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Text File | 1990-08-10 | 9.8 KB | 347 lines | [TEXT/UNIX]

Preview: A sample color-mapped image file ------------------------------------------ Block ------------------- 1 | "SCMI" | " 1" | <--- File Identification ------------------- --------------------- 2A | "AT" | " 50" | <--- Section header (attributes) --------------------- ----------------------------------------------------------------------- 2B | " 512" | " 464" | " 128" | "GDA 1 222.21 (-114.5 54.8) (17.2 84.0)" | ----------------------------------------------------------------------- --------------------- | 3A | "CM" | " 384" | <--- Section header (color map) | --------------------- Attributes: ------------- width, height, 3B | b | b | b | <--- Color map, color[0] # colors, ------------- associated data | b | b | b | <--- color[1] ------------- ... ['b' => 8-bit byte] ------------- | b | b | b | <--- color[n-1] ------------- --------------------- 4A | "PD" | " 237568" | <--- Section header (pixel data) --------------------- --------------------- 4B | bbbbb... ...bbbbb | <--- Pixel data --------------------- Sample associated data in attributes: "GDA 1 222.21 (-114.5 54.8) (17.2 84.0)" \ \ \ \ \ \ \ \ \ Longitute/Latitude @ top right \ \ \ --- Longitude/Latitude @ bottom left \ \ --- Scale, pixels per radian of longitude \ --- Version # of application data format --- Application identification "Img" Image File Formats This is the beginning of a larger specification document for an overall subsystem to handle simple image processing. The subsystem is tentatively named "Img". This document includes only these specification sections: 1 Img File Formats 1.1 Color Mapped Images 1.2 24-bit RGB Images Additional sections have not been written, but would describe the imglib routines and the set of programs which manipulate images in this format. Most of this information can be extracted from source modules by using the ccomex comment extractor. As with the file format, imglib functions were designed to make the interface from application code as simple as possible. Most of imglib is independent of the underlying graphics or window system. One module supplies such system-dependent support for X11R3. 1 Img File Formats 1.1 Color Mapped Images Purposes of this particular file format are: 1. Represent color-mapped images which use no more than 256 colors. 2. Conserve disk space. 3. Conserve processing time for reading and displaying images. 4. Minimize complexity in basic image manipulation functions. 5. Allow extensibility in future versions. The format defined here represents an uncompressed image file. Such images would normally be stored in compressed format, as used by the UNIX* compress/uncompress utility. (* UNIX is a trademark of AT&T.) When this is true, the filename ends with ".Z". One simple way to read compressed files is to pipe input through uncompress, approximately as follows: if ( compressed ) { sprintf ( command, "uncompress <%s", filename ); img_file = popen ( command, "r" ); } else img_file = fopen ( filename, "r" ); In addition to easy availability of compress/uncompress, use of this particular compress tool set generally produces better compression than run length encoding. It also improves on storage of bitmaps: Using one bit per byte in compressed format usually produces better compression than packing 8 bits per byte. Some design rules applied to all parts of this file format are: 1. All data is a byte stream: The format has no dependence on byte or word order for any particular machine. 2. Pixel data, which are indices to the color map, and RGB component values are 8-bit unsigned binary values. 3. Attribute data, except RGB values in the color map, are stored in fixed-size ascii fields. Integers use either 4-byte or 8-byte fields; programs can read or write them via scanf/printf functions using, for example, "%4u" or "%4i" format. 4. Whenever the length of a section of the file may vary depending on image contents, that section is preceded by a prefix which identifies it and gives its length in bytes. This facilitates use of block-oriented i/o operations to optimize i/o speed; it does this in a way which allows future extension of the image format to include sections whose size does not depend on data in the attribute section. 1.1.1 Top Level Structure A color-mapped image file contains the following blocks: 1. File identification 2A. Attribute prefix 2B. Attributes 3A. Color map prefix 3B. Color map 4A. Pixel data prefix 4B. Pixel data 1.1.2 Section Prefixes Each "prefix" block identifies a section and report reports its size. Format: +0 +2 ----------------- | xx | nnnnnnnn | ----------------- \ \___ Length of data in next section: %8d or %8u \ \___ Section identifier: AT => Attributes CM => Color Map PD => Pixel Data Other prefixes could be used to represent data sections in formats defined outside this specification. 