Intro to Amiga IFF ILBM Files and Amiga Viewmodes
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            Intro to Amiga IFF ILBM Files and Amiga Viewmodes
            =================================================
          Carolyn Scheppner - Commodore Amiga Technical Support

The IFF (Interchange File Format) for graphic images on the Amiga
is called FORM ILBM (InterLeaved BitMap).  It follows a standard
parsable IFF format.

Sample hex dump of beginning of an ILBM:
========================================

 Important note!  You can NOT ever depend on any particular ILBM chunk
 being at any particular offset into the file!  IFF files are composed,
 in their simplest form, of chunks within a FORM.  Each chunk starts
 starts with a 4-letter chunkID, followed by a 32-bit length of the
 rest of the chunk.  You PARSE IFF files, skipping past unneeded or
 unknown chunks by seeking their length (+1 if odd length) to the
 next 4-letter chunkID.

0000: 464F524D 00016418 494C424D 424D4844    FORM..d.ILBMBMHD
0010: 00000014 01400190 00000000 06000100    .....@..........
0020: 00000A0B 01400190 43414D47 00000004    .....@..CAMG....
0030: 00000804 434D4150 00000030 001000E0    ....CMAP...0....
0040: E0E00000 20000050 30303050 50500030    .... ..P000PPP.0
0050: 90805040 70707010 60E02060 E06080D0    ..P@ppp.`. `.`..
0060: A0A0A0A0 90E0C0C0 C0D0A0E0 424F4459    ............BODY
0070: 000163AC F8000F80 148A5544 2ABDEFFF    ..c.......UD*...
0080: FFBFF800 0F7FF7FC FF04F85A 77AD5DFE    ...........Zw.].
etc.


Interpretation:

      'F O R M' length  'I L B M''B M H D'<-start of BitMapHeader chunk
0000: 464F524D 00016418 494C424D 424D4844    FORM..d.ILBMBMHD

       length  WideHigh XorgYorg PlMkCoPd <- Planes Mask Compression Pad
0010: 00000014 01400190 00000000 06000100    .....@..........

      TranAspt PagwPagh 'C A M G' length  <- start of C-AMiGa View modes chunk
0020: 00000A0B 01400190 43414D47 00000004    .....@..CAMG....

      Viewmode 'C M A P' length   R g b R <- Viewmode 800=HAM | 4=LACE 
0030: 00000804 434D4150 00000030 001000E0    ....CMAP...0....

       g b R g  b R g b  R g b R  g b R g <- Rgb's are for reg0 thru regN
0040: E0E00000 20000050 30303050 50500030    .... ..P000PPP.0

       b R g b  R g b R  g b R g  b R g b
0050: 90805040 70707010 60E02060 E06080D0    ..P@ppp.`. `.`..

       R g b R  g b R g  b R g b 'B O D Y' 
0060: A0A0A0A0 90E0C0C0 C0D0A000 424F4459    ............BODY

       length   start of body data        <- Compacted (Compression=1 above)
0070: 000163AC F8000F80 148A5544 2ABDEFFF    ..c.......UD*...
0080: FFBFF800 0F7FF7FC FF04F85A 77AD5DFE    ...........Zw.].
etc.

Notes on CAMG Viewmodes:  HIRES=0x8000  LACE=0x4  HAM=0x800  HALFBRITE=0x80

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

(Note - Also watch for AUTH Author chunks and (c) Copyright chunks
 and preserve any copyright information if you rewrite the ILBM)

   BMHD - info about the size, depth, compaction method
          (See interpreted hex dump above)

   CAMG - 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.

   CMAP - RGB values for color registers 0 to n
          (each component left justified in a byte)

   BODY - 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
             etc.

 
Body Compression
================

The BODY contains pixel data for the image.  Width, Height, and depth
(Planes) is specified in the BMHD.

