Tiles and autotiling

Seamless tiles

1. The tile size belongs to the game, not to you. Ask. 16 is the safe default, 8 and 32 are common, 48 is RPG Maker MV/MZ. Edge doubling quadruples the area (one 32 holds four 16s), so never mix sizes in one set. Tiles are perfect squares with no gaps; a big object is split into tile-sized pieces.
2. Draw a master tile that wraps on every edge first: rough it in, offset by half on both axes, rework the middle (the old seams), offset back. Side-view strips need horizontal wrap only; top-down fields need both axes.
3. Never judge a tile alone. Repeat it 3×3 (2×2 minimum) and look at game scale. Seams are invisible in isolation.
4. What makes a grid show: one distinct element, a gradient across the tile, a strong shadow or high-contrast pixels, one feature filling the tile, detail piled in one corner, pixels piled along the edge, shapes repeating every N px in aligned lines. Break it by letting clusters cross the border. A visible grid is acceptable only for naturally gridded materials (floor slabs, ridged metal).
5. Variants change only the centre. Keep the outer 1–2 px identical to the master (for 16 px: leave x and y in 0–1 and 14–15 untouched, derived) so every variant still joins every other. Make 3–4 variants of any heavily used tile and place them semi-randomly.
6. Layer textures that do not fill a tile. Tufts, rocky dirt, cracks go on transparent and sit over a different base: texture combinations without baking each pair. Draw order bottom → top: dirt, grass, shadows, trees and rocks, cliffs, shadows 2, cliffs 2.
7. Shadows are short and uniform. Cast on one or two faces only, never longer than one tile, the same length for every wall height. Bake a shadow into a tile only when it fits inside it; otherwise keep shadow tiles on their own layer.
8. Tiles are quieter than sprites: desaturated, same base hues. A palette-swapped copy of a set is a legitimate way to get a second town.
9. Tall things (trees, towers, walls): the footprint stays on the grid, only the top may exceed it. Split into tile-sized pieces so they can sort by y.

Transitions

10. Cut transitions out of the two full tiles. Take pixels from terrain A where a mask says A and from B elsewhere, then run one boundary pass. Both sides then join their full tile with no seam. Do not repaint the sides.
11. Fix the boundary offset and make both ends equal. In the templates the boundary sits 5 px in from the tile edge and wobbles ±1 px in runs of 2+; the profile starts and ends at 5 so neighbours agree. Corner arcs end on the same 5 px point.
12. Boundary pass with light from upper-left: 1 px dark outline on the edge-most pixels of the upper terrain, a light rim on the pixels just inside when the edge faces north or west, a 1 px shadow on the lower terrain on the south and east sides. Flipping or rotating a lit edge puts the rim on the wrong side: after a flip re-run the pass, swapping rim and shadow (recolor).
13. Side-view top edge: grass lip overhangs the dirt (4 bands: light rim, base, dark underside, dirt); the shadow row under the lip is on the dirt. Collision stays below the lip.

Autotile sets — layouts in the section below

14. Pick the set by the terrain, not by habit. Rectangles and UI boxes: 9-slice (+ 4 inner corners). Pipes, roads, wall lines: 4-bit, 16 tiles. Smooth terrain at minimum art: dual-grid, 16 tiles. Full terrain with the tileset export: blob47. RPG Maker: A2 (20 minitiles → the same 47 shapes).
15. A diagonal neighbour counts only when both cardinals beside it are present. That rule collapses 256 masks to exactly 47. Bit order, clockwise from north: N=1, NE=2, E=4, SE=8, S=16, SW=32, W=64, NW=128.
16. The tileset tool expects canonical order: the 47 reduced masks in ascending numeric order at Aseprite tile indices 1–47 (index 0 is the reserved empty tile). Wangtile id = index − 1. Any other order autotiles wrongly; the export refuses an incomplete set.
17. Build the body tile first, then edges, then corners; draw no detail until every connection works. Squeeze every tile: avoid rare cases. Test with a single cell, a 3×3 patch (9 different tiles), an L shape (inner corner), a ring (hole), a plus.
18. Two terrains on one map: give them priorities and draw overlays lowest to highest; the higher terrain's open sides read the lower one as its background.
19. Animated tiles cycle frames inside one autotile index; the index changes only when a neighbour changes.
20. Platformer kit (any tile size): 3×3 block (floor, ceiling, walls, corners) + inner-corner set = 12 tiles at 8 px gives every level without slopes.

