feat: add keyboard shortcuts for pan/zoom/iterations/AA

Arrow keys pan, Z/S zoom in/out, +/- adjust iteration count, R resets
the view, H/I toggle the Help/Info windows, and A toggles 2x2
antialiasing. Disabled while a text field has focus.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018GFBP9qrNg6NGJWzzP36xe

CLAUDE.md

@@ -0,0 +1,139 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## What this is
+
+A deep-zoom fractal explorer (Rust + wgpu + egui + WGSL). It zooms past the
+~10¹³× limit of plain `f64` using **perturbation theory**: one high-precision
+reference orbit is computed on the CPU (arbitrary precision via `dashu-float`),
+and every pixel is rendered on the GPU as a cheap `f32` delta from it, with
+rebasing to avoid glitches. The `f32` GPU tier reaches roughly 10³⁰×. Runs
+natively (Vulkan/Metal/DX12) and in the browser (WebGPU only — WebGL2 can't do
+storage buffers, which the fragment shader needs for the reference orbit).
+
+## Commands
+
+```sh
+cargo run --release          # native, run (release matters: fractal math is hot)
+cargo test                   # reference-orbit math, share-link round-trip, WGSL validation
+cargo test --test shader_valid   # just the WGSL parse/validate tests (naga, no GPU needed)
+cargo clippy
+cargo fmt                    # rustfmt.toml just pins edition = "2024"
+```
+
+Web build (WebGPU):
+
+```sh
+rustup target add wasm32-unknown-unknown
+cargo install wasm-bindgen-cli --version 0.2.128   # must match the wasm-bindgen crate version
+./build-web.sh                                     # -> ./dist
+python3 -m http.server -d dist 8080
+```
+
+Native CLI flags (`src/cli.rs`, applied in `FractalApp::apply_cli`): `--kind`,
+`--power`, `--julia re,im`, `--share <fragment>`,
+`--view re,im,half_height[,iterations]`, `--de`, `--buddhabrot`,
+`--buddha-palette`, `--export` (+ `--export-path out.png`). `--headless`
+(`src/headless.rs`) skips the window entirely: it builds the same view from
+the other flags, creates its own offscreen wgpu device, and renders straight
+to a PNG (`--width`/`--height`, default 1920×1080) — implies `--export`'s
+save behavior without needing a GPU-backed window/event loop. Not yet
+supported with `--buddhabrot`. Run `mandelbrot --help` for the full list.
+
+There's no GPU in most sandboxes: `cargo check`/`cargo test --test shader_valid`
+are the fast, headless way to validate a change. `cargo test` also runs but
+doesn't touch the GPU — the reference-orbit tests are pure CPU math (see
+below), and `shader_valid` parses/validates WGSL with `naga` statically instead
+of creating a pipeline.
+
+## Architecture
+
+### The perturbation pipeline (the core mechanism, spans several files)
+
+For a pixel at parameter `c = C_ref + dc`, its orbit is written as
+`y_n = X_n + e_n`, where `X_n` is the (shared, high-precision) reference orbit
+and `e_n` is a small `f32` delta. Whenever `|y_n| < |e_n|` (or the reference
+runs out), rebase: `e ← y_n − X_0`, restart the reference index at 0. This is
+what makes deep zoom cheap — one expensive high-precision orbit, then every
+pixel is a handful of `f32` complex multiplies.
+
+- `src/view.rs` — `ViewState`; center is arbitrary-precision `FBig` (`Big`
+  type alias), pixel scale stays `f64` (still in-range at 10³⁰×). Precision
+  (bits) scales with zoom depth (`precision_for`).
+- `src/fractal/reference.rs` — `FractalKind` enum (Mandelbrot, Burning Ship,
+  Tricorn, Multibrot, Celtic, Perpendicular, Buffalo, Phoenix, Lambda) and
+  `compute_reference`/`compute_set_reference`: iterate the chosen formula at
+  high precision on the CPU, emitting `Z_n` as `f32` pairs — that's the
+  reference orbit the GPU perturbs from.
+- `src/shaders/mandelbrot.wgsl` — the perturbation fragment shader.
+  `advance_delta(z, e)` is the per-kind delta step (`z` = reference point,
+  `e` = current delta); the caller adds `step_add` (= `dc`) afterward — this
+  relies on `c` being additive in every current kind's formula (a kind where
+  it isn't, e.g. a rational map with `c` in a denominator, would need its own
+  step function that consumes `dc` internally instead, plus extra per-step
