FACT from ElectricImage

Contributed by Dr. Edward Smith-Rowland

Perl script to translate a FACT file to Inventor/VRML

The Electric Image .fact format is an IFF type format. In other words it follows similar conventions as the Lightwave object file format.

Summary of IFF

The IFF file metaformat is summarized as follows: The file must start with the keyword FORM and an unsigned long indicating the size of the file in bytes (excepting the 8 bytes already read). Then there must be a four byte ASCII file type ID indicating the type of IFF file. If you don't recognize the file type you can punt. Then follow chunks of data. Each chunk consists of a four byte ASCII chunk ID followed by a four-byte unsigned long indicating the length of the data in bytes to corresponding to the chunk (again excepting the 8-byte header). If you don't recognize the chunk type you can skip it. In this way, IFF files are extensible. All numerical fields in an IFF file are big-endian (the high-order bytes occur earlier in the file).

ElectricImage .fact format

The file type ID for the Electric Image .fact file is 3DFL.

The large scale structure of the file is a file header chunk identified by FORM followed by a tree-like structure of part chunks also identified by FORM. The file header chunk ID (FORM) is followed by the size and the form header ID FHDR. Then follows the FINF chunk and it's size. The FINF chunk consists of three unsigned longs indicating the number of vertices in the whole model, the number of facets in the whole model, and the number of groups (parts) in the whole model. This is followed by six floats describing the bounding box of the whole model. Last is four longs that seem to be zeroed out.

The data is contained in a sequence of group header chunks with ID GRUP. The file header chunk ID (FORM) is followed by the form size and the form ID GHDR. Then follows the GINF and it's size. The GINF chunk consists of three unsigned longs indicating the number of vertices in the group, the number of facets in the group, and the number of subgroups in the group. Then there are six floats describing the bounding box of the object. After four bytes of crud there is 1 char indicating the length of the object name followed by the object name in ASCII with a bunch of null padding. The last four bytes in this chunk seem to have something in them.

The the GATR chunk follows with its size and can be skipped (or parsed by someone smarter than me). Then follows the geometry data in CORD and ELEM chunks.

The vertex coordinate chunk has the ID CORD. The size of the CORD chunk should be such that size/12 = n_grp_points where n_grp_points is the number of vertices specified in the corresponding group header. The CORD chunk size is immediately followed by triplets of vertex coordinates in IEEE floating point format (4-bytes) for the X, Y, and Z coordinates.

The element (facet) chunk has the ID ELEM. Each facet begins with a 6-byte header whose meaning is not entirely clear to me except that there is an ID (byte 2) indicating whether the element is a quad (id = 0) or a polygon (id = 1). If the element is a polygon then the size of the rest of the data in bytes appears in byte 6 of the header and can be divided into polygon vertices.

For quads, the element header is followed by four unsigned integers indicating the unit-offset vertex indices into the list of vertices. The vertex indices are NOT byte offsets but indicate the vertex number. Index 0 (zero) indicates an invalid index so that if the last index of the four in a facet is zero then the facet is a triangle indexed by the first three indices. (I don't know if wires or points are allowed.) The trick is that the byte size of the indices is determined by the number of vertices in the group. If the number of vertices in the group is less than 256 then the vertex indices are unsigned char. If the number of vertices in the group is less than 65536 then the vertex indices are unsigned short. If the number of vertices in the group is less than 4294967296 then the vertex indices are unsigned long.

For polygons the first 8 bytes of data following the header is unknown (by me). The rest of the stream consists of integer indices into the vertex array defining the polygon. The integer size follows the same conventions as for quads.

It seems that Z points into the screen and that the polygons are specified with clockwise vertex order. I will negate all Z coordinates and bounding box components so that facets have normals facing outwards and so that the Z bounding box coordinates have z_max > z_min.


Here is a hex dump for your viewing pleasure:

0000000 464f524d 0007d13a 3344464c 464f524d
         F O R M  file_sz  3 D F L  F O R M
0000020 00000040 46484452 46494e46 00000034
                  F H D R  F I N F      len
0000040 000043b4 00005452 00000006 c1b921f9
           n_pts   n_fcts   n_prts    min_x
0000060 c1217666 426f9fb3 41b92321 405a532a
           min_y    min_z    max_x    max_y
0000100 c2855c71 00000000 00000000 00000000
           max_z      pad      pad      pad
0000120 00000000 464f524d 00001176 47525550
             pad  F O R M      len  G R U P
0000140 464f524d 00000086 47484452 47494e46
         F O R M      len  G H D R  G I N F
0000160 0000004e 000000b8 000000d2 00000000
         n_g_pts n_g_fcts n_g_prts      pad
0000200 c08652e3 bf9b87f9 c23cdb28 40861355
         min_g_x  min_g_y  min_g_z  max_g_x
0000220 3feeb94e c24a2ea8 71000000 04657965
         max_g_y  max_g_z           4 e y e // char size of ascii object name and string.
0000240 73000000 00000000 00000000 00000000
0000260 00000000 00000000 00000000 000037cf
0000300 cdac4741 54520000 00242280 ff000000
             G A  T R                      
0000320 ff000000 ff202020 4063ab85 1eb851ec
0000340 ffff0000 003ff000 00000000 0000434f
                                        C O
0000360 52440000 08a0c07a 8eeb3fa4 80d8c243
         R D          x_1      y_1      z_1
0000400 eef8c086 52e33f2a ec39c244 0630c081
             x_2      y_2      z_2      x_3

. . . . .

0004600 13553f2a ec39c244 06de4086 1355bc91
0004620 2fd3c244 06de454c 454d0000 08340000
     yN       zN      E L  E M      len<---
0004640 ff7f7f7f 01020304 0000ff7f 7f7f0506
        q_head-> <q_vrts> <-q_head --><