Data for overworld background graphics begins at 1890000. Offsets 1890018-89153F are the pointer table. There are 0x2A5 (1890008-189000B) pairs of pointers. The first in a pair is the component's debug name (1891540-1892C6F). The second points to the beginning of the component's data. There are 4 types of components- palettes, tiles, chunk assembly, and image assembly.
The types of components used for each graphic fall into 4 main groups:
Group 0- The basic group. It has a palette, tile/chunk data, and image assembly.
Group 1- Graphics with an excerpt. It has a palette, excerpt data, tile/chunk data, and image assembly. Examples include Rub-a-Dub River, Hill of the Ancients, and town areas.
Groups 2 and 3- Group 0 graphics with an overlay of group 0 or 1, respectively. They have a palette, data for their overlay, tile/chunk data, and image assembly. Examples include underwater friend areas and Luminous Cave.
Dream World/Dream Eater, the Aged Chambers, Crater, and the team base are special cases. Dream World and Dream Eater share tile/chunk and image assembly data but have different palettes. The Aged Chambers share tile/chunk data but have different image assembly data and palettes. Crater is group 2, but with a group 1 base layer. The team base is a series of type 1 graphics, each with multiple excerpts. The graphic used is determined by the player's type and construction progress while the excerpts used are determined by the player's species. Details are unknown.
|0x00||2||uint16||palcount||The number of palettes|
|0x04||palcount*60||RGBX||colortbl||A series of 16-color palettes|
*Many graphics have several palettes that are identical but shifted over. This is likely to swap out palettes in order to animate tiles that change color but don't move.
Graphics are broken into chunks, which are further broken into 8x8-pixel tiles. Each tile in a chunk and each chunk in a graphic is arranged in left-to-right rows going top-to-bottom.
|0x00||2||uint16||chunkw||The chunk width, in tiles|
|0x02||2||uint16||chunkh||The chunk height, in tiles|
|0x04||2||uint16||tilecount||The number of tiles defined|
|0x0E||2||uint16||chkcount||The number of chunks defined|
|varies||0-3||Padding to realign with 4-divisible offsets|
Block data is 4bpp linear in reverse order. Some animated sections of a graphic are placed before the header of the main tile data. These include flowers and lily pads seen in town, the sun at Hill of the Ancients, and the water wheel at Rub-a-Dub River. They have their own header consisting of 2 shorts representing the excerpt's width and height in tiles followed by what is likely a series of integers representing the duration of each frame. What determines frame count is unknown.
In the chunk data, each tile gets 2 little-endian bytes. This table is listed from the lsb (first byte).
|0x0A||1||Flip the tile horizontally|
|0x0B||1||Flip the tile vertically|
*Tiles are assigned consecutive indices based on their order in data. The first index is usually 1. However, in fully animated graphics and the aforementioned animated excerpts, the first index appears to vary. For excerpts, the indices continue where the main part ended.
Fully animated graphics such as attack animations precede the tiling data of each frame with an unidentified integer followed by a 16-byte header identical to the block data header (which is omitted from its typical location).
The data that encodes chunk order is preceded by a 12-byte header.
|0x00||1||byte||camx||The horizontal position of the camera|
|0x01||1||byte||camy||The vertical position of the camera|
|0x02||1||byte||chunkw||The width of chunks, in tiles, redundant?|
|0x03||1||byte||chunkh||The height of chunks, in tiles, redundant?|
|0x04||1||byte||imgwidth||The width the image, in chunks|
|0x05||1||byte||imgheight||The height of the image, in chunks|
The each row has a series of Pair-24 NRL compressed values, one for each chunk column (rounded up to the nearest even number). These vaues are xor'ed with the respective indices of the previous row to get the actual indices of the chunks in the current row. The first row assumes all previous indices were 0.
This section works differently for the backgrounds of rest stops and boss rooms. It is believed it codes for mobility types instead. Locations of actual image assembly data in these cases is unknown.
Unknown data follows. It is likely NRL compressed and believed to code collision.