| 46 |
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There are two ways to support the language, by linking the parser to |
| 47 |
|
the program itself, or by linking the parser to a translator program |
| 48 |
|
that expresses MGF entities in the native scene description format. |
| 49 |
< |
The differences in the two approaches are slight, and we will explain |
| 49 |
> |
The differences in the two approaches are slight, and we will mention |
| 50 |
|
them following a general explanation of the parser and support library. |
| 51 |
|
|
| 52 |
|
The Parser |
| 63 |
|
Initializing the parser is the most important part of writing an MGF |
| 64 |
|
program, and it is done through the mg_ehand array and a call to mg_init. |
| 65 |
|
The global mg_ehand variable is an array of pointers to entity handler |
| 66 |
< |
functions. The arguments to these functions is always the same, an |
| 66 |
> |
functions. The arguments to these functions are always the same, an |
| 67 |
|
argument count and an array of argument pointers (ala main). The return |
| 68 |
|
value for these integer functions is one of the error codes defined in |
| 69 |
|
parser.h, or MG_OK if the entity was handled correctly. You must |
| 107 |
|
c_cmaterial variable will be pointing to a structure with all the |
| 108 |
|
current settings. (Note that you would have to also set the color |
| 109 |
|
mg_ehand entries to c_hcolor if you intended to support color |
| 110 |
< |
materials.) |
| 110 |
> |
materials.) A list of related mg_ehand assignments is given below: |
| 111 |
|
|
| 112 |
+ |
mg_ehand[MG_E_COLOR] = c_hcolor; |
| 113 |
+ |
mg_ehand[MG_E_CCT] = c_hcolor; |
| 114 |
+ |
mg_ehand[MG_E_CMIX] = c_hcolor; |
| 115 |
+ |
mg_ehand[MG_E_CSPEC] = c_hcolor; |
| 116 |
+ |
mg_ehand[MG_E_CXY] = c_hcolor; |
| 117 |
+ |
mg_ehand[MG_E_ED] = c_hmaterial; |
| 118 |
+ |
mg_ehand[MG_E_MATERIAL] = c_hmaterial; |
| 119 |
+ |
mg_ehand[MG_E_NORMAL] = c_hvertex; |
| 120 |
+ |
mg_ehand[MG_E_POINT] = c_hvertex; |
| 121 |
+ |
mg_ehand[MG_E_RD] = c_hmaterial; |
| 122 |
+ |
mg_ehand[MG_E_RS] = c_hmaterial; |
| 123 |
+ |
mg_ehand[MG_E_SIDES] = c_hmaterial; |
| 124 |
+ |
mg_ehand[MG_E_TD] = c_hmaterial; |
| 125 |
+ |
mg_ehand[MG_E_TS] = c_hmaterial; |
| 126 |
+ |
mg_ehand[MG_E_VERTEX] = c_hvertex; |
| 127 |
+ |
|
| 128 |
|
In addition to the three handler functions, context.c contains a |
| 129 |
|
few support routines that make life simpler. For vertices, there |
| 130 |
|
is the c_getvertex call, which returns a pointer to a named vertex |
| 137 |
|
simply returns 1 or 0 based on whether the passed color structure |
| 138 |
|
is close to grey or not. Finally, there is the c_clearall routine, |
| 139 |
|
which clears and frees all context data structures, and is the |
| 140 |
< |
principle action of the parser's mg_clear function. |
| 140 |
> |
principal action of the parser's mg_clear function. |
| 141 |
|
|
| 142 |
|
Transform Support |
| 143 |
|
================= |
| 164 |
|
The number of names is stored in the global obj_nnames variable. To clear |
| 165 |
|
this array (freeing any memory used in the process), call obj_clear. |
| 166 |
|
|
| 167 |
+ |
Loading vs. Translating |
| 168 |
+ |
======================= |
| 169 |
+ |
As mentioned in the introduction, the parser may be used either to load |
| 170 |
+ |
data into a rendering program directly, or to get MGF input for translation |
| 171 |
+ |
to another file format. In either case, the procedure is nearly identical. |
| 172 |
+ |
The only important difference is what you do with the parser data structures |
| 173 |
+ |
after loading. For a translator, this is not an issue, but rendering |
| 174 |
+ |
programs usually need all the memory they can get. Therefore, once the |
| 175 |
+ |
input process is complete, you should call the mg_clear function to free |
| 176 |
+ |
the parser data structures and return to an initialized state (i.e. it |
| 177 |
+ |
is never necessary to recall the mg_init routine). |
| 178 |
+ |
|
| 179 |
+ |
Also, if you use some of the support functions, you should call their |
| 180 |
+ |
specific clearing functions. For the transform module, the call is |
| 181 |
+ |
xf_clear. For the object support module, the call is obj_clear. The |
| 182 |
+ |
context routines use the c_clearall function, but this is actually |
| 183 |
+ |
called by mg_clear, so calling it again is unnecessary. |
| 184 |
+ |
|
| 185 |
+ |
Linking Vertices |
| 186 |
+ |
================ |
| 187 |
+ |
Although the MGF language was designed with linking vertices in mind, |
| 188 |
+ |
there are certain aspects which make this goal more challenging. |
| 189 |
+ |
