GEM Metafile file format

GEM Meta file    *.GEM

Files ending in '.GEM' are graphic metafiles created by GDOS. They are usually used to
represent vector graphics but may also be used to store links to bitmap images and
textual information. Two primary versions of GEM files exist. Version 1 files are
guaranteed not to contain bezier curves whereas version 3 files may. Version 3.xx files
are also commonly referred to as GEM/3 files.

The Metafile Header (see META_HEADER structure)

GEM metafiles begin with a header as follows:

 WORD   Contents
  0     Magic number (0xFFFF).
  1     Header length in WORDs.
  2     Version number (major * 100 + minor).
  3     NDC Flag as follows: Value Meaning 0 (0, 0) in lower-left corner (NDC) 
                                           2 (0, 0) in upper-left corner (RC)
  4     Minimum X extent.
  5     Minimum Y extent.
  6     Maximum X extent.
  7     Maximum Y extent.
  8     Page width in tenths of millimeters.
  9     Page height in tenths of millimeters.
 10     Lower Left X value of coordinate system.
 11     Lower Left Y value of coordinate system.
 12     Upper Right X value of coordinate system.
 13     Upper Right Y value of coordinate system.
 ...    Other information may appear in the header following which is
        currently undefined. Use WORD #1 to skip any unknown information.

The definition of WORDs 4-13 is defined by the creator of the file using three metafile
commands. WORDs 4-7 are set with the v_meta_extents() function. WORDs 8-9 are defined
with the vm_pagesize() function. WORDs 10-13 are defined with vm_coords(). If the creator
fails to specify defaults for any of these values, the appropriate values will be set to
0 in the header. If zeros appear for WORDs 10-13, the default NDC coordinate system
should be assumed.

Metafile Records:

Following the header will appear a list of records of varying length which, when
translated, can be 'played back' on the destination VDI device. Each record is formatted
as follows:

        WORD    Meaning
        0       Opcode of VDI function.
        1       Number of PTSIN elements.
        2       Number of INTIN elements.
        3       Function sub-ID.
        4...    PTSIN elements.
        ...     INTIN elements.

The list of records is terminated with an opcode of 0xFFFF (this record is written when a
v_clswk() call is made by the creator). When playing back GEM files, the application must
translate all coordinates from the metafile coordinate system to that of the destination
device. In addition, text metrics should be appropriately converted. If an unknown opcode
is discovered it should be played after any elements of the PTSIN array are translated
(making the assumption that they should be).

Metafile Sub-Opcodes:

GEM metafiles support the use of special sub-opcodes for implementing reserved and
user-defined functions. GEM metafile translators should ignore sub-opcodes they don't
understand. Each sub-opcode can be identified with the primary opcode of 5, function ID
of 99 and the first (required) member of INTIN being the sub-opcode ID. The currently
defined sub-opcodes are as follows:

    INTIN[0]    Meaning
      10        Start Group.
      11        End Group.
      49        Set No Line Style.
      50        Set Attribute Shadow On.
      51        Set Attribute Shadow Off.
      80        Start Draw Area Type Primitive.
      81        End Draw Area Type Primitive.

None of the pre-defined sub-opcodes use additional INTIN or PTSIN elements though
user-defined sub-opcodes may.

Opcodes from 0-100 are reserved for use by Atari. Sub-opcodes from 101-65535 are
available for use by developers but should be registered with Atari to avoid possible
conflicts.

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