PC DOS Graphics Formats

PC DOS Graphics Formats

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8086 / 16-bit real-mode formats for the GOG Might and Magic II PC build. Not Amiga — see 06-gfx-loading.md for .32 planar sheets.

Artifact	Path
Disassembly	EXTRACTED/pc/ — mm2.capstone.asm, cga.capstone.asm, ega.capstone.asm
Decoder	tools/decode_pc_gfx.py
Encoder + round-trip validation	tools/encode_pc_gfx.py (--roundtrip)
Gallery export	tools/pc_gfx_export.py → wiki/public/gallery/pc/
Extracted PNGs	EXTRACTED/pc_gfx/{cga,ega}/ walls; EXTRACTED/pc_gfx/monsters/{cga,ega}/ combat

Regenerate listings:

python tools\disasm_pc_x86.py "C:\Program Files (x86)\GOG Galaxy\Games\Might and Magic 2\MM2.EXE" `
  "C:\Program Files (x86)\GOG Galaxy\Games\Might and Magic 2\CGA.DRV" `
  "C:\Program Files (x86)\GOG Galaxy\Games\Might and Magic 2\EGA.DRV"
python tools\decode_pc_gfx.py "C:\Program Files (x86)\GOG Galaxy\Games\Might and Magic 2" --batch
python tools\pc_gfx_export.py

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File inventory

Extension	Mode	Driver	Examples
.4	CGA 320×200×4 (2 bpp)	CGA.DRV	CASTLE.4, TOWN.4, MONSTERS.4, SKY.4
.16	EGA 16-colour (4 bpp linear on disk)	EGA.DRV	CASTLE.16, MONSTERS.16, …

Filename table in MM2.EXE @ 0x8815 (same stems as Amiga .32 list, different extension).

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LZW container (all .4 / .16 blobs)

Field	Size	Notes
uncompressed_size	u32 LE	Expected decompressed byte count
LZW stream	rest	lzw_decompress @ MM2.EXE 0x2A42

Decompressor uses 9→12 bit codes, early-change dictionary growth, clear code 0x100. Root character for new dictionary entries is expand(code)[0] (not the last char).

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Wall / environment sheets

Used for first-person view walls, floors, skies, UI sprites (THROW, TOWN, SKY, …).

Decompressed layout

Offset	Field
+0	frame_count = dec[0] & 0x3F; class = dec[0] >> 6 (gfx_format_class_check @ 0x5C5A)
+1	flags — low nibble + bits 4–5 (frame_index_scale @ 0x42EA)
+2	Offset table: u32 LE `(end<<16)
per offset	Frame blob

Per-frame blob

Field	Size
width	u16 LE
height	u16 LE
pixels	(width+3)/4 bytes/row (CGA) or (width+1)/2 bytes/row (EGA) × height

CGA: 2 bpp, MSB-first, 4 pixels/byte (CGA.DRV plot @ 0x34C).
Palette 1 (black / cyan / magenta / white) via INT 10h AH=0Bh BX=0x0101 in drv_init @ 0x2A8.

EGA: 4 bpp linear on disk (hi nibble = left pixel). Not planar despite EGA blit using map-mask for screen writes.

Runtime frame selection (pick_frame_from_sheet @ 0x5D4A)

1. load_wall_gfx_sheet @ 0x5234 LZW-decompresses the file into a resource record.
2. frame_index_scale @ 0x42EA derives a frame index from dec[1] flags and viewport state (word ptr [0x9E0C]). For sheets with dec[1]=0 and scale 0, the default logical frame budget is 8 (mov word ptr [0x9e0c], 8 @ 0x433A) — matches MASTER.16.
3. pick_frame_from_sheet walks driver token stream (parse_resource_token @ 0x366E) to resolve the sheet slot, then indexes the offset table at logical_index × 2 (shl ax, 1 @ 0x5E60 before the table pointer add).
4. CGA.DRV blit @ 0x65A / EGA equivalent copies rows to video or off-screen buffer.

Packed u32 tables: sheets like MASTER.16 store each entry as (frame_end << 16) | frame_start (u32 LE). dec[0] & 0x3F is the frame count (15 on GOG MASTER.16). Frame 14 is the 320×196 EGA title screen ("Might and Magic Book Two" + pegasus); frames 0–13 are menu/UI sprites stored in the same blob. The region after the first eight small frames is not a separate script block — it holds additional packed frame records and pixel data until the title bitmap @ 0x2B10. MM2.EXE indexes with index×4 @ 0x7018.

Grouped u16 view: the same bytes can be read as interleaved u16 [start, end] pairs (the first eight frames only). Runtime pick_frame_from_sheet uses index×2 for that view @ 0x5E60. Tools prefer the packed-u32 interpretation when it decodes more frames with matching end offsets.