Specifically, the ability to redefine values for a previously named |
| 190 |
+ |
vertex is troublesome for the programmer, since the same vertex can |
| 191 |
+ |
have different values at different points in the input. Likewise, the |
| 192 |
+ |
effect of the transform entity on surfaces rather than vertices means |
| 193 |
+ |
that the same named vertex can appear in many positions. |
| 194 |
+ |
|
| 195 |
+ |
It is not possible to use the parser data structures directly for |
| 196 |
+ |
linking vertices, but we've taken a couple of steps in the support |
| 197 |
+ |
routines to make the task of organizing your own data structures a |
| 198 |
+ |
little easier. First, there is a clock member in the C_VERTEX |
| 199 |
+ |
structure that is incremented on each change. (The same member is |
| 200 |
+ |
contained in the C_COLOR and C_MATERIAL structures.) Second, the |
| 201 |
+ |
current transform (pointed to by xf_context) contains a unique |
| 202 |
+ |
identifier, xf_context->xid. This is a long integer that will be |
| 203 |
+ |
different for each unique transform. (It is actually a hash key on the |
| 204 |
+ |
transformation matrix, and there is about 1 chance in 2 billion that |
| 205 |
+ |
two different matrices will hash to the same value. Is this a bug? |
| 206 |
+ |
I guess it depends on how long the programmer lives -- or vice versa.) |
| 207 |
+ |
|
| 208 |
+ |
There are two ways to use of this additional information. One |
| 209 |
+ |
is to record the vertex clock value along with it's id and the |
| 210 |
+ |
current xf_context->xid value. If another vertex comes along with |
| 211 |
+ |
the same name, but one of these two additional values fails to match, |
| 212 |
+ |
then it (probably) is a different vertex. Alternatively, one can reset |
| 213 |
+ |
the clock member every time a new vertex is stored. That way, it is |
| 214 |
+ |
only necessary to check the clock against zero rather than storing this |
| 215 |
+ |
value along with the vertex name and transform id. If the name and |
| 216 |
+ |
transform are the same and the clock is zero, then it's the same vertex |
| 217 |
+ |
as last time. |
| 218 |
+ |
|
| 219 |
+ |
Yet another approach is to ignore the parser structures entirely and |
| 220 |
+ |
focus on the actual vertex values. After all, the user is not compelled |
| 221 |
+ |
to reuse the same vertex names for the same points. It is just as likely |
| 222 |
+ |
that the same vertices will appear under different names, so that none |
| 223 |
+ |
of the above would help to merge them. The most sure-fire approach to |
| 224 |
+ |
linking identical vertices is therefore to hash the point and normal |
| 225 |
+ |
values directly and use the functions in lookup.c to associate them. |
| 226 |
+ |
You will have to write your own hash function, and we recommend making |
| 227 |
+ |
one that allows a little slop so that nearly identical points hash to |
| 228 |
+ |
the same value. |
| 229 |
+ |
|
| 230 |
|
Examples |
| 231 |
|
======== |
| 232 |
|
Two example translator programs are included with this package. |
| 233 |
|
|
| 234 |
|
The simplest is a translator from MGF to MGF called mgfilt.c, which |
| 235 |
|
produces on the standard output only those entities from the standard |
| 236 |
< |
input that are supported according to the command line arguments. For |
| 237 |
< |
example, one could remove everything but the raw, flat polygonal |
| 236 |
> |
input that are supported according to the first command line argument. |
| 237 |
> |
For example, one could remove everything but the raw, flat polygonal |
| 238 |
|
geometry with the following command: |
| 239 |
|
|
| 240 |
< |
mgfilt v p f xf < any.mgf > faces.mgf |
| 240 |
> |
mgfilt v,p,f,xf any.mgf > faces.mgf |
| 241 |
|
|
| 242 |
|
Note that the xf entity must also be included, for its support is |
| 243 |
|
required by all geometric entities. |
| 255 |
|
worked out. The intent is to offer it free of charge to all those who |
| 256 |
|
wish to use it (with no guarantees, of course). However, we may decide |
| 257 |
|
that copyright protections are necessary to prevent unauthorized versions |
| 258 |
< |
of the parser that do not properly support the MGF standard from getting |
| 259 |
< |
spread around. Since this is a pre-release, we trust that you will not |
| 260 |
< |
share it with anyone without getting our permission first. |
| 258 |
> |
of the parser, which do not properly support the MGF standard, from |
| 259 |
> |
getting spread around. Since this is a pre-release, we trust that you |
| 260 |
> |
will not share it with anyone without getting our permission first. |
| 261 |
|
|
| 262 |
|
Questions |
| 263 |
|
========= |
