GEM VDI

Also Known As: GEM Vector, VDI, .GDI

Type	Metafile
Colors	256
Compression	Uncompressed
Maximum Image Size	32Kx32K
Multiple Images Per File	No
Numerical Format	Big-endian
Originator	Digital Research
Platform	GEM GUI running under MS-DOS, some Ataris
Supporting Applications	GEM Artline, GEM Draw Plus, GEM Scan, others
See Also	GEM Raster

Usage
Illustration, drawing, and desktop publishing applications, and some data interchange.

Comments
A vector format with good support at one time, associated with the GEM GUI from Digital Research. If you are thinking of supporting this format, be prepared to draw Bezier curves. It also has support for embedded bitmaps.

Vendor specifications are available for this format.

Although often called the GEM Vector format, GEM VDI is actually a metafile format and is closely associated with the functioning of the GEM user interface. The GEM system provides a metafile driver that is accessed from within the GEM programming system through a documented API. Display requests to the driver result in items being written to a metafile buffer in GEM's standard metafile format. Metafile elements thus consist of calls to the GEM display system.

Contents:
File Organization
File Details
For Further Information

Supporting GEM VDI is similar to supporting many other metafile formats. Be prepared to duplicate the functionality of the host system, in this case GEM, or at least a reasonable subset of it, before you're through.

File Organization

We would like to have more information on this format. Information provided by DISCUS (see "For Further Information" below) indicates that the file consists of a header followed by a stream of standard-format metafile items.

File Details

The structure for the GEM VDI header is shown below.

typedef struct _GemVdiHeader
{
    WORD Identifier;     /* Magic number. Always FFFFh */
    WORD LengthOfHeader  /* Length of the header in 16-bit WORD */
    WORD Version;        /* Format version number */
    WORD Transform;      /* Image origin */
    WORD Coords[4];      /* Size and position of image */
    WORD PageSize[2];    /* Physical page size */
    WORD Bounds[4];      /* Limits of coordinate system */
    WORD Flags;          /* Bit image opcode flag */
} GEMVDIHEADER;

Identifier is the magic number of GEM VDI image files. This value is always FFFFh.

LengthOfHeader is the size of the header described as the number of 16-bit WORDs it contains. This value is typically 0Fh.

Version is the version number of the file format. This value is calculated using the formula: 100 * major version number + minor version number.

Transform is the NDC/RC transformation mode flag. This value is 00h if the origin of the image is in the lower-left corner of the display ("Normalized Device Coordinates") and 02h if the origin is in the upper-left corner ("Raster Coordinates").

Coords are four WORDs indicating the minimum and maximum coordinate values of data in the file. These values indicate the size of the image and its position on the display and are stored as: minimum X, minimum Y, maximum X, maximum Y.

PageSize is the size of the physical printed page the image will cover in 1/10 of millimeters. This value is 00h if the page size is undefined by the application creating the image.

Bounds are four WORDs that describe the maximum extent of the coordinate system used by the image and defined by the application. These values are stored as: lower-left X, lower-left Y, upper-right X, upper-right Y.

Flags contains the bit image opcode flag. The values for this field are 00h if no bit image is included in the file and 01h if a bit image is included. Bits 2 through 15 in Flags should always be set to 0.

Standard format metafile items consist of control, integer, and vertex parameters. This structure is described below.

Word	Value	Description
0	control[0]	Opcode
1	control[1]	Vertex count
2	control[3]	Integer parameter count
3	control[5]	Sub-opcode or zero
4	ptsin[0-n]	Input vertex list (if provided)
...
n+4	intin[0-m]	Input integer (if provided)
...

Table GEM VDI-1 shows the correspondence of standard metafile items and their opcodes to the display commands accepted by the GEM display subsystem. Arguments appear to be documented only in the GEM Programmer's Toolkit, but you might be able to recover them through diligent application of trial and error.