There is no compositing within a wall sheet — each frame is a complete image. Extraction writes:

• raw/frameNN.png — individual frames as stored
• sheet.png — all frames tiled in a grid (reconstructed atlas view)

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Monster combat atlas (MONSTERS.4 / MONSTERS.16)

Indexed by picture number — the same id Amiga uses for NN.anm combat sprites (monsters.dat byte @ 0x15 & 0x7F). PC and Amiga share the same monsters.dat (GOG MONSTERS.DAT is LZW-compressed on disk but decompresses to the Amiga file).

Gallery tools

Output	Command
Per-slot PNG/GIF (01/…)	python tools/decode_pc_gfx.py MONSTERS.16 --monsters --extract-all
Slot compare 01–74	python tools/gen_gfx_compare_html.py → EXTRACTED/pc_gfx/compare/
Per-monster Amiga/CGA/EGA GIFs	python tools/export_monster_variants.py
Wiki table (256 monsters)	python wiki/scripts/export-monster-variants.py

File header (GOG)

The prefix is not u16 count + u16[count]. It is a flat table of u32 LE blob file offsets:

Entry	File bytes	GOG value	Meaning
entry[0]	0..3	300	Header size in bytes and offset of first blob
entry[1]	4..7	3134	Blob for picture id 2
entry[N−1]	(N−1)×4 …	0 or offset	Blob for picture id N

• 75 entries on GOG (300 / 4); bytes 0..299 are the table, first LZW blob @ 300.
• entry[k] = 0 → no blob for picture id k + 1 in this file.
• monsters.dat picture id N → seek u32[(N − 1) × 4] (1-based picture id, 0-based table index). Confirmed: pic 1 (spider) → entry 0; pic 19 (cat) → entry 18.

Runtime loader (MM2.EXE @ 0x6824)

1. Open MONSTERS.4 or MONSTERS.16 (filename patched @ DS:0x4D0).
2. Read 0x11C (284) bytes into RAM @ 0x6052 — 71 u32 entries in the buffer.
3. bl = byte [0x392]; seek u32[bl × 4] from the buffered header.
4. Read u32 decompressed size @ blob start, LZW-decompress.

The loader has no zero-check; zero offset seeks to file start and fails decompression. When a slot is empty, the caller advances to the next non-empty picture (below).

0x68ED opens a different file (path @ DS:0x52D2) with a u16 table — not the primary MONSTERS.* u32 header. Do not use it for combat atlas export.

Missing-slot fallback (observed in-game)

When header entry is 0, the DOS build advances to the next non-empty picture:

Monster	monsters.dat pic	PC slot	In-game PC sprite
Merchant	34	empty	pic 35 (mage)
Mountain Man	43	non-zero	blob = Amiga 42.anm (amazon); 43.anm absent from PC

Some non-empty slots also differ from Amiga for the same picture id (port reshuffle). Export marks fallback rows → pic NN. See Monster variants.

Per-picture LZW blob

Offset	Field
+0	u32 LE decompressed_size
+4	LZW bitstream
dec +0	frame_count = dec[0] & 0x3F
dec +1	flags
dec +2	u16 LE [frame_count] inner frame offsets
mid	Animation script sequences (see below)
per offset	Frame records

Animation scripts

Between the u16 offset table and the first frame offset, the blob contains one or more script sequences:

• Each sequence: byte pairs (frame_index, delay) … terminated by 0xFF
• End of all scripts: 0xFF 0xFF
• delay is a combat tick hold count (used by combat overlay timing)
• Typical pattern: script 0 = idle sway overlays; script 3 = walk cycle (3,0)(4,0)…(7,0)…

Combat load path: load_combat_monster_gfx @ 0x6544 → pick_frame_from_sheet → overlay draw thunk @ 0x78C6 → CGA.DRV/EGA.DRV sprite decode.

Per-frame record

Offset	Field
+0	x — blit X on 96×96 combat canvas
+1	y — blit Y
+2	width
+3	height
+4	Nibble-RLE token stream

Frame 0 is the full base sprite (usually x=0, y=0). Later frames are delta patches. Header (x,y,w,h) matches Amiga TV-prelude slot N−1 for frame N (verified on spider pic 1).

Compositing (export + runtime)

Same rule as Amiga composite_anm_frame (07-anm-tv-format.md):

1. Blit frame 0 onto the 96×96 canvas.
2. For delta N: clear (x,y,w,h) to transparent, then blit patch.
3. Script step (frame_idx, delay): base only if frame_idx == 0, else base + delta.

GIF export flattens to opaque (13,3,3) with disposal 2.

Nibble-RLE sprite codec

Implemented in CGA.DRV 0xAF2 and EGA.DRV 0x1422. Draws into a 96×96 off-screen buffer (0x900 bytes CGA / 0x1200 EGA per slot), then op 0x17 blits to screen.

Each token byte:

Bits	Meaning
high nibble	Run count; run length = count + 1
low nibble	Code (see below)

CGA (.4): code 0–3 = literal colour run; 4–15 = transparent run (skip pixels).
EGA (.16): code 5 = transparent; else colour = xlat[code] where xlat @ EGA.DRV 0x121B = [0,1,2,9,6,8,10,3,4,5,7,11,12,13,14,15].

