207 lines
8.7 KiB
Plaintext
207 lines
8.7 KiB
Plaintext
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Intro to Amiga IFF ILBM Files and Amiga Viewmodes
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Downloaded from Magic Tower BBS, +64 4 753 561 (24 hours)
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Intro to Amiga IFF ILBM Files and Amiga Viewmodes
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=================================================
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Carolyn Scheppner - Commodore Amiga Technical Support
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The IFF (Interchange File Format) for graphic images on the Amiga
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is called FORM ILBM (InterLeaved BitMap). It follows a standard
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parsable IFF format.
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Sample hex dump of beginning of an ILBM:
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========================================
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Important note! You can NOT ever depend on any particular ILBM chunk
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being at any particular offset into the file! IFF files are composed,
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in their simplest form, of chunks within a FORM. Each chunk starts
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starts with a 4-letter chunkID, followed by a 32-bit length of the
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rest of the chunk. You PARSE IFF files, skipping past unneeded or
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unknown chunks by seeking their length (+1 if odd length) to the
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next 4-letter chunkID.
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0000: 464F524D 00016418 494C424D 424D4844 FORM..d.ILBMBMHD
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0010: 00000014 01400190 00000000 06000100 .....@..........
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0020: 00000A0B 01400190 43414D47 00000004 .....@..CAMG....
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0030: 00000804 434D4150 00000030 001000E0 ....CMAP...0....
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0040: E0E00000 20000050 30303050 50500030 .... ..P000PPP.0
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0050: 90805040 70707010 60E02060 E06080D0 ..P@ppp.`. `.`..
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0060: A0A0A0A0 90E0C0C0 C0D0A0E0 424F4459 ............BODY
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0070: 000163AC F8000F80 148A5544 2ABDEFFF ..c.......UD*...
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0080: FFBFF800 0F7FF7FC FF04F85A 77AD5DFE ...........Zw.].
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etc.
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Interpretation:
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'F O R M' length 'I L B M''B M H D'<-start of BitMapHeader chunk
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0000: 464F524D 00016418 494C424D 424D4844 FORM..d.ILBMBMHD
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length WideHigh XorgYorg PlMkCoPd <- Planes Mask Compression Pad
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0010: 00000014 01400190 00000000 06000100 .....@..........
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TranAspt PagwPagh 'C A M G' length <- start of C-AMiGa View modes chunk
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0020: 00000A0B 01400190 43414D47 00000004 .....@..CAMG....
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Viewmode 'C M A P' length R g b R <- Viewmode 800=HAM | 4=LACE
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0030: 00000804 434D4150 00000030 001000E0 ....CMAP...0....
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g b R g b R g b R g b R g b R g <- Rgb's are for reg0 thru regN
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0040: E0E00000 20000050 30303050 50500030 .... ..P000PPP.0
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b R g b R g b R g b R g b R g b
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0050: 90805040 70707010 60E02060 E06080D0 ..P@ppp.`. `.`..
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R g b R g b R g b R g b 'B O D Y'
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0060: A0A0A0A0 90E0C0C0 C0D0A000 424F4459 ............BODY
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length start of body data <- Compacted (Compression=1 above)
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0070: 000163AC F8000F80 148A5544 2ABDEFFF ..c.......UD*...
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0080: FFBFF800 0F7FF7FC FF04F85A 77AD5DFE ...........Zw.].
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etc.
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Notes on CAMG Viewmodes: HIRES=0x8000 LACE=0x4 HAM=0x800 HALFBRITE=0x80
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------
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ILBM is a fairly simple IFF FORM. All you really need to deal with
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to extract the image are the following chunks:
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(Note - Also watch for AUTH Author chunks and (c) Copyright chunks
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and preserve any copyright information if you rewrite the ILBM)
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BMHD - info about the size, depth, compaction method
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(See interpreted hex dump above)
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CAMG - optional Amiga viewmodes chunk
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Most HAM and HALFBRITE ILBMs should have this chunk. If no
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CAMG chunk is present, and image is 6 planes deep, assume
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HAM and you'll probably be right. Some Amiga viewmodes
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flags are HIRES=0x8000, LACE=0x4, HAM=0x800, HALFBRITE=0x80.
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CMAP - RGB values for color registers 0 to n
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(each component left justified in a byte)
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BODY - The pixel data, stored in an interleaved fashion as follows:
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(each line individually compacted if BMHD Compression = 1)
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plane 0 scan line 0
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plane 1 scan line 0
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plane 2 scan line 0
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...
