gen1recomp/docs/modding.md

Native modding

The modding book lives on the project wiki.

• Getting started — install a mod, write a first one, enable and disable it.
• Tutorials — twelve dependency-ordered rungs, each a runnable mod.
• Cookbook — task-sized recipes.
• Registry reference — every registry, generated from src/mods/Schemas.lua.

Regenerate the reference. With no argument it writes in-repo, to docs/modding/reference/registries.md; name a wiki checkout to write the wiki's own page name into it instead:

luajit tools/gen_registry_docs.lua
luajit tools/gen_registry_docs.lua ../gen1recomp.wiki

Mods and Gold (Gen 2)

The mod API is one API across both generations, but Gold runs its own battle engine, overworld, script VM and save format, so a mod says which games it is for and Gold serves a declared subset of the surface.

• docs/preparing-your-mod-for-gen2.md the migration guide: what breaks, the games manifest key, the module adapter, the patterns no adapter can fix, and a worked before/after.
• docs/mod-api-gen2-compat.md the reference: every registry, hook and event, whether Gold serves it, and the record-shape differences where it does.

Start with the checker, which reads your manifest and scans your Lua against the adapter's own coverage table:

python3 tools/modkit.py gen2check mods/my_mod

Editing maps in Tiled

Maps are data, not assets, so they can be authored in a real map editor and exported as a mod. tools/tiled_export.py builds a Tiled workspace out of the imported ROM cache:

python3 tools/tiled_export.py          # -> build/tiled/ (gitignored)

Open build/tiled/gen1.tiled-project, edit any of the 222 maps (or kanto.world for the stitched overworld), and export with the gen1-mod-export extension — one map file, or a whole loadable mod folder. An edited vanilla map becomes a mod.content.maps:patch carrying only the fields that moved; a new map becomes a :register. See docs/new-features.md and the extension's own README.

Read-only map overviews

mod.world:mapOverview() returns collision rows at map-cell resolution, optional visual tileRows at 2x resolution, and optional tileDetailRows at 4x resolution. Visual rows contain Game Boy shades from "0" (lightest) to "3" (darkest); their matching width and height fields describe the grid. markers contains active { kind, x, y } points in map-cell coordinates for warp, visible item, and untaken hidden locations. All fields are read-only snapshots; mods choose which layers to render.

Rendering pipelines

Most registries hand the engine content. render_pipelines hands it drawing: a pipeline is a display mode a mod owns, which may replace the overworld's world pass with geometry of its own and/or post-process the finished image. mods/voxel_world is the worked example — a 3D diorama overworld plus a tilt-shift miniature pass, in about 120 lines of glue over its renderer.

A record declares what the mode is; the engine (src/render/Pipelines.lua) supplies everything about being a display mode: the OFF/1/2/3 ladder, an options row next to TILT, a hotkey, persistence in save.options.pipelines, and the rule that a world pipeline and the engine's own TILT are mutually exclusive.

mod.content.render_pipelines:register("diorama", {
  label = "DIORAMA",                    -- options row label
  levels = { "OFF", "15", "35", "50" }, -- ladder; defaults to OFF/ON
  hotkey = "6",                         -- checked after the engine's keys
  priority = 20,                        -- highest eligible wins the world
  available = function() return Renderer3D.ok() end,
  update = function(dt, level) Camera.ease(dt, level) end,
  drawWorld = function(ctx) return renderScene(ctx) end,
})

Three draw stages, each optional; a record needs at least one:

stage	signature	runs
drawWorld	(ctx) -> canvas | nil	instead of the flat/tilt world pass
worldPresent	(canvas, ctx) -> canvas	over the world, before the UI composites
present	(canvas, ctx) -> canvas	over the whole frame, world and UI alike

worldPresent is the one to reach for when an effect must leave dialog boxes and menus crisp — a depth-of-field or colour grade on the world only. present is for effects that genuinely own the screen, like a CRT curve.

ctx carries the frame: state, cam, vw/vh (world-pixel view), width/height (window pixels), scale, level, paletteFor(map) and spriteColors(map). It also carries ctx.drawFx(project, scale) — call it with your own projection and the engine draws every active field effect (the "!" bubble, the Poké Center heal machine, the Fly bird, the fishing rod, Rock Tunnel darkness) at its correct anchor under your camera. There is exactly one copy of each effect, so a new engine effect works in your pipeline without you touching anything.

