The background tool of the opensnes-* family (The opensnes-* tools: conventions). A picture drawn at the screen's size becomes the three things a BG layer loads: the tileset (.pic), the tilemap (.map) and the palette (.pal). The converter is gfx4snes -m's, linked as a library: the bytes are its own.

Convert

opensnes-tileset convert res/town.png --colors 16            # 4 bpp, Mode 1: tiles, map, 16-colour palette
opensnes-tileset convert res/title.png --bpp 8 --mode 7      # Mode 7: packed .pc7 / .mp7
opensnes-tileset convert res/level.png --pages --flip --save # a 64x64 scrolling map in 32x32 pages

In: an indexed PNG or BMP, cut into --size blocks (8 by default, the BG tile; 16 for metatiles). Duplicate blocks are stored once; --flip also folds mirrored blocks, the map carrying the flip bits.

Out, beside the input or in --out:

File	What it is
town.pic	the unique tiles, --bpp deep (2, 4 or 8); --lz compresses; --pack writes packed pixels (--mode 7 implies it, as .pc7)
town.map	one 16-bit entry per block: tile number (+ --offset), palette bank, --priority bit, flips; --pages lays it out in 32x32 pages; Modes 5 and 6 halve the width; Mode 7 writes one byte per tile (.mp7)
town.cmap	only with --column-major: the entries of town.map column after column (see below)
town.pal	the palette (--colors entries, 256 by default); --no-palette skips it
town.inc, town_data.as	the glue, in the lib's asset.h naming: town.inc declares town_tiles, town_map, town_pal (each with _end) and a ready BgAsset town (DECLARE_BG_ASSET, when the map is 32x32, 64x32, 32x64 or 64x64) so bgLoad(0, &town, slot, tiles_vram, map_vram) is the whole load; town_data.as is the .incbin fragment the build gathers into assets_gen.asm

A map larger than the screen, scrolled in both axes. Such a game keeps a window of the map in VRAM and feeds it a row and a column at a time. A row of town.map is one contiguous block; a column is not — its entries are a whole row apart. --column-major (column-major = true in the settings file) also writes town.cmap, the same entries column after column, each column from top to bottom, and declares town_cols[] / town_cols_end[] in town.inc: the entry of column c, row r is at byte (c * height + r) * 2, height being the map's height in entries. A column is then one block, pushed to VRAM with a vertical increment (vramQueuePush(town_cols + (c * height + r) * 2, addr, n * 2, VRAM_QUEUE_COLUMN)), straight from ROM: no copy in RAM, nothing built at boot. It costs the map a second time in ROM, in the asset banks. Refused where there is no column order to write: Mode 7 (one byte per entry), --pages, and a picture of exactly 64x32, 32x64 or 64x64 entries, which is written in 32x32 screens as the PPU reads it. Asked by the first game that scrolled a 80x144 map on the SDK: it was building that table in RAM at boot, 23 KB and 38 frames.

Palette banks. At 4 bpp a map entry names one of eight 16-colour banks, so every opaque pixel of a tile must sit in one bank of the palette. A tile that straddles two is refused with its position. --rearrange regroups the picture's colours into banks and renumbers the tiles accordingly (the palette then holds the eight banks, 128 colours at 4 bpp); --palette FILE imposes an authored .pal so indices never drift when the picture changes. --palette-entry N offsets the bank numbers for a layer that shares CGRAM.

Colour 0 of a bank is never drawn. On a background a tile pixel of 0 is transparent, whatever the palette holds at that index (SNESdev wiki, Palettes: "a tile pixel of 0 is always transparent"): what shows there is the layer behind, or the backdrop. A picture that comes from a machine without that rule (an Amiga bitmap, say) and uses index 0 as its black will have holes; a cell that must be opaque black needs another index set to black. Asked by the first game built on the SDK, for a status bar that had to hide what scrolls under it.

The hardware limits are refused, not masked: a tilemap addresses 1024 tiles (the 10-bit field), a Mode 7 map 256 (one byte per tile).

Inspect

opensnes-tileset inspect res/*.png

# town.png: 256x256 px, 1024 blocks of 8 (up to 1024 tiles before deduplication, 32768 bytes of VRAM at 4 bpp, map 2048 bytes), 14 colours used, highest index 13 (bank 0)

The upper bound before deduplication, the map's size, and whether the picture already lives in one palette bank.

Settings beside the asset

--save writes town.png.toml ([convert]: colors, mode, flip, pages…); every later run reads it, the command line overrides for one run.

From gfx4snes

gfx4snes	opensnes-tileset
gfx4snes -s 8 -o 16 -u 16 -p -m -i town.png	convert town.png --colors 16
-u 4 / -u 256	--bpp 2 / --bpp 8
-M 7 (and the .pc7/.mp7 pair)	--mode 7
-a, -c fixed.pal, -e N	--rearrange, --palette fixed.pal, --palette-entry N
-f N, -g, -y, -R, -F	--offset N, --priority, --pages, --no-reduce, --flip
-b, -z, -k, -d	--blank, --lz, --pack, --round

Sprites and metasprites are opensnes-sprite — sheets, metasprites and Aseprite clips; Tiled maps with objects and collision are opensnes-level's, next in the family.