effector/CLAUDE.md

# CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

## Build and test

```bash
cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release
cmake --build build
ctest --test-dir build --output-on-failure
```

Everything is warning-clean under `-Wall -Wextra -Wpedantic`; keep it that way.

The test suite is a single hand-rolled binary (no framework), so there is no
per-test filter: run `./build/tests/effector_tests` directly to see the section
headings (`rng`, `registry`, `scanlines`, `tiles`, `blur`, `chain`, `project`,
`image io`, `batch loading`) and the `ok`/`FAIL` lines. To iterate on one area, comment
out the other blocks in `tests/core_tests.cpp` or add a new
`std::cout << "section\n"` block — checks are plain
`check(condition, "description")` calls and `main()` returns the failure count.

Headless render (no display needed, the fastest end-to-end smoke test):

```bash
build/effector -o /tmp/out.png -s 800x600 --strip-height 6 --seed 1 a.jpg b.png
```

### Running the GUI from this shell

GTK picks Wayland from an inherited `WAYLAND_DISPLAY` and will not draw into an
Xvfb display, which silently yields black screenshots. Under Xvfb always use:

```bash
env -u WAYLAND_DISPLAY GDK_BACKEND=x11 DISPLAY=:79 build/effector ...
```

## Architecture

### The plug-in contract is declarative

`Effect` (`src/core/effect.hpp`) exposes `params()` returning `std::vector<ParamSpec>`
(`src/core/params.hpp`). Those specs are the *only* thing the GUI and CLI know
about an effect: `src/gui/window.cpp` builds SpinButtons/Switches/DropDowns/file
choosers from them and `src/cli/options.cpp` generates flags from them
(`strip_height` → `--strip-height`). Never add effect-specific branches to the
GUI or the parser — add a `ParamSpec` instead. A `Path` param names a file on
disk; like a source path it is written relative to the project folder. A `Text`
param is free-form text the user types, and a `Color` param is a colour written
`#rrggbb` (`parseColor`/`formatColor`/`paramColor` in `src/core/params.hpp`),
shown as a colour button in the interface. Values
travel as `ParamValues` (a
`map<string, variant<long,double,bool,string>>`), read back with
`paramInt`/`paramDouble`/`paramBool`/`paramChoice`.

Whether an effect works from the loaded photographs is part of the same
contract: `Effect::needsSources()` (default true, false for blur). Only the
first stage ever sees the sources, so `chainNeedsSources` (`src/core/chain.cpp`)
just asks the first stage — a chain led by blur renders with no images loaded at
all, and both front-ends gate on that rather than on "are there any images".

Adding an effect: new `src/effects/<name>.{hpp,cpp}` subclassing `Effect`, one
line in the registry list in `src/core/registry.cpp`, one line in the
`effector_core` sources in `CMakeLists.txt`. Nothing else — it shows up in the
CLI, in the chain menu and as a cascade stage on its own.

### Effects cascade

Effects are never applied on their own: `renderChain` (`src/core/chain.cpp`)
runs a `std::vector<ChainStage>` in order, handing stage 0 the loaded sources
and every later stage the previous stage's output as its *only* source. That is
the whole of the cascade — an effect neither knows nor cares whether its input
came from disk. All stages share one `Rng` and run strictly in sequence, so a
seed still reproduces the cascade byte for byte.

A stage carries a `ChainStage::enabled` flag, and `enabledStages`
(`src/core/chain.cpp`) is the one place that reads it: a disabled stage is
skipped whole — handed no image and drawing nothing from the shared rng — so
switching one off renders exactly what the chain rendered before it was added.
`renderChain`, `chainNeedsSources`, `chainTitle` and `chainSlug` all go through
it, which is why the status line and the file name never mention a stage that
did not run. A chain with every stage disabled throws rather than rendering
nothing.

The interface builds the chain in the sidebar ("Effect chain": add from the
menu, reorder, remove, switch off with the check on the row, one stage selected
at a time with its params below), the CLI builds it from repeated `-e`:
`-e scanlines --strip-height 6 -e blur --radius 5`, with `--disable NAME`
leaving a stage in place but skipping it. Flags share one namespace across all effects, so `buildChain`
(`src/cli/options.cpp`) hands each flag to the stage whose effect declares it —
two effects must not declare the same param key.