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plane n scan line 0
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plane 0 scan line 1
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plane 1 scan line 1
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etc.
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Body Compression
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================
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The BODY contains pixel data for the image. Width, Height, and depth
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(Planes) is specified in the BMHD.
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If the BMHD Compression byte is 0, then the scan line data is not compressed.
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If Compression=1, then each scan line is individually compressed as follows:
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More than 2 bytes the same stored as BYTE code value n from -1 to -127
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followed by byte to be repeated (-n) + 1 times.
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Varied bytes stored as BYTE code n from 0 to 127 followed by n+1 bytes
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of data.
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The byte code -128 is a NOP.
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Interpreting the Scan Line Data:
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================================
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If the ILBM is not HAM or HALFBRITE, then after parsing and uncompacting
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if necessary, you will have N planes of pixel data. Color register
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used for each pixel is specified by looking at each pixel thru the planes.
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IE - if you have 5 planes, and the bit for a particular pixel is set in
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planes 0 and 3:
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PLANE 4 3 2 1 0
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PIXEL 0 1 0 0 1
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then that pixel uses color register binary 01001 = 9
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The RGB value for each color register is stored in the CMAP chunk of the
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ILBM, starting with register 0, with each register's RGB value stored as
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one byte of R, one byte G, and one byte of B, with each component left
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justified in the byte. (ie. Amiga R, G, and B components are each stored
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in the high nibble of a byte)
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BUT - if the picture is HAM or HALFBRITE, it is interpreted differently.
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=== =========
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Hopefully, if the picture is HAM or HALFBRITE, the package that saved
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it properly saved a CAMG chunk (look at a hex dump of your file with
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ascii interpretation - you will see the chunks - they all start with
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a 4-ascii-char chunk ID). If the picture is 6 planes deep and has no
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CAMG chunk, it is probably HAM. If you see a CAMG chunk, the "CAMG" is
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followed by the 32-bit chunk length, and then the 32-bit Amiga Viewmode
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flags.
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HAM pics will have the 0x800 bit set in CAMG chunk ViewModes.
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HALBRITE pics will have the 0x80 bit set.
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To transport a HAM or HALFBRITE picture to another machine, you must
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understand how HAM and HALFBRITE work on the Amiga.
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How Amiga HAM mode works:
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=========================
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Amiga HAM (Hold and Modify) mode lets the Amiga display all 4096 RGB
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values. In HAM mode, the bits in the two last planes describe an R G or
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B modification to the color of the previous pixel on the line to create
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the color of the current pixel. So a 6-plane HAM picture has 4 planes
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for specifying absolute color pixels giving up to 16 absolute colors
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which would be specified in the ILBM CMAP chunk. The bits in the last
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two planes are color modification bits which cause the Amiga, in HAM mode,
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to take the RGB value of the previous pixel (Hold and), substitute the 4
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bits in planes 0-3 for the previous color's R G or B component (Modify)
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and display the result for the current pixel. The color modification bits
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in the last two planes are interpreted as follows:
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00 - no modification. Use planes 0-3 as normal color register index
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10 - hold previous, replacing Blue component with bits from planes 0-3
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01 - hold previous, replacing Red component with bits from planes 0-3
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11 - hold previous. replacing Green component with bits from planes 0-3
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How Amiga HALFBRITE mode works:
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===============================
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This one is simpler. In HALFBRITE mode, the Amiga interprets the
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bit in the last plane as HALFBRITE modification. The bits in the other
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planes are treated as normal color register numbers (RGB values for each
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color register is specified in the CMAP chunk). If the bit in the last
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plane is set (1), then that pixel is displayed at half brightness.
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This can provide up to 64 absolute colors.
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Other Notes:
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============
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Amiga ILBMs images must be a even number of bytes wide. Smaller
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images (such as brushes) are padded to an even byte width.
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ILBMs created with Electronic Arts IBM and Amiga "DPaintII" packages
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are compatible (though you may have to use a '.lbm' filename extension
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on an IBM). The ILBM graphic files may be transferred between the
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machines (or between the Amiga and IBM sides your Amiga if you have
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a CBM Bridgeboard card installed) and loaded into either package.
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--
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==========================================================================
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Carolyn Scheppner -- CATS Commodore Amiga Technical Support
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PHONE 215-431-9180 UUCP ...{uunet,allegra,rutgers}!cbmvax!carolyn
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Oh I'm a numberjack and I'm OK, I code all night and I work all day...
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==========================================================================
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And downloaded from The Cave BBS (Wellington) for the library of
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The Pinnacle Club, Auckland...................................B.
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==========================================================================
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