Four tiles from one grass/dirt pair (ground-grass-16, ground-dirt-16): o is the boundary outline, rim l and shadow D come from the pass in rule 12.

• tiles-edge-n-16 — blob47 mask 124 (N open: grass below, dirt above). West edge = the same mask transposed, then re-run the pass.
• tiles-corner-nw-16 — mask 28 (N and W open, corner rounded). The straight runs end on the same 5 px as the edges.
• tiles-inner-nw-16 — mask 127 (only NW open): a dirt notch with the same 5 px runs.
• tiles-platform-top-16 — side-view ground top, horizontal wrap only.

o = grass-outline #1f4a38
d = grass-dark    #2d6a45
m = grass-mid     #46984a
l = grass-light   #86c653
D = dirt-dark     #5b3a31
B = dirt-mid      #8c5d3e
L = dirt-light    #b98a58
---
BBBBBBBBBBBBBBBB
BBBBBBBBBDDDBBBB
BBLLBBBBBBBBBBBB
BBLDDBBBBBBBBBBB
BBBooDBBBBBooDBB
ooollooDBoollooo
lllmmlloollmmlll
mmlmmmmlllmlmmmm
mmlmmmmmmlmlmmmm
mmdmmmmmmdmdmddd
dmmmmlmlmmmmmmdd
mmmmmlmlmmmmmmmm
mllmmdmdmmmmlmlm
mmllmmmmmmlmlmlm
mmmmmdddmmdmdmdm
mmmmmmdddmmmmmmm

o = grass-outline #1f4a38
d = grass-dark    #2d6a45
m = grass-mid     #46984a
l = grass-light   #86c653
D = dirt-dark     #5b3a31
B = dirt-mid      #8c5d3e
L = dirt-light    #b98a58
---
BBBBBBBBBBBBBBBB
BBBBBBBBBDDDBBBB
BBLLBBBBBBBBBBBB
BBLDDBBBBBBBBBBB
BBBBBBBBBBBooDBB
BLLBBBBBooollooo
BBBBBBoolllmmlll
BBBBBBolmlmlmmmm
BBBBBolmmlmlmmmm
BBBBBolmmdmdmddd
BDDDBollmmmmmmdd
BBBBolmlmmmmmmmm
BBBBolmdmmmmlmlm
BBBBBolmmmlmlmlm
BBBBBoldmmdmdmdm
BBBBBolddmmmmmmm

o = grass-outline #1f4a38
d = grass-dark    #2d6a45
m = grass-mid     #46984a
l = grass-light   #86c653
D = dirt-dark     #5b3a31
B = dirt-mid      #8c5d3e
L = dirt-light    #b98a58
---
BBBBBolmmmmmmmml
BBBBBolmmdddmmml
BBLLBolmmmdddmmd
BBLDolmmmmmmmmmm
BBBolmmmmmmmmmml
ooolmllmmmmmmlml
lllmmmllmmmmmdmd
mmlmmmmmmlmlmmmm
mmlmmmmmmlmlmmmm
mmdmmmmmmdmdmddd
dmmmmlmlmmmmmmdd
mmmmmlmlmmmmmmmm
mllmmdmdmmmmlmlm
mmllmmmmmmlmlmlm
mmmmmdddmmdmdmdm
mmmmmmdddmmmmmmm

