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ILBM is a fairly simple IFF FORM.  All you really need to deal with to
extract the image are the following chunks:

    Information about the size, depth, compaction method (see interpreted
    hex dump above).

    Optional Amiga ViewModes chunk.  Most HAM and HALFBRITE ILBMs should
    have this chunk.  If no CAMG chunk is present, and image is 6 planes
    deep, assume HAM and you'll probably be right.  Some Amiga ViewModes
    flags are HIRES=0x8000, LACE=0x4, HAM=0x800, HALFBRITE=0x80.  2.0
    ILBM writers should write a full 32-bit mode ID in the CAMG.  See the
    IFF Manual for more information on writing and interpreting 32-bit
    CAMG values.

    RGB values for color registers 0 to N.  Previously, 4-bit RGB
    components each left justified in a byte.  These should now be stored
    as a full 8-bit RGB values by duplicating 4-bit values in the high
    and low nibble of the byte.

    The pixel data, stored in an interleaved fashion as follows (each
    line individually compacted if BMHD Compression = 1):

                plane 0 scan line 0
                plane 1 scan line 0
                plane 2 scan line 0
                plane n scan line 0
                plane 0 scan line 1
                plane 1 scan line 1

Optional Chunks
    Also watch for AUTH Author chunks and (c) copyright chunks and
    preserve any copyright information if you rewrite the ILBM.