1.1.2 File Identification This identifies the file type and the version number of the file format. +0 +4 --------------- | SCMI | nnnn | --------------- \ \___ File format version #: %4d or %4u \ \___ File type code: "SCMI" 1.1.3 Attributes The "attributes" section defines image characteristics other than the content of its color map. This consists of the image width and height, number of colors, and associated data for application use. For purposes of this file format, associated data is only a sequence of bytes. An appropriate convention for higher level specifications would be to begin any associated data with an application code and version number to identify its format. ----------------- | AT | nnnnnnnn | <--- Section header ----------------- +0 +4 +8 +12 --------------------------------------------- | nnnn | nnnn | nnnn | [... assoc data ...] | --------------------------------------------- \ \ \ \__Associated data, if any \ \ \__Number of colors: %4d or %4u \ \__Height, pixels: %4d or %4u \__Width%4d or %4u Size of associated data is block size - 12 bytes. (Block size from section header) 1.1.4 Color Map The color map is an array with one entry for each color in the image. For grey-scale images, each RGB component is identical. Black-and-white images have a two-entry color table. ----------------- | CM | nnnnnnnn | <--- Section header ----------------- Red Grn Blue / / / ------------- | b | b | b | <--- color[0] @ +0 ------------- | b | b | b | <--- color[1] @ +3 ------------- ... ------------- | b | b | b | <--- color[k] @ +3*k ------------- ... ------------- | b | b | b | <--- color[n-1] @ +3*(n-1) ------------- Each color component byte (b) is an unsigned 8-bit integer. Block size is 3*n, where n is the number of colors defined in the attributes. 1.1.5 Pixel Data Pixel data is an array with one 8-bit entry for each pixel. Pixels are in conventional scan line order. ----------------- | CM | nnnnnnnn | <--- Section header ----------------- --------------------- | bbbbb... ...bbbbb | <--- Pixel data --------------------- Each pixel value byte (b) is an 8-bit unsigned integer which is the index of a color map entry. Valid range is [0,n-1], where n is the number of colors defined in the attributes. Size of the pixel data section is width*height bytes. Order of pixels is row major -- Top to bottom by rows, left to right within each row. 1.2 24-bit RGB Images Purposes of this particular file format are: 1. Represent images using 24 bits per pixel: 8 bits each for red, green, and blue values. 2. Conserve disk space. 3. Maintain reasonable consistency with existing techniques for acquiring RGB image data 4. Minimize complexity in basic image manipulation function. 5. Allow using an ordinary text editor to define image attributes. As with color mapped images, 24-bit RGB images can optionally be stored in compressed format. Any file which is compressed has ".Z" appended to its filename. 1.2.1 Top Level Structure A 24-bit RGB image is defined by four separate files which are identified by file naming conventions: 1. Attributes: <name>.a[.Z] 2. Red component: <name>.r[.Z] 3. Green component: <name>.g[.Z] 4. Blue component: <name>.b[.Z] For any given image, the various component files may be either compressed or uncompressed on an individual basis. 1.2.2 Attribute File The attribute file contains precisely the data which would appear in the attribute section (section 2B) of a color-mapped image file, except that the color count is meaningless: +0 +4 +8 +12 --------------------------------------------- | nnnn | nnnn | xxxx | [... assoc data ...] | --------------------------------------------- \ \ \ \__Associated data, if any \ \ \__Reserved for color count, 4 chars \ \__Height, pixels: %4d or %4u \__Width%4d or %4u Size of associated data is the file size - 12. Although attribute files can be stored in compressed format, keeping them in easily editable uncompressed format can simplify generating and maintaining image data bases. 1.2.3 RGB Component Files Each RGB component file contains 8-bit unsigned pixel values for the red, green, or blue component of the image. Each file represents width*height bytes, where width and height are defined in the corresponding attribute file. Order of pixels is row major -- Top to bottom by rows, left to right within each row.