Runs wrap across scan-lines; decode stops after height rows. CGA 2bpp shift table @ 0xAE4 = 06 04 02 00 (MSB-first within byte).

Extraction layout

EXTRACTED/pc_gfx/monsters/ega/
  01.gif
  01/
    frames/f00.png
    base.png
    s5_step00.png
    s5.gif

GOG coverage (picture ids 1–74)

File	Thumbs	Empty in both
MONSTERS.4	53 / 74	+ CGA-only gaps at 5,6,29,36,45,71
MONSTERS.16	59 / 74	7,23,28,30,32,34,38,42,47,49,52,54,64,70,73,75

Amiga repo: 72 .anm files (01–74 with gaps).

Blob sharing (multiple picture ids → same LZW blob):

EGA blob @	Picture ids
0x287F	03, 06
0x35CC	04, 72
0x3F2E	05, 74
0x778F	11, 22
0xEE6F	23, 46
0xF99C	24, 48

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Title screen (PC DOS)

There is no intro.4 / intro.16 on the GOG PC install (Amiga intro.32 pegasus background is not a separate PC sheet). Title/menu presentation uses:

Asset	Role
NWCP.4 / NWCP.16	“New World Computing Presents” logo (320×83, filename table index 8)
BOOK.4 / BOOK.16	Menu book ornament frames
1MENU1.OVL / 1MENU2.OVL	Title/menu overlay code (text, input, flow)

See Amiga title docs (38-title-screen-and-intro-assets.md) for the full pegasus + introclips attract sequence — PC CGA/EGA uses the assets above instead.

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Overlays (.OVL)

Combat logic lives in external 2COMBAT.OVL (flat code, no MZ header). MM2.EXE overlay manager @ 077D:0344 loads .OVL files listed in the descriptor table (seg 0x6BF). Title/menu flow uses 1MENU1.OVL / 1MENU2.OVL (see title screen table above).

The sprite decode itself is in the video drivers (CGA.DRV / EGA.DRV), not the overlay.

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Encoder + round-trip validation (tools/encode_pc_gfx.py)

Inverse operations for everything above, plus a --roundtrip mode that decodes every real .4/.16 file in a GOG install, re-encodes, decodes again, and compares:

python tools\encode_pc_gfx.py --roundtrip
python tools\encode_pc_gfx.py --roundtrip --gog "C:\...\Might and Magic 2" --out-root EXTRACTED\pc_gfx_roundtrip

Codec	Status	Verified against
LZW compressor (inverse of lzw_decompress @ MM2.EXE 0x2A42)	Byte-identical, 60/60 wall sheets + 111/111 monster picture blobs	full decompressed payload of every real GOG .4/.16
CGA 2bpp / EGA 4bpp pixel pack	Byte-identical, 756/756 frames	every wall-sheet frame in the GOG install
Monster nibble-RLE encoder	Pixel-identical, 807/807 frames	every combat-sprite frame (base + delta)
Monster atlas container rebuild	Pixel-identical, 111/111 pictures	full MONSTERS.4/MONSTERS.16 rebuild (fresh blob offsets; frame content decodes identically)
Wall-sheet container rebuild (frame-level, byte-exact)	28/60 sheets byte-identical; rest have a documented gap (below)	parse_wall_sheet frame list rebuilt at original offsets

The LZW compressor's key subtlety (found by fuzzing + testing against every real file): the decompressor's dictionary is always exactly one entry behind the compressor's own (the code >= dict_next "KwKwK" branch is what makes that safe for code values), so the compressor's code-width growth check must trigger one entry later than the decompressor's own check (dict_next > dict_limit, not >=) to stay bit-aligned. Getting this wrong silently corrupts output starting at the first width transition (~code 256) while looking correct on small test files that never reach it.

Known gap: wall-sheet payload rebuild (encode path)

Packed-u32 wall sheets (CASTLE, CAVE, TOWN, MASTER, outdoor biomes, *B border sets, …) decode all frames correctly via parse_wall_sheet / PcGfx.cpp, but encode_pc_gfx.rebuild_wall_sheet_payload is not yet byte-identical to the original decompressed blob for those sheets (interstitial bytes between packed table entries). This affects container rebuild only — LZW round-trip and per-frame pixel encode/decode remain correct.

Grouped-u16 sheets (CASTLET, CAVET, TOWNT on some builds) share the same [start, end] interleaving; tools pick grouped u16 or packed u32 by score. Every frame the decoder extracts still re-encodes pixel-perfectly.

Maze walker export (tools/export_map_walker.py) uses parse_wall_sheet for PC .4/.16 PNGs under wiki/maze-walker/pc/ (386 frames per CGA/EGA variant on GOG). The browser lazy-loads those trees; bump PC_GFX_BUILD_ID in wiki/maze-walker/version.js after re-export.

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Related Amiga docs

Topic	Doc
.32 planar sheets	06-gfx-loading.md
3D view composition	15-3d-view-and-game-screen.md
Combat overview	26-combat-overview.md
Monster records	16-monster-ability-format.md