Sonic the Hedgehog (16-bit)/Level Editing

SCHG: Sonic the Hedgehog
Main Article Art Editing Object Editing Level Editing Text Editing RAM Editing Miscellaneous Editing

Object placement

There are six bytes in one object definition. The first two bytes are the X position of the object. The next two bytes, broken down in bits, have the format AB00 YYYY YYYY YYYY, where A is the vertical flip flag, B is the horizontal flip flag and YYYY YYYY YYYY is the Y position of the object. The 5th byte is the reference number on the object pointer list, with the most significant bit if set indicating that the object should be assigned an entry on the object respawn table, and the 6th byte is an optional declaration to use for defining that object's behavior and/or animation. This will depend on the object.

The 6th byte, the object subtype, is loaded in byte $28 of the status table of that object (see SCHG:Sonic the Hedgehog/RAM Editing#Object Status Table Format for more info).

Level layout

Level layouts are pretty simple. There is a two byte header, with the first byte being width - 1, and the second being height - 1. After that, there is one byte per 256x256 tile to place on the map. The blocks are put together from left to right, top to bottom. Unlike Sonic 2 and 3, which have two solidity layers, Sonic 1 uses a special flag to mark loop tiles. If the most significant bit (0x80) of the byte is set, then that chunk will be marked as a loop tile, and will be switched with the tile following it in the list as Sonic passes through it.

Sonic 1 loads level layouts into RAM in a format that allows level sizes up to 64 by 8 chunks.

16x16 block mappings

This data is Enigma compressed.
16x16 block mappings consist of four 8x8 tiles, arranged in the shape:

0	1
2	3

Each 8x8 tile is represented by one word, which like all SEGA Mega Drive VDP pattern indices, is a bitmask of the form PCCY XAAA AAAA AAAA. P is the priority flag, CC is the palette line to use, X and Y indicate that the sprite should be flipped horizontally and vertically respectively, and AAA AAAA AAAA is the actual tile index, i.e. the VRAM offset of the pattern divided by $20.

256x256 block mappings

This data is Kosinski compressed.
256x256 block mappings consist of 256 16x16 tiles, arranged in the shape:

0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47
48	49	50	51	52	53	54	55	56	57	58	59	60	61	62	63
64	65	66	67	68	69	70	71	72	73	74	75	76	77	78	79
80	81	82	83	84	85	86	87	88	89	90	91	92	93	94	95
96	97	98	99	100	101	102	103	104	105	106	107	108	109	110	111
112	113	114	115	116	117	118	119	120	121	122	123	124	125	126	127
128	129	130	131	132	133	134	135	136	137	138	139	140	141	142	143
144	145	146	147	148	149	150	151	152	153	154	155	156	157	158	159
160	161	162	163	164	165	166	167	168	169	170	171	172	173	174	175
176	177	178	179	180	181	182	183	184	185	186	187	188	189	190	191
192	193	194	195	196	197	198	199	200	201	202	203	204	205	206	207
208	209	210	211	212	213	214	215	216	217	218	219	220	221	222	223
224	225	226	227	228	229	230	231	232	233	234	235	236	237	238	239
240	241	242	243	244	245	246	247	248	249	250	251	252	253	254	255

Each 16x16 tile has a two-byte value with the format 0SSY X0II IIII IIII. SS is the solidity of the tile - 00 means not solid, 01 means top solid, 10 means left/right/bottom solid, and 11 means all solid. Y is the Y-flip flag, X is the X-flip flag, and II IIII IIII is the index of the 16x16 tile to use.