Table GEM VDI-1: GEM Metafile Items and Commands

Routine	Code	Subcode	Metafile Object
v_alpha_text	05	19	Output Printer Alpha Text
v_alpha_text	0b	19	Output Printer Alpha Text
v_arc	0b	02	Arc
v_bar	0b	01	Bar
v_bit_image	05	17	Output Bit Image File
v_bit_image	0b	17	Output Bit Image File
v_circle	0b	04	Circle
v_clear_disp_list	05	16	Clear Display List
v_clrwk	03	NA	Clear Workstation
v_ellarc	0b	06	Elliptical Arc
v_ellipse	0b	05	Ellipse
v_ellpie	0b	07	Elliptical Pie Slice
v_entercur	05	03	Enter Alpha Mode
v_entercur	0b	13	Enter Alpha Mode
v_exitcur	05	02	Exit Alpha Mode
v_fillarea	09	NA	Fill Area
v_form_adv	05	14	Form Advance
v_form_adv	0b	14	Form Advance
v_justified	0b	0a	Justified Graphics Text
v_line	06	NA	Polyline
v_output_window	05	15	Output Window
v_pieslice	0b	03	Pie
v_pieslice	0b	08	Rounded Rectangle
v_pmarker	07	NA	Polymarker
v_qtext	08	NA	Text
v_rfbox	0b	09	Filled Rounded Rectangle
v_updwk	04	NA	Update Workstation
vr_recfl	73	NA	Fill Rectangle
vs_color	0e	NA	Set Color Representation
vsf_clip	81	NA	Set Clipping Rectangle
vsf_color	19	NA	Set Fill Color Index
vsf_interior	17	NA	Set Fill Interior Style
vsf_perimeter	68	NA	Set Fill Perimeter Visibility
vsf_style	18	NA	Set Fill Style Index
vsf_updat	70	NA	Set User Fill
vsl_color	11	NA	Set Polyline Color Index
vsl_ends	6c	NA	Set Polyline End Styles
vsl_type	0f	NA	Set Polyline Type
vsl_udsty	71	NA	Set User Line Style
vsl_width	10	NA	Set Polyline Width
vsm_color	14	NA	Set Polymarker Color Index
vsm_height	13	NA	Set Polymarker Height
vsm_type	12	NA	Set Polymarker Type
vst_alignment	27	NA	Set Graphic Text Alignment
vst_color	16	NA	Set Text Color Index
vst_effects	6a	NA	Set Graphics Text Effects
vst_font	15	NA	Set Text Font
vst_height	0c	NA	Set Character Height
vst_point	6b	NA	Set Character Height (points)
vst_rotation	0d	NA	Set Character Baseline Vector
vswr_mode	20	NA	Set Writing Mode

There are also two non-standard metafile items:

v_opnwk	Open Workstation
v_clswk	Close Workstation

There are three metafile escape functions:

v_meta_extents	Update Metafile Extents
v_write_meta	Write Metafile Item
vm_filename	Change GEM VDI Filename

There are several inquire functions:

vq_chcells	Inquire Addressable Character Cells
vq_color	Inquire Color Representation
vq_attributes	Inquire Current Polyline Attributes
vq_extnd	Extended Inquire

Several metafile sub-opcodes are reserved for the Digital Research GEM Output application:

Physical Page Size

Coordinate Window

Several metafile sub-opcodes are also reserved for the Digital Research GEM Draw Plus application:

Group

Set No Line Style

Set Attribute Shadow On

Set Attribute Shadow Off

Start Draw Area Type Primitive

End Draw Area Type Primitive

Also associated with the GEM VDI format is a standard keyboard mapping.

For Further Information

The GEM VDI format originated at Digital Research, which is now owned by Novell, and GEM VDI is currently being supported by DISCUS Distribution Services, a service organization. Note that DISCUS will provide support only if you have first purchased the GEM Programmer's Toolkit from Digital Research. You can contact DISCUS at:

DISCUS Distribution Services, Inc.
8020 San Miguel Canyon Road
Salinas, CA 93907-1208
Voice: 408-663-6966

You may still be able to get some information from Novell/Digital Research at:

Novell/Digital Research, Inc.
P.O. Box DRI
Monterey, CA 93942
Voice: 408-649-3896
Voice: 800-848-1498
BBS: 408-649-3896

We have also been able to include information on Atari support of GEM VDI on the CD-ROM.

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