+  reference data since the orbit point alone wouldn't be enough to recover an
+  exact delta). `fprime(z)` is the derivative used for distance-estimation
+  (DE) shading; exact for holomorphic kinds, an approximation (`~2Z`) for the
+  abs-based ones. A `KIND_*` constant here must match the matching
+  `FractalKind` variant's discriminant exactly.
+- `src/fractal/renderer.rs` — `FractalRenderer` (wgpu pipelines, uniform +
+  storage buffers, bind groups), `Uniforms` (repr(C) layout that must match
+  the WGSL `Uniforms` struct field-for-field, including padding), and
+  `FractalCallback` (the `egui_wgpu::CallbackTrait` impl: `prepare()` uploads
+  changed buffers and decides whether to re-run the iterate pass, the cheap
+  colourise pass, or just blit the cached texture). Also `ExportRender`, a
+  self-contained tiled renderer used for PNG export off the UI thread.
+- `src/worker.rs` — native background thread for reference-orbit computation
+  (coalesces bursts of requests so a fast drag doesn't compute every
+  intermediate view). The wasm32 build computes inline instead (see the
+  `#[cfg(target_arch = "wasm32")]` branch in `app.rs::ensure_reference`) —
+  **any signature change to `compute_reference`/`compute_set_reference` or
+  `RefRequest`/`RefResult` must be applied to both call sites.**
+- `src/app.rs` — `FractalApp` (the egui app + all UI). Key methods:
+  `should_request`/`ensure_reference` (decide when the reference is stale and
+  dispatch/collect it), `make_uniforms` (assemble the per-frame `Uniforms`),
+  `tick_animations` (drives the "morph c/p/λ" and auto-zoom animations),
+  `default_view_for` (per-kind starting view). `KINDS`, `JULIA_PRESETS`, and
+  `SET_PRESETS` are sized as `[T; FractalKind::<last variant> as usize + 1]` —
+  adding a new `FractalKind` means bumping all three (and adding an empty
+  `&[]` slot to the two preset arrays if the kind has none).
+- `src/fractal/share.rs` — `ShareState`: encodes the full view (mode, kind,
+  full-precision decimal center, zoom, iterations, per-kind constants,
+  coloring) as a `#`-fragment URL for bookmarking/sharing deep-zoom locations.
+
+### Adding a new `FractalKind`
+
+Touches, in order: `reference.rs` (enum variant + CPU iteration formula, and a
+test comparing against a naive `f64` iteration), `mandelbrot.wgsl` (matching
+`KIND_*` const + `advance_delta`/`fprime` arms), `buddhabrot.wgsl` (matching
+arm in `advance()`, if the kind makes sense as a Buddhabrot), `renderer.rs`
+`Uniforms` (only if the kind needs a new per-kind constant, e.g. Phoenix's
+`phoenix_p`), `share.rs` (encode/decode string tag), `app.rs` (`KINDS` label,
+`JULIA_PRESETS`/`SET_PRESETS` slot, `default_view_for` entry, and optionally a
+UI control for its constant + an animation toggle, following the
+Phoenix/Lambda pattern). If `c` doesn't enter the formula additively (e.g. a
+rational map with `c` in a denominator), the `advance_delta`/`step_add` split
+doesn't work — that needs its own step function plus extra per-step reference
+data uploaded in a second GPU buffer alongside the orbit.
+
+### Buddhabrot is a separate pipeline
+
+`src/fractal/buddhabrot.rs` + `src/shaders/buddhabrot.wgsl` implement the
+Monte-Carlo orbit-density histogram. It does **not** use the perturbation/
+reference-orbit machinery: a Buddhabrot sample's orbit scatters across the
+whole image rather than staying in one pixel, so it's plain `f32` iteration
+from the live view (no deep zoom) via a compute pass that accumulates into a
+histogram buffer, tone-mapped by a fragment pass every frame. Its own
+`KIND_*` iteration formulas in `advance()` must be kept in sync with
+`reference.rs` by hand (there's no shared code path).
+
+### Two-pass render + caching (`renderer.rs`)
+
+The interactive path splits iteration (expensive, perturbation) from
+colourising (cheap, palette remap) into separate offscreen textures, so
+palette/color-scale/offset tweaks skip re-iteration entirely (`geom_differs`
+vs `color_differs` in `renderer.rs` decide which pass reruns). While the user
+is actively panning/zooming, the app renders downscaled with AA off
+(`INTERACT_DOWNSCALE`) and snaps back to full resolution once input settles
+(`INTERACT_SETTLE`).

