, which |
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produces on the standard output only those entities from the standard |
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input that are supported according to the command line arguments. For |
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example, one could remove everything but the raw, flat polygonal |
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geometry with the following command: |
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mgfilt v p f xf < any.mgf > faces.mgf |
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|
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Note that the xf entity must also be included, for its support is |
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required by all geometric entities. |
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|
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The second translator converts from MGF to the Radiance scene description |
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language, and is a more practical example of parser use. Unfortunately, |
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we did not include all of the support functions required by this translator, |
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so it serves as a source code example only. If you wish to get the rest |
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of it because you intend to run it, contact Greg Ward <[email protected]> |
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and he'll be happy to provide you with the missing pieces. |
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|
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Copyright |
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========= |
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At this point, the legal issues related to this parser have not been |
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worked out. The intent is to offer it free of charge to all those who |
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wish to use it (with no guarantees, of course). However, we may decide |
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that copyright protections are necessary to prevent unauthorized versions |
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of the parser, which do not properly support the MGF standard, from |
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getting spread around. Since this is a pre-release, we trust that you |
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will not share it with anyone without getting our permission first. |
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|
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Questions |
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========= |
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Questions should be directed to Greg Ward <[email protected]>, who will be |
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happy to offer any reasonable assistance in using this standard. (Greg's |
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telephone is 1-510-486-4757, fax 1-510-486-4089.) |
