ILBM is a type of IFF file used to store raster graphics data. The format supports interlaced scanning, the color image is represented by a set of one-bit arrays, each of which stores 1 data bit for one pixel in the image. Such arrays are called bit-plans ( eng. Bitplanes ), or bitmaps ( eng. Bitmaps ). Also, the format supports horizontal and vertical image data compression by the RLE algorithm.
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The ILBM format was developed for the most complete disclosure of the capabilities of the classic Amiga chipset .
OCS and ECS chipsets have 32 palette registers and support up to 6 bit plans, which theoretically should limit the number of simultaneously displayed colors: 2 6 = 64. However, this is only true when using the head-on format, according to the canonical scheme adopted today for all PC- compatible hardware. The 32 palette registers on the Amiga would not allow outputting more than 32 colors if the 6th bit plan did not store image brightness data (the so-called Half-Bright mode), which increases the number of displayed colors to 64. Another feature of the classic Amiga chipset is technology Hold and Modify (literally: “Hold and Change”, the technology of drawing only really modified parts of the image instead of the entire screen) allows you to simultaneously display up to 4096 colors using only 16 palette registers.
The AGA chipset allows up to 8 bit plans and has 256 palette registers. Thus, in the forehead, it allows you to display 2 8 = 256 colors, and in the modified HAM8 mode - up to 262,144 colors from a palette of 16.8 million colors (24-bit graphics).
All these features are most fully supported by the ILBM format, since, based on the Interchange File Format, it uses the term “chunk”. A chunk in ILBM consists of a 4-byte header indicating the type of data, 4 bytes storing the length of the data block and a structure dependent on the data type. This makes it possible to expand the data storage format and skip unrecognized data, since the length of their blocks is known in advance.
The ILBM format describes the following chunk types:
- BMHD ( B it M ap H ea D er) is a header describing the bit plan;
- CMAP ( C olor MAP ) - a standard chunk that stores palette register data;
- GRAB ( GRAB ) - saved pointer to data in HAM mode;
- CAMG ( C ommodore AM i G a computer) - reserved;
- BODY - the data of all bit-plans and their masks, interleaved.
Of course, the ILBM format files may contain any IFF data in the chunks: information about the author, version, copyright , etc. For example, files created in the DPaint raster graphic editor contain a chunk:
- DPAN ( DPA i N t) - saved DPaint settings.
There is an extension to the ILBM format called ANIM , which is intended for saving animated images. In this format, the following additional chunks were added:
- ANHD ( AN imation H ea D er) - header that stores animation data;
- DLTA - the difference ( delta ) in the size of the structures of compressed animation frames.