Cargo.toml

@@ -13,6 +13,8 @@ png = "0.18.1"
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 env_logger = "0.11.11"
+clap = { version = "4.5.51", features = ["derive"] }
+pollster = "1.0.1"
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 futures-channel = { version = "0.3.34", default-features = false, features = ["alloc", "std"] }

src/app.rs

@@ -2,19 +2,23 @@ use std::sync::{Arc, Mutex};
 
 use eframe::CreationContext;
 use eframe::egui_wgpu;
+#[cfg(target_arch = "wasm32")]
 use eframe::egui_wgpu::wgpu;
 
+#[cfg(not(target_arch = "wasm32"))]
+use crate::cli::Cli;
 use crate::fractal::{
     BuddhabrotCallback, BuddhabrotRenderer, BuddhabrotUniforms, ExportRender, FractalCallback,
-    FractalKind, FractalRenderer, MAX_REF_POINTS, ShareState, Uniforms,
+    FractalKind, FractalRenderer, MAX_REF_POINTS, ShareState, Uniforms, compute_reference,
+    compute_set_reference,
 };
-#[cfg(target_arch = "wasm32")]
-use crate::fractal::{compute_reference, compute_set_reference};
 use crate::lights::Light;
 use crate::view::{
     Big, DEFAULT_HALF_HEIGHT, ViewState, big_from_decimal_str, big_from_f64, big_to_decimal_str,
     precision_for,
 };
+#[cfg(not(target_arch = "wasm32"))]
+use clap::Parser;
 
 const BAILOUT_SQ: f32 = 1.0e6;
 /// Cap on exported image dimension (px), to stay within GPU texture limits.
@@ -63,6 +67,34 @@ fn kind_label(kind: FractalKind) -> &'static str {
         .unwrap_or("Mandelbrot")
 }
 
+/// The iteration formula for a kind, in human-readable notation (mirrors the
+/// doc comments on `FractalKind`'s variants). `power` is only used by
+/// Multibrot.
+fn kind_formula(kind: FractalKind, power: u32) -> String {
+    match kind {
+        FractalKind::Mandelbrot => "z → z² + c".to_string(),
+        FractalKind::BurningShip => "z → (|Re z| + i|Im z|)² + c".to_string(),
+        FractalKind::Tricorn => "z → conj(z)² + c".to_string(),
+        FractalKind::Multibrot => format!("z → z^{power} + c"),
+        FractalKind::Celtic => "z → |Re(z²)| + i·Im(z²) + c".to_string(),
+        FractalKind::Perpendicular => "z → (x² − y²) − 2x|y|i + c".to_string(),
+        FractalKind::Buffalo => "z → |Re(z²)| − i|Im(z²)| + c".to_string(),
+        FractalKind::Phoenix => "z → z² + c + p·z_prev".to_string(),
+        FractalKind::Lambda => "z → λ·z(1 − z)".to_string(),
+    }
+}
+
+/// Shorten a long decimal string for compact display, keeping the leading
+/// (most significant) digits and marking the cut with "…".
+fn truncate_digits(s: &str, max_len: usize) -> String {
+    if s.chars().count() <= max_len {
+        s.to_string()
+    } else {
+        let head: String = s.chars().take(max_len).collect();
+        format!("{head}…")
+    }
+}
+
 type JuliaPreset = (&'static str, f64, f64, u32, Option<(f64, f64)>);
 