d = grass-dark    #2d6a45
m = grass-mid     #46984a
l = grass-light   #86c653
D = dirt-dark     #5b3a31
B = dirt-mid      #8c5d3e
L = dirt-light    #b98a58
---
................
..ll...l..ll....
llmmlllmllmmllll
mmmmmmmmmmmmmmmm
mmmmmmmmmmmmmmmm
ddmmddmmmddmmddd
DDddDDmmdDDddDDD
BBDDBBdmDBBDDBBB
BBBBBBDdBLLBBBBB
BBBBBBBDBLDDBBBB
BDDDBBBBBBBBBBBB
BBBBBBBBBBBBBDBB
BBBBBLLBBBBBBBDD
BBBBBLDDBBBBBBBB
BBBBBBBBBBLLBBBB
BBBBBBBBBBBBBBBB

# = neighbour is the same terrain, . = not, centre always terrain. Each cell is index=mask; the index is the Aseprite tile index, the mask is the sum of the set bits. Author in this order.

 1=0     2=1     3=4     4=5     5=7     6=16    7=17    8=20
...     .#.     ...     .#.     .##     ...     .#.     ...
.#.     .#.     .##     .##     .##     .#.     .#.     .##
...     ...     ...     ...     ...     .#.     .#.     .#.

 9=21   10=23   11=28   12=29   13=31   14=64   15=65   16=68
.#.     .##     ...     .#.     .##     ...     .#.     ...
.##     .##     .##     .##     .##     ##.     ##.     ###
.#.     .#.     .##     .##     .##     ...     ...     ...

17=69   18=71   19=80   20=81   21=84   22=85   23=87   24=92
.#.     .##     ...     .#.     ...     .#.     .##     ...
###     ###     ##.     ##.     ###     ###     ###     ###
...     ...     .#.     .#.     .#.     .#.     .#.     .##

25=93   26=95   27=112  28=113  29=116  30=117  31=119  32=124
.#.     .##     ...     .#.     ...     .#.     .##     ...
###     ###     ##.     ##.     ###     ###     ###     ###
.##     .##     ##.     ##.     ##.     ##.     ##.     ###

33=125  34=127  35=193  36=197  37=199  38=209  39=213  40=215
.#.     .##     ##.     ##.     ###     ##.     ##.     ###
###     ###     ##.     ###     ###     ##.     ###     ###
###     ###     ...     ...     ...     .#.     .#.     .#.

41=221  42=223  43=241  44=245  45=247  46=253  47=255
##.     ###     ##.     ##.     ###     ##.     ###
###     ###     ##.     ###     ###     ###     ###
.##     .##     ##.     ##.     ##.     ###     ###

Families (derived): 0 cardinals = 1 tile; 1 = 4 end caps; 2 = 2 straights, 4 outer corners, 4 outer corners with the inside diagonal filled (the 4 notches); 3 = 4 T-shapes × 4 corner fills; 4 = 1 plus-without-corners, 4, 6, 4, 1 full. Total 1 + 4 + 10 + 16 + 16 = 47.

4-bit edge (Wang edge): index = N·1 + E·2 + S·4 + W·8, no inner corners.

Wang corner / dual grid: the data grid is offset half a tile from the display grid; each display tile looks at the 4 data cells at its corners, index = TL·1 + TR·2 + BR·4 + BL·8. The boundary runs through the middle of the tile, so inner corners come free.

Indices 5 and 10 are saddle tiles: decide once which terrain connects through the middle.

Think in half tiles (minitiles, 8 px at 16). A block is 2 × 3 tiles = 4 × 6 minitiles; an A2 sheet is 8 × 4 blocks (16 × 12 tiles; 256 × 192 px at 16 px).

minitile (col,row)     col: 0   1 | 2   3
row 0                      P   P | I   I     P = palette preview, unused in maps
row 1                      P   P | I   I     I = inner (concave) corner pieces
                           ------+------
rows 2-5  = box rows 0-3   B   B   B   B     B = outer corners, rims, centre
                           B   B   B   B
                           B   B   B   B
                           B   B   B   B

Every minitile always lands in the same quarter of the assembled tile (a top-left piece in the top-left quarter). A quarter depends on two cardinals and one diagonal: both cardinals open → outer corner; one open → rim along that side; both closed and the diagonal open → inner corner; all closed → full. Source piece per quarter (box coords col,row; derived from the geometry):

Edge contract: draw edges per minitile, not per tile. The right edge of every top-left piece must match the left edge of every top-right piece it can touch; same for top and bottom pairs. 4 quarters × 5 states = 20 minitiles cover all 47 shapes.

