MIL-STD-2301 Computer Graphics Metafile Cgm Implementation Standard For The National Imagery Transmission Format StandardThis standard establishes the requirements necessary to implement Computer Graphics Metafiles CGM used for the representation of symbol graphics in the National Imagery Transmission Format Standard NITFS.

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MIL-STD-2301A

3.6  Integer parameters.  For MIL-STD-2301A, all integer parameters are 16-bit two's complement signed integers except where specified.  Each 16-bit word is numbered from most significant bit to least significant bit using 15 to zero (as illustrated on figure 1).  When a 16-bit two's complement integer is used as a parameter in a CGM metafile, the high order byte of the integer is represented as the 8 most significant bits.  That is, bits 15 through 8.  Bits 7 through zero represent the low order byte of the integer.  Note: This is also known as the "Big- Endian" or "Network Byte Order" representation for 16-bit integers.

BITS               BITS

15                 8    7                   0

HIGH ORDER     LOW ORDER

16-BIT WORD 1                            16-BIT WORD 2                            16-BIT WORD 3

FIGURE 1.  Integer parameters.

3.7  Virtual Device Coordinates (VDC) space.  The VDC space defines a coordinate system that is overlaid onto an image to which CGM elements are referenced.

4.  GENERAL REQUIREMENTS

4.1  CGM commands.  The CGM implementation for NITFS is a subset of the Federal Information Processing Standard FIPS PUB 128 and complies with the specifications established in FIPS PUB 128.  The CGM structure is explained fully in the FIPS PUB 128 document.  The following describes the required CGM commands that the CGM implementation for NITFS must support for input interpretation and output generation and are grouped by element class.  All CGM commands used in this subset are listed by element class along with their encoding in the CGM Interface Input Requirements and CGM Interface Output Requirements sections of this document.  When CGM commands are encountered that are beyond the minimally compliant CGM

implementation listed below, the interpreter is responsible for interpreting, discarding, or substituting for these commands.

a.  Metafile Delimiter Elements BEGIN METAFILE BEGIN PICTURE

BEGIN PICTURE BODY END PICTURE

END METAFILE

b.  Metafile Descriptor Elements METAFILE VERSION (version = 1) METAFILE ELEMENT LIST METAFILE DESCRIPTION

FONT LIST

c.  Metafile Picture Descriptor Elements

COLOR SELECTION MODE (1 = direct) EDGE WIDTH SPECIFICATION MODE

(0 = absolute)

LINE WIDTH SPECIFICATION MODE (0 = absolute)

VDC EXTENT

4