HNM4

2007-04-11T04:37:24Z

MlyOjl:

HNM4 is the third generation of the Cryo HNM video format. At least two decoder-incompatible variation of this format is known. Main version, operates in 320x200x256 resolution and simplified 640x480x256 spin-off, used in ALIENS game. Both formats share same container structure.

== File Format ==
File header:
dword signature - file signature, "HNM4"
dword unknown1 - probably format version
word width - width of the frame
word height - height of the frame
dword filesize - size of the entrie hnm file
dword frames - number of frames
dword taboffset - offset of the TAB chunk
word bits - sound sample resolution
word channels - number of sound channels
dword framesize - frame allocation size (width * height)
byte unknown2[16] - seems to be unused
byte copyright[16] - "-Copyright CRYO-"

The rest of the file organized on manner similar to [[HNM (1)]] format. All single-frame related chunks combined to superchunks, where each superchunk starts with 32-bit value, lower 24 bits of it indicates superchunk size and highest 8 bits - flags (unknown). Each chunk starts with another 24 8 bits length header followed by 4 bytes of chunk ID (where only 2 bytes can be actually used), and chunk data. Near the end of file usually TAB chunk located, it doesn't have an upper-level superchunk.

=== PL Chunk ===
Palette. Stored in same format as used in [[HNM (1)]].
* ALIENS: However, non-zero bit 23 of ChunkId indicates that palette stored in complete 8-bits-per-component format, unlike regular 6-bits otherwise.

=== IZ Chunk ===
Keyframe. See video decompression below.

=== IU Chunk ===
Interframe. See video decompression below.

=== SD Chunk ===
Sound data. [[PCM#Differential_PCM|DPCM]]-encoded sound fragment.

== Decoding Video ==
=== Intraframes ===
Intraframe decompression is simialar to [[HNM (1)]] 0xFE frames. But with a few changes. Chunk data starts with small 4-bytes header:
word width - width of the frame
byte height - height of the frame (only lower byte of it)
byte mode - rendering mode (always 0xFE ?)

Then immediately followed by LZ-packed raster (without 6-byte compression header as was used before). LZ decompression algorithm remains the same, but bitreader use 32-bits queue and processing bits from highest to lowest.

* You can safely ignore width/height/mode fields of the frame header as original decoder always renders whole frame regardless specified values.

=== Interframes ===
==== HNM4 Interframe ====
Packed interframe consist of one or more compression codes. Depending on the code's count field following cases possible:

bits
0..4 : count = 0
5..7 : tag:
0: copy next two bytes of input to the output
1: skip (next byte of input) * 2 bytes of output
2: skip (next word of input) * 2 bytes of output
3: fill (next byte of input) * 2 of output with (next byte of input)
else: finish the unpacking

- or -

0..4 : count <> 0
5 : previous
6 : backline
7 : backward
8 : swap
9..23 : offset

In this case for (count) pixel pairs, copy them from the (output offset * 2 - 32768) of the current frame to the output. Flag bits modify various parameters of this scheme (multiple flags can be used):
* previous - copy pairs from the previous frame
* backline - first pixel located 2 lines above (swap)
* backward - advance source pixel pairs in backward order
* swap - swap pair's pixels

==== HNM4A Interframe ====
HNM4A interframe is a simplified version of the HNM4 interframe. Code format:
bits
0..5 : count = 0
6..7 : tag:
0: skip (next byte of input) bytes of output
1: draw 1x2 column of the output with next 2 bytes of input
2: advance to the next line of output (but keep current x position)
3: finish the unpacking

- or -

0..5 : count <> 0
6 : previous
7 : delta
8..23 : offset

For (count) 1x2 columns copy them from the (output offset) to the output.
* previous - copy column from the previous frame
* delta - copy column from (output offset - 65536)

=== Postprocessing ===
For HNM4 (but not HNM4A movies) you need to perform extra frame postprocessing by swapping pixels using following self-explaining example (assuming 4xN image):

Just unpacked frame Final frame
p0 p1 p2 p3 ==> p0 p2 p4 p6
p4 p5 p6 p7 p1 p3 p5 p7
... ...

== Decoding Audio ==
'''TODO'''

== Games Using HNM4 ==
* Lost Eden
* Hardline
* Aliens

[[Category:Game Formats]]

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HNM4