 /// Nice-looking Julia constants offered as presets.
@@ -299,6 +331,11 @@ pub struct FractalApp {
     /// Whether the app is in fullscreen (browser Fullscreen API on web, viewport
     /// fullscreen on native). Kept in sync with the real state each frame.
     fullscreen: bool,
+    /// Whether the "Fractal Info" popup (formula/constants/zoom for the
+    /// current view) is open.
+    info_open: bool,
+    /// Whether the Help window (about + mouse/touch controls) is open.
+    help_open: bool,
     /// Time-based animation of colours / Julia c / Phoenix p / zoom.
     anim: AnimState,
 
@@ -338,6 +375,9 @@ pub struct FractalApp {
     export_requested: bool,
     /// Progress/handle for an in-flight PNG export, if any.
     export: Option<Arc<Mutex<ExportShared>>>,
+    /// Output path for `--export` (native CLI only); falls back to a
+    /// timestamped name when unset.
+    export_path: Option<String>,
     /// Short status line (saved path, "link copied", errors).
     status: Option<String>,
 
@@ -388,6 +428,27 @@ impl FractalApp {
             guard.callback_resources.insert(buddhabrot_renderer);
         }
 
+        let mut app = Self::default_state();
+
+        // On the web, restore a shared view from the URL fragment (#...).
+        #[cfg(target_arch = "wasm32")]
+        if let Some(frag) = web_location_hash() {
+            if let Some(state) = ShareState::decode(&frag) {
+                app.apply_share(&state);
+            }
+        }
+
+        // Debug/testing hooks, driven by CLI flags.
+        #[cfg(not(target_arch = "wasm32"))]
+        app.apply_cli(Cli::parse());
+
+        app
+    }
+
+    /// Build the app's default state (no window, no GPU, no CLI applied yet).
+    /// Shared by the windowed app (`new`, which then layers CLI/share-link
+    /// overrides on top) and headless rendering.
+    pub(crate) fn default_state() -> Self {
         let view = ViewState::default();
         let ref_center_re = view.center_re.clone();
         let ref_center_im = view.center_im.clone();
@@ -397,7 +458,7 @@ impl FractalApp {
         let center_im_edit = big_to_decimal_str(&view.center_im, sig);
         let zoom_edit = format_magnification(view.magnification());
 
-        let mut app = Self {
+        Self {
             view,
             mode: FractalMode::Mandelbrot,
             kind: FractalKind::Mandelbrot,
@@ -424,6 +485,8 @@ impl FractalApp {
             buddha_accumulate: true,
             controls_open: true,
             fullscreen: false,
+            info_open: false,
+            help_open: false,
             anim: AnimState::default(),
             fps: 0.0,
             fps_frames: 0,
@@ -442,78 +505,59 @@ impl FractalApp {
             last_interact_time: -1.0e9,
             export_requested: false,
             export: None,
+            export_path: None,
             status: None,
             center_re_edit,
             center_im_edit,
             zoom_edit,
             zoom_edited: false,
-        };
+        }
+    }
 