### The canvas is a format or plain pixels

`src/core/canvas.cpp` holds the one table of standard sizes both front-ends
offer: web formats are a fixed pixel size, print formats a sheet in millimetres
that becomes pixels at a dpi. Adding a format is a line in `canvasFormats()` —
the dropdown, `--format` and `--help` all walk that table. The pixels stay the
truth everywhere (`renderChain` never hears about formats); a format is the
*presentation* of a canvas, carried alongside the pixels in `Options` and
`Project` and empty for a custom size. Where a format is named it decides the
pixels, so a project saved as A4 reopens as A4.

### Projects are plain JSON

`src/core/project.cpp` reads and writes the whole setup — source paths, the
chain with its params (and its `"enabled": false`, written only for a stage
that is switched off), canvas and seed — as a small JSON file, with its own
minimal reader rather than a JSON dependency. Writing is driven by the effects'
`ParamSpec`s, not by whatever happens to sit in a stage's `ParamValues`, so a
key an effect does not declare is never written and never read back: an old
project keeps the defaults for params that did not exist when it was saved.
Source paths under the project's own folder are written relative, so a folder
can be moved or shared whole.

The interface opens and saves projects from the header-bar menu (New, Open,
Save, Save As, with the usual Ctrl+N/O/S accelerators); the window title shows
the project name. Opening one replaces the sources, so a decode still in flight
is discarded rather than mixed into the new project (`dropLoadedImages_`).

### Two link targets, deliberately

`effector_core` (static) holds image IO, params, effects, rng and links only
`gdk-pixbuf-2.0`, `pangocairo` (the text effect's glyphs; Pango and Cairo need
no display) + Threads — **no gtkmm, no display**. The `effector` executable
adds `src/main.cpp`, `src/cli/`, `src/gui/` and links gtkmm. Keep GTK out of
`src/core/` and `src/effects/`; the tests link the core alone and rely on this.

### Determinism is a contract

`Rng` (`src/core/rng.hpp`) is a hand-written xoshiro256\*\* rather than
`<random>`, because libstdc++ distributions are not portable across
implementations. Same sources + settings + seed must produce byte-identical
output on any machine — the tests assert this. The one thing outside this is the
text effect's glyphs, which are whatever shapes the machine's fonts and
fontconfig give: everything it is handed, the rng included, is still used the
same way. Effects must draw *every* random
value from the `Rng&` they are handed, and must not depend on thread scheduling:
the scanlines effect first draws the whole strip plan serially from the rng,
then copies pixels in parallel.

### Threading model

- `parallelFor` (`src/core/parallel.hpp`) splits fixed contiguous chunks across
  hardware threads — used for pixel work, where items cost the same.
- `loadImages` (`src/core/image.cpp`) deliberately does *not* use `parallelFor`:
  files vary wildly in size, so workers pull the next index from a shared
  `std::atomic` cursor. It reports through `LoadProgress::onFile` (called from
  decoding threads: must be thread-safe and non-blocking) and aborts when
  `LoadProgress::cancel` is set, in which case the partial result is discarded
  by the caller, not used.
- The GUI never blocks the main thread. Both decoding and rendering run on a
  `std::thread` and signal back with a `Glib::Dispatcher`
  (`loadProgress_`/`loadFinished_`/`renderDone_` in `src/gui/window.hpp`); the
  handlers are the only code that touches widgets. Renders are coalesced through
  a debounce timer (`requestRender()` → `startRender()`), and the progress
  window is created on a 250 ms delay so short batches never flash a dialog.
  `loadWindow_` is rebuilt from scratch each time, so `loadBar_`/`loadFileLabel_`
  are raw pointers that must be nulled in `closeLoadWindow()`.
- Sources are held as an immutable `shared_ptr<const SourceList>` and edited
  copy-on-write via `mutateSources()`, so a render in flight can never race with
  an add/remove.

### Image buffer

`Image` is tightly packed 8-bit RGB with stride exactly `width * 3` — effects
rely on this for `memcpy`, and GDK uploads the buffer as is. Decoding applies
EXIF orientation. `Image::load` throws; `loadImages` collects per-file
`LoadFailure`s instead.

## Reference

`python-reference/` is the original Python prototype, kept only for comparison.
Do not modify it or import from it.