• Supported layouts: grid and blob47. blob47 writes a Tiled wangset (format="tiled", two explicit colours, wangid order N, NE, E, SE, S, SW, W, NW) and a blob47 index → mask list in format="json". format="godot" writes the atlas without terrain data: set peering bits in the editor from the same mask table.
• The set must have 48 tiles including the empty tile 0, in canonical order, all pixel-distinct.
• Wang-16, dual-grid, 4-bit and A2 have no wangset export: export grid, or for RPG Maker export the sheet with export op png. State your index convention to the user.
• pack assigns indices by first appearance scanning the mockup row by row, skipping empty cells, and deduplicates identical cells. So paint the 47 tiles in canonical order, left to right, 8 per row, and make sure no two are identical.
• stamp places tiles by grid cell (x, y in cells), the way to test a set with a patch.
• To build blob47 from A2 pieces: for each of the 47 masks, take each quarter's state from the table, draw op blit the four minitiles into its cell of the mockup, then pack.

1. Fix the tile size and palette (rules://21-limited-and-platform-palettes for platform sets). Make the canvas a whole number of tiles: pack refuses otherwise.
2. Master tile: draw it, blit the four quadrants swapped to move the seam to the middle (transform op translate does not wrap), rework, swap back.
3. Wrap test: draw op blit the tile into its eight neighbours on a scratch layer, look op preview, then look op ascii the first and last columns and rows and check they continue each other.
4. Transitions: composite A and B through the mask, then the boundary pass (rule 12). One edge, one outer corner, one inner corner cover all four sides once flipped and re-lit.
5. Autotile set: paint the mockup cells in canonical order, body tile (mask 255, index 47) first. For every pair of tiles that show terrain on touching sides, read_pixels the border column or row and check it is identical.
6. tileset op pack (tileWidth, tileHeight). Read tileCount and cellCount: tileCount 47 means all distinct. Fewer means two tiles are identical: redraw the pair.
7. Test patches with tileset op stamp on a tilemap layer, look op preview: single cell, 3×3, L, ring, plus.
8. tileset op export layout="blob47" format="tiled"; then format="json" and compare a few blob47 entries to the diagram above by eye.
9. validate layers, strays.

• Every tile repeated 3×3 shows no seam, no grid, no aligned pixel lines.
• Variants differ in the centre only; borders are identical.
• Transition tiles join both full tiles without a visible step; boundary offsets agree on every side.
• Light direction is the same on all edges; no rim on a south or east edge.
• Set order is canonical; tileCount is 47 (blob47) and the patch tests (single, 3×3, L, ring, plus) look correct.
• Tiles are less saturated than the sprites that walk on them.

• Silber, Pixel Art for Game Developers, ch. 9: seamless textures, 9-slice and three-tile transitions, platform tiles.
• Slynyrd, Pixelblog 20, 28, 35, 43, 45, 47, 60 (tile sets, layered tiles, 3×3 + inner corner kits) — slynyrd.com/pixelblog-catalogue.
• Saint11 tutorial Tiles — saint11.art/blog/pixel-tutorials.
• Red Blob Games, tile-mask autotiling write-ups; cr31 blob-47 template and comments (OpenGameArt); Godot docs, Using tilesets (terrain peering modes).
• RPG Maker A2 autotile block diagrams (RPG Maker blog, nerulio); Tiled manual, terrain/wang sets.
• Tsugumo, So You Want To Be A Pixel Artist? ch. 2 (the grid), 4, 5.
• Dawe, Make Your Own Pixel Art, pp. 105–111 (tilesets, seams, plus-shaped starter set).