MIL-HDBK-1300A
reconstruct the image correctly. If the image is band sequential (IMODE = S), there will be multiple block image masks--one for each band. The block image masks will be arranged in the same order as the image bands, with each mask containing number of bits per row (NBPR) x number of bits per column (NBPC) records. Block image masks can be used in conjunction with a transparent pixel mask, as described below. A block image mask may also be used to provide random access within the blocked image data for large images even if all blocks are recorded.
5.3.2 Symbols (graphics). The symbol data type used in the NITF to store a two-dimensional graphical symbol represented as a bit-map, as an NITF-defined "object," or as a CGM. A symbol may be black and white, gray scale, or color. Examples of symbols are circles, ellipses, rectangles, arrows, lines, triangles, logos, unit designators, object designators (ships, aircraft), alphanumeric labels, and special characters. A symbol is stored as a distinct unit in the NITF file allowing it to be manipulated and displayed nondestructively relative to the images, labels, and other symbols in the file. Computer graphic metafile symbols form a subset of national and international standards, and provide the user with more display flexibility than either bit-mapped or object graphics. CGM often requires less
stored data than bit-mapped graphics and fewer limitations than object graphics. MIL-STD-2301 defines the subset of commands (correlated with the minimum implementation subset of commands specified in MIL-D-28003A) applicable for graphic annotation of imagery within the NITFS.
5.3.3 Labels. The label data type in the NITF is used to store a label composed of printable ASCII characters plus carriage returns and line feeds. The intent is for the label to be nondestructively overlaid upon one or more images and/or symbols to serve as textual annotation. However, it is recommended that label information be represented using the CGM symbol data type construct rather than the label data type construct. The label data type may not be included in future releases of the standard.
5.3.4 Text. The text data type in the NITF is used to store a file or item of text, such as a word processing file or document. Text data types are intended to convey information about the image product contained in the NITF file. The format of the text is conveyed by information in the Text Subheader.
5.4 Format extensions. Extension of the NITF is supported by built-in mechanisms and procedures that allow inclusion of user-defined data and data characteristics without changing the standard. In addition, there are Extension Segments that provide space within the file structure for entirely unspecified future purposes. Variations of the same basic extension mechanism, tagged records, are used for all specified types of extensions. The three varieties of tagged record extensions are registered extensions, controlled extensions, and encapsulated extensions.
5.4.1 Registered extensions. Each registered tagged record extension consists of three required fields: an identifier, or tag; a length; and a data field. These extensions are entirely user-defined; only the six character tag field is registered with the NTB. The purpose of registering the tags is to avoid having two users use the same tag to mean different extensions. The NITF provides several places within a NITF file to embed registered tagged record extensions.
5.4.2 Controlled extensions. These extensions are defined and submitted to the NTB for approval, and once accepted, are subject to formal configuration management. The tagged record
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