If the BMHD Compression byte is 0, then the scan line data is not compressed.
If Compression=1, then each scan line is individually compressed as follows:

   More than 2 bytes the same stored as BYTE code value n from  -1 to -127 
     followed by byte to be repeated (-n) + 1 times.
   Varied bytes stored as BYTE code n from 0 to 127 followed by n+1 bytes 
     of data.
   The byte code -128 is a NOP.


Interpreting the Scan Line Data:
================================

If the ILBM is not HAM or HALFBRITE, then after parsing and uncompacting
if necessary, you will have N planes of pixel data.  Color register
used for each pixel is specified by looking at each pixel thru the planes.
IE - if you have 5 planes, and the bit for a particular pixel is set in 
planes 0 and 3:

       PLANE     4 3 2 1 0
       PIXEL     0 1 0 0 1

   then that pixel uses color register binary 01001 = 9

The RGB value for each color register is stored in the CMAP chunk of the 
ILBM, starting with register 0, with each register's RGB value stored as
one byte of R, one byte G, and one byte of B, with each component left
justified in the byte.  (ie. Amiga R, G, and B components are each stored
in the high nibble of a byte)



BUT - if the picture is HAM or HALFBRITE, it is interpreted differently.
                        ===    =========

Hopefully, if the picture is HAM or HALFBRITE, the package that saved
it properly saved a CAMG chunk (look at a hex dump of your file with
ascii interpretation - you will see the chunks - they all start with
a 4-ascii-char chunk ID).  If the picture is 6 planes deep and has no
CAMG chunk, it is probably HAM.   If you see a CAMG chunk, the "CAMG" is
followed by the 32-bit chunk length, and then the 32-bit Amiga Viewmode
flags.  

HAM pics will have the 0x800 bit set in CAMG chunk ViewModes.
HALBRITE pics will have the 0x80 bit set.

To transport a HAM or HALFBRITE picture to another machine, you must
understand how HAM and HALFBRITE work on the Amiga.

How Amiga HAM mode works:
=========================

   Amiga HAM (Hold and Modify) mode lets the Amiga display all 4096 RGB
values. In HAM mode, the bits in the two last planes describe an R G or
B modification to the color of the previous pixel on the line to create
the color of the current pixel.  So a 6-plane HAM picture has 4 planes
for specifying absolute color pixels giving up to 16 absolute colors
which would be specified in the ILBM CMAP chunk.  The bits in the last
two planes are color modification bits which cause the Amiga, in HAM mode,
to take the RGB value of the previous pixel (Hold and), substitute the 4
bits in planes 0-3 for the previous color's R G or B component (Modify)
and display the result for the current pixel.  The color modification bits
in the last two planes are interpreted as follows:

   00 - no modification.  Use planes 0-3 as normal color register index
   10 - hold previous, replacing Blue component with bits from planes 0-3 
   01 - hold previous, replacing Red component with bits from planes 0-3
   11 - hold previous. replacing Green component with bits from planes 0-3


How Amiga HALFBRITE mode works:
===============================

   This one is simpler.  In HALFBRITE mode, the Amiga interprets the
bit in the last plane as HALFBRITE modification.  The bits in the other
planes are treated as normal color register numbers (RGB values for each
color register is specified in the CMAP chunk).  If the bit in the last 
plane is set (1), then that pixel is displayed at half brightness. 
This can provide up to 64 absolute colors.


Other Notes:
============

   Amiga ILBMs images must be a even number of bytes wide.  Smaller
images (such as brushes) are padded to an even byte width.

   ILBMs created with Electronic Arts IBM and Amiga "DPaintII" packages
are compatible (though you may have to use a '.lbm' filename extension
on an IBM).  The ILBM graphic files may be transferred between the
machines (or between the Amiga and IBM sides your Amiga if you have
a CBM Bridgeboard card installed) and loaded into either package.


-- 
==========================================================================
  Carolyn Scheppner -- CATS  Commodore Amiga Technical Support
  PHONE 215-431-9180   UUCP  ...{uunet,allegra,rutgers}!cbmvax!carolyn 

 Oh I'm a numberjack and I'm OK, I code all night and I work all day...
==========================================================================
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