Three rules worth knowing:

• gate governs input, never the draw. It decides whether the player may change the mode (default: free-roam overworld only). A mode that stopped rendering during a warp would flash the flat 2D world every time the player walked through a door.
• available is re-read every frame and is the only thing that decides whether the mode can render at all. Answer false on a headless run or a driver with no depth canvas and the engine silently keeps the vanilla 2D path — which is why shipping a pipeline enabled is safe.
• A callback that throws retires its pipeline, attributed to your mod in the manager's error feed, and the frame falls back to 2D. A broken renderer costs the player a display mode, never the game.

Returning nil from drawWorld is a normal answer meaning "not this frame"; the engine draws the vanilla world instead.

Variable-size overworld sprites

The sprites registry keeps the vanilla 16x16 grounded walker as its default, but a mod can describe any frame rectangle and anchor for player characters, NPCs, followers, mounts, vehicles, bosses, or other field actors:

mod.content.sprites:register("SPRITE_COMPANION", {
  image = "mods/example/companion.png", -- one frame per row
  frames = 6,
  walker = true,
  frameWidth = 32,
  frameHeight = 32,
  anchorX = 16, -- frame-relative bottom-center anchor
  anchorY = 32,
})

frameWidth and frameHeight are sheet pixels. anchorX and anchorY are measured from each frame's top-left; when omitted they default to the frame's horizontal center and bottom edge, so a larger sprite grows upward while its feet stay on the same world cell. Omitting all four fields is exactly the vanilla 16x16 placement. The normal player/NPC/follower draw paths consume these values automatically, including horizontal flips and the fishing pose.

Custom render pipelines can use the same geometry without reproducing the pose rules:

local geometry = sprite:getPoseGeometry(facing, walkPhase, stepFlip)
-- geometry.quad, .x/.y/.width/.height, .anchorX/.anchorY, .mirror
local originX, originY = sprite:getScreenOrigin(px, py, camX, camY)

getFrameGeometry(frame) is the corresponding accessor for a specific zero-based sheet frame. Both accessors return fresh tables and share the renderer’s frame selection and mirror conventions.

Battle sprite scaling

The enemy's front pic draws at 1x and the player's back pic at 2x, the way the Game Boy did. A mod can override either, per species or per image.

Per species, on the pokemon record:

-- MEW's back pic renders 1.5x; its front pic is untouched
mod.content.pokemon:patch("MEW", { battleScaleBack = 1.5 })

battleScaleFront scales the enemy pic, battleScaleBack the player pic; both take a number in 0.25 .. 4.0.

Per image, on the battle_sprite_scales registry, keyed by the asset path exactly as the data references it:

mod.content.battle_sprite_scales:register("abra_back", {
  path = "assets/generated/battle/back/abrab.png",
  scale = 1.5,
})

An image-level entry beats the species scale for that one pic, and it is the only way to scale a pic that is not species-keyed — the player's trainer back sprite, held on screen until "Go!", is a bare image path.

The resolution order at draw time is image-level → species-level → default (1x front, 2x back).

• The pic stays grounded at every scale. The player pic keeps its feet flush on the text-box top (y = 96); the enemy pic keeps its bottom edge and horizontal centre pinned in its 7×7 slot. A larger pic grows upward and outward from that anchor, never off the shelf.
• Scaling composes with the send-out grow. The AnimateSendingOutMon ball-to-pic grow multiplies your scale through each stage, so a rescaled mon still grows into place from the ball, grounded the whole way.

Durable tool storage and runtime checkpoints

mod.save remains the right place for state that should travel with the next normal Pokémon SAVE. Tools that need independently written, larger data-only records can use mod.storage; the engine scopes every logical key by game version, opaque playthrough identity, and mod id, and routes it through the same standard or portable persistence backend as saves:

local context, code, message = mod.storage:context(game)
local ok, code, message = mod.storage:write(game, "history/quick/q0001", {
  format = 1, createdAt = os.time(), payload = { money = 3000 },
})
local value, code, message = mod.storage:read(game, "history/quick/q0001")
local keys, code, message = mod.storage:list(game, "history/quick")
local deleted, code, message = mod.storage:delete(game, "history/quick/q0001")

context returns { engineVersion, gameVersion, playthroughId }. The engine version is compatibility metadata; physical launcher-slot and path identity stays private.

