Isometric Tiles (.let)

Individual isometric tiles are stored in ./iso/let/<name>.let (TileFormat=let in iso.ini). A .let is a small (~100–800 byte) descriptor that binds a tile id to a rectangle inside an .aim atlas; it holds no pixel data itself.

Fixed header (36 bytes)

The fields at 0x0C–0x18 are the atlas source rectangle — the crop the renderer takes out of the .aim bitmap. For example klippen_auf2_8.let stores (544, 174, 136, 87), i.e. that rectangle inside KLIPPEN_0.png (a 1024x1024 atlas holding a 7x11 grid of 136x87 cliffs). A plain ground tile stores the whole small bitmap, e.g. wasser_neu = (0, 0, 34, 21). All the uint16 fields are followed by 2 padding bytes; cols/rows by 3 padding bytes.

After the header

0x24  ExtHeader   : u32 count, then u32 entries[count]
      FrameTable  : (cols + rows - 1) frames, 16 bytes each: s32 x, y, w, h
      Primary     : u32 length, then "images\module_stadtkarte/NAME.aim" (NUL-terminated)
      u32 variant_count
      Variants    : shadow / snow / scaffolding chunks (type-specific layout)

• Frame table — the per-animation sub-slices inside the source rectangle. A 1x1 tile has one frame; a building/cliff has cols + rows - 1 vertical slices. Their bounding box equals the header source rect for 868 of 934 files; for the rest the header rect is a few transparent rows taller at the top (same bottom edge). Cropping the header rect is the reliable choice.
• Primary — the length-prefixed path to the .aim atlas actually rendered. The converted PNG is <NAME>_0.png (the whole atlas canvas); the header rect selects the sub-image.
• Variants — additional length-prefixed .aim paths for the shadow, seasonal snow and under-construction (BGeruest) versions. Each variant chunk has a type-specific layout and still ends in a Primary-style path.

Example — 1_1.let (water, a 1x1 ground tile)

0000  00 52 1b 01 0e 00 00 00  ff 00 ff ff 00 00 00 00
      |  R  TexID Ty  pad      TranspType   src_x src_y
0010  00 00 00 00 22 00 00 00  15 00 00 00 01 00 00 00
      src_y..    width=34      height=21    cols=1
0020  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
      rows=1     ext_count=0   frame.x=0    frame.y=0
0030  22 00 00 00 15 00 00 00  28 00 00 00 69 6d 61 67   "imag
      frame.w=34 frame.h=21    len=0x28=40  i  m  a  g
0040  65 73 5c 6d 6f 64 75 6c  65 5f 73 74 61 64 74 6b   es\module_stadtk
0050  61 72 74 65 2f 77 61 73  73 65 72 5f 6e 65 75 2e   arte/wasser_neu.
0060  61 69 6d 00 00 00 00 00                            aim

TextureID = 0x011B = 283, source rect (0, 0, 34, 21), one frame, pointing at images\module_stadtkarte/wasser_neu.aim.

The tile size 34 x 21 matches TileSize in iso.ini. Tiles named 1_1..9_9 are ground tilesets; the remaining named files are objects. Variants are bundled by tile groups.