RotM August 2019 - Gex 3: Deep Cover Gecko

[David]

Summary

This post will be a simple summary of what was found in the discord server over this month.

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1.) Compression Format

Gex 3 uses raw deflate for compressing files. 

If you are using python, then you can use the standard zlib library to uncompress the data.

Code:

import zlib

def decompress_raw_deflate(compressed_bytes):
   return zlib.decompress(compressed_bytes, wbits=-15)

def compress_raw_deflate(bytes):
   return zlib.compress(bytes, level=9)[2:] # Remove zlib header.

2.) Font Sheet

The main font used by the game uses a 128x64 I4 texture.



3.) Objects

There are 963 objects in the game. They are defined in the ROM range 0x818C0 to 0x854F0, where each one is associated with an 8 character string ID.

// 0x10 bytes in size
struct ObjectInfo {
   /* 0x00 */ char[8] name_id;
   /* 0x08 */ u32 rom_offset_to_compressed_data_start;
   /* 0x0C */ u32 rom_offset_to_compressed_data_end;
}

4.) Levels

There are 30 levels in the game.

Show ContentLevel Names

Level ID	Name	Internal Name
0	Totally Scrooged	snow96
1	Clueless in Seattle	clue1
2	Holy Moses!	eqypt01
3	War is Heck	war01
4	The Organ Trail	gtown2
5	Cutchesse Island	pirate45
6	Unsolved Mythstories	myth64
7	Red Riding in the Hood	beane1
8	When Sushi goes Bad	anime1
9	My Three Goons	mob01
10	Superzeroes	city1
11	Snowboard Bonus	dsnw1
12	Western Bonus	burro1
13	Pirate Bonus	gator13
14	Greek Bonus	roo11
15	Army Bonus	tank11
16	Indy secret	spider01
17	Pirate secret	push44
18	Lifeguard secret	joyz1
19	Scuba secret	water16
20	Wrestling Boss	wwgex1
21	Wizard of Oz Boss	oz01
22	Final Boss	endboss1
23	Mission control	gexcave6
24	Lake Flacid	gexcave5
25	Slappy Valley	gexcave7
26	Funky Town	gexcave8
27	Title Screen	fly77
28	Boot Screen	opening1
29	New Game Cutscene	opening3

At ROM offset 0x8013C and RAM address 0x8007F53C is an array of level information structures. Offsets 0x1C & 0x20 define the ROM region

// 0x54 bytes in size
struct LevelInfo {
   /* 0x00 */ char* internal_name;
   /* 0x04 */ s16 level_title_text_id;
   /* 0x06 */ s16 channel_title_1_text_id;
   /* 0x08 */ s16 channel_title_2_text_id;
   /* 0x0C */ s16 tv_channel_img_id; // Used in hub areas. 0x06 = Snow, 0x05 = Mystery, etc.
   /* 0x0E */ s16 mission_1_text_id;
   /* 0x10 */ s16 mission_2_text_id;
   /* 0x12 */ s16 mission_3_text_id;
   /* 0x14 */ s32 unk_14;
   /* 0x18 */ s32 unk_18;
   /* 0x1C */ u32 rom_offset_to_compressed_level_data_start;
   /* 0x20 */ u32 rom_offset_to_compressed_level_data_end;
   /* 0x24 */ u32 unk24_rom_offset_to_asm;
   /* 0x28 */ u32 unk28_rom_offset_to_asm;
   /* 0x2C */ s32* unk2C_ptr;
   /* 0x30 */ s32* unk30_ptr;
   /* 0x34 */ s32 unk_34;
   /* 0x38 */ u32 unk38_rom_offset;
   /* 0x3C */ u32 rom_offset_to_channel_ci4_texture;
   /* 0x40 */ u32 rom_offset_to_N64WaveTables;
   /* 0x44 */ u32 rom_offset_to_N64PtrTablesV2;
   /* 0x48 */ u32 unk48_rom_offset;
   /* 0x4C */ u32 unk4C_rom_offset_to_compressed_data;
   /* 0x50 */ u32 unk50_rom_offset_to_compressed_data;
}

I made a simple javascript level geometry viewer that will let you view the level geometry data. You can also download the levels as 3D models as either glTF or extended OBJ.

You can download it from the github here: https://github.com/hack64-net/rotm/tree/...gex3viewer