Values must be tables containing serializable data only. Keys are conservative slash-separated segments (letters, digits, _, -); paths and filesystem handles are never exposed. Writes are staged and decode-verified, reads recover from a valid staged/backup generation, and methods return structured errors for normal data or I/O failures. The playthrough identity is allocated lazily on the first storage/checkpoint call, so an unused API changes no save bytes.

mod.checkpoints captures and reconstructs engine-owned semantic runtime state:

local capability = mod.checkpoints:inspect(game)
if capability.canCapture then
  local checkpoint, code, message = mod.checkpoints:capture(game)
  -- Store the detached data-only checkpoint through mod.storage.
end

local ok, code, message = mod.checkpoints:restore(game, checkpoint)

Checkpoint format 1 supports settled overworld control and proven battle player-decision safe points. Battle checkpoints are limited to ordinary single-player wild/trainer origins with no suspended script; link, Safari, ghost, demo, scripted, animation, message, queue, and forced-action phases fail closed. New checkpoints preserve gameplay RNG, while legacy overworld records without RNG remain loadable. Capture excludes global options and runtime objects. Restore validates format, game/playthrough identity, content, coordinates, battle relationships, continuation, and RNG before mutation; preserves current options; suppresses normal map-entry/save-load/intro side effects; verifies a recapture; and rolls back runtime plus RNG in memory if reconstruction fails. Callers that need crash recovery should durably capture their own recovery checkpoint before restore.

Checkpoint ownership follows the persistence model rather than mod identity:

• canonical game.save progress, including every mod's save.modData / mod.save bucket and data-only fields added to saved Pokémon, rewinds;
• global and per-mod options remain at their current values;
• independently written mod.storage records do not rewind; and
• mod-owned runtime objects, references, and caches are never serialized.

Successful restore emits checkpoint.restored only after reconstruction and differential recapture have committed. Mods that cache rewound progress or hold references to reconstructed runtime objects can re-read their own public state and rebuild at that point:

mod.events:on("checkpoint.restored", function(ev)
  -- ev.kind is "overworld" or "battle"; ev.game is fully reconstructed.
  cachedQuestStage = mod.save:get("quest_stage", 0)
  rebuildRuntimeFor(ev.game, ev.kind)
end)

The event is not emitted for validation failure, failed reconstruction, or a successful rollback. Its payload contains no checkpoint data or other mod's private state. A mod that deliberately stores progress-coupled truth in mod.storage must version and reconcile that relationship itself; the engine cannot distinguish it safely from independent history, configuration, or cache data.

See RFC 0003, RFC 0004, and RFC 0005 for exact contracts and error codes.

Developer console

Boot with developer mode on to unlock the in-game console and hot-reload hotkeys. Either set POKEPORT_DEV=1 in the environment or pass --developer on the command line:

love . --developer

While developer mode is active:

• ` (backtick) opens the console overlay — a Lua REPL with game, data and mods in scope. Press ` again to close it.
• F5 hot-reloads mods and asset caches without restarting.

The console understands these verbs (anything else is evaluated as Lua):

• warp MAP [x y] — teleport to a map (default cell 5,5).
• give ID [n|level] — add an item (count) or a Pokémon (level).
• flag NAME [on|off] — read or set an event flag.
• party — dump the current party.
• mods — list loaded mods and their state.
• reload — hot-reload mods (same as F5).
• trace PAT | trace off — trace events/hooks matching a glob pattern.
• help — list the verbs.

Tool input and title-menu hooks

Tool mods that need to act once per game logic tick can wrap input.step. It runs immediately before queued button edges are promoted, so input added by the wrapper is visible during that same fixed step. The callback receives (next, game, dt) and must call next(game, dt).

input.pointer delivers uncaptured gameplay pointer events -- touches and real mouse input alike. The callback receives (next, game, ev) where ev is { phase, source, id, x, y, dx, dy, pressure, button }: phase is "pressed", "moved", "released" or "cancelled"; source is "touch" or "mouse"; id is the LÖVE touch id or "mouse"; and the coordinates are LOVE window units, the same space render.hud's viewport and the touch overlay lay out in. The on-screen touch controls keep first refusal: a pointer that begins on a virtual control belongs to the pad for its whole lifecycle and never reaches the hook, while one that begins outside stays visible even if it later crosses a control. A real mouse reaches the hook without POKEPORT_TOUCH (synthesized istouch mouse twins are dropped, so a mobile touch fires once), and focus or visibility loss and input recovery deliver a "cancelled" for every pointer the hook saw pressed but not yet released. Return true without calling next to consume the event.

mod.input presses GB buttons source-safely. mod.input:tap(game, btn) queues exactly one wasPressed edge for the next fixed step and holds nothing; local token = mod.input:press(game, btn) holds the button until mod.input:release(token). Buttons are up, down, left, right, a, b, start and select. Every press is its own input source inside the engine's multi-source bookkeeping, so releasing a token never clears a hold the keyboard, a controller, the touch overlay or another mod still owns; release is idempotent and refuses tokens taken by another mod. Outstanding tokens are released automatically on entry-chunk rollback, hot reload and input recovery.

ui.title_menu.items receives (next, game, items) and follows the same decorate-after-next convention as ui.start_menu.items. It is the safe place for a tool to offer a fresh-session action before gameplay begins.

Ephemeral tools can wrap save.write(next, game) and return false to veto a progress write before world state is captured or any bytes reach disk.

render.hud receives (next, game, viewport) after the finished game frame is composited and before touch controls draw. The window-space viewport contains width, height, gameX, gameY, gameWidth, gameHeight, scale, dpiX, and dpiY, so a tool can use the letterbox margins without drawing over the playfield or pushing an updating game state.

render.compose wraps the whole-window composite in Renderer:endFrame. It receives (next, renderer, ctx); returning true without calling next hands the mod full control of the window, while calling next runs the engine's normal single-window composite so the mod can decorate around it. ctx carries the finished worldCanvas and uiCanvas with their SGB zones / worldZones, worldActive, the frame metrics (ww, wh, pw, ph, ox, oy, vpw, vph, scale, Sx, Sy, dpiX, dpiY), renderer:blitCanvas(...) for a palette-correct blit of either canvas into an arbitrary screen rect, and the secondScreen bridge (available() / push(imageData, w, h) / pollTouch() / setEnabled) for driving a second physical display. pollTouch() returns the oldest queued event as "action,x,y" in submitted-frame coordinates, or nil. This is what lets a mod lay the two passes out as two stacked Game Boy screens, or push one onto a second screen, without the engine knowing the layout.

screen.render_visible receives (next, state) while the main screen is being composed. Return false to omit that state from drawing, opacity selection and palette-zone ownership. The state remains on the stack and keeps its normal update and input ownership, so a mod can mirror a native menu on another display without reimplementing it. The default is true. Treat the wrapper as a pure predicate: the renderer may ask it more than once per frame.

Scrollable list states expose state.kind for use with this hook. Generic lists fall back to their title; PC lists use stable, localization-independent identifiers: pc_box_withdraw, pc_box_deposit, pc_box_release, pc_box_change, pc_item_withdraw, pc_item_deposit, and pc_item_toss.

battle.bottom_ui_visible and battle.status_hud_visible independently control the battle text/menu layer and the HP/status panels. Both receive (next, state) and default to true, so vanilla rendering is unchanged. Pushed text boxes also pass through battle.bottom_ui_visible; a wrapper that only owns battle presentation should return false only for its active battle or text-box state.

core.logic_speed receives (next, game) once per Game:logicSpeed() call (once per frame). Vanilla behavior resolves the per-category GAME SPEED option (GameSpeed.CATEGORIES: overworld/battle/menu) for whichever category Game.speedCategoryInStack says is active right now. A mod may call next(game) and return its result to pass that resolution through, or return a different number outright to override it for that frame (a bot mod forcing 1X for one route segment, say, regardless of the category or saved option). The result is clamped to the nearest valid GameSpeed.LEVELS entry regardless of what a subscriber returns, so a bad value (0, negative, nil) cannot destabilize the fixed-step accumulator. This hook runs after link play's 1X lock and the --speed/equivalent run-argument override, both of which stay unconditional and are never visible to a subscriber.

Developer mode also arms the mod loader's dev tripwire, which flags mods that reach outside their permission set.

Process-lifecycle hooks

These exist so a platform-specific launcher integration (a native shell that embeds this engine and wraps its window in platform UI) can live entirely in a mod instead of hand-patching main.lua, which every other engine change also touches.

core.update receives (next, game, dt) once per frame from love.update. Vanilla behavior is game:update(dt), unconditionally. A mod may skip calling next(game, dt) to pause the simulation for that frame (e.g. while a native settings sheet is on top), and may run additional per-frame polling before or after that call regardless of whether it calls next -- useful for one-shot flags that must be observed every frame even while paused.

core.quit_to_launcher receives (next) once from love.quit(). next() returns the engine's own decision for whether closing the window should return to the Lua launcher instead of exiting; a mod may return false outright, without ever calling next, to veto that and let the process really quit -- for a platform host that owns its own "return to launcher" UI and would otherwise get looped straight back into the game it just quit.

A manifest may also declare force_enable_env, an environment variable name that re-enables the mod regardless of a saved disable in options.mods when that variable is set to "1". This is for a mod that cannot function disabled on the one build where its env var is set (a platform-bridge mod bundled only with that build's launcher, for example).

Neither hook needs a Runtime.wantsHook guard before calling it: Hooks:call already falls straight through to the vanilla function when no mod has wrapped the name, at negligible cost.