-        // On the web, restore a shared view from the URL fragment (#...).
-        #[cfg(target_arch = "wasm32")]
-        if let Some(frag) = web_location_hash() {
-            if let Some(state) = ShareState::decode(&frag) {
-                app.apply_share(&state);
+    /// Apply native CLI flags on top of the default state: fractal kind/mode,
+    /// a restored share link or explicit view, coloring toggles, and export
+    /// options. Shared by the windowed app and headless rendering.
+    #[cfg(not(target_arch = "wasm32"))]
+    pub(crate) fn apply_cli(&mut self, cli: Cli) {
+        if let Some(k) = cli.kind {
+            self.kind = k.into();
+            if let Some(p) = cli.power {
+                self.power = p.clamp(2, 8);
+            }
+            self.view = Self::default_view_for(self.mode, self.kind);
+        }
+        if let Some(jc) = cli.julia {
+            let p: Vec<&str> = jc.split(',').collect();
+            if let (Some(Ok(re)), Some(Ok(im))) = (
+                p.first().map(|s| s.trim().parse::<f64>()),
+                p.get(1).map(|s| s.trim().parse::<f64>()),
+            ) {
+                self.mode = FractalMode::Julia;
+                self.julia_c = (re, im);
+                self.view = Self::default_view_for(FractalMode::Julia, self.kind);
             }
         }
-
-        // Debug/testing hooks.
-        #[cfg(not(target_arch = "wasm32"))]
+        if let Some(frag) = cli.share
+            && let Some(state) = ShareState::decode(&frag)
         {
-            if let Ok(k) = std::env::var("MANDEL_KIND") {
-                app.kind = match k.trim().to_ascii_lowercase().as_str() {
-                    "burningship" | "burning_ship" | "ship" => FractalKind::BurningShip,
-                    "tricorn" | "mandelbar" => FractalKind::Tricorn,
-                    "multibrot" | "multi" => FractalKind::Multibrot,
-                    "celtic" => FractalKind::Celtic,
-                    "perpendicular" | "perp" => FractalKind::Perpendicular,
-                    "buffalo" => FractalKind::Buffalo,
-                    "phoenix" => FractalKind::Phoenix,
-                    _ => FractalKind::Mandelbrot,
-                };
-                if let Ok(p) = std::env::var("MANDEL_POWER")
-                    && let Ok(p) = p.trim().parse::<u32>()
-                {
-                    app.power = p.clamp(2, 8);
-                }
-                app.view = Self::default_view_for(app.mode, app.kind);
-            }
-            if let Ok(jc) = std::env::var("MANDEL_JULIA") {
-                let p: Vec<&str> = jc.split(',').collect();
-                if let (Some(Ok(re)), Some(Ok(im))) = (
-                    p.first().map(|s| s.trim().parse::<f64>()),
-                    p.get(1).map(|s| s.trim().parse::<f64>()),
-                ) {
-                    app.mode = FractalMode::Julia;
-                    app.julia_c = (re, im);
-                    app.view = Self::default_view_for(FractalMode::Julia, app.kind);
-                }
-            }
-            if let Ok(frag) = std::env::var("MANDEL_SHARE")
-                && let Some(state) = ShareState::decode(&frag)
-            {
-                app.apply_share(&state);
-            }
-            if let Ok(spec) = std::env::var("MANDEL_VIEW") {
-                app.apply_view_spec(&spec);
-            }
-            if std::env::var("MANDEL_DE").is_ok() {
-                app.de_coloring = true;
-            }
-            if std::env::var("MANDEL_BUDDHABROT").is_ok() {
-                app.buddhabrot = true;
-            }
-            if let Ok(p) = std::env::var("MANDEL_BUDDHA_PALETTE")
-                && let Ok(p) = p.trim().parse::<u32>()
-            {
-                app.buddha_palette = p.min(BUDDHA_PALETTE_NAMES.len() as u32 - 1);
-            }
-            if std::env::var("MANDEL_EXPORT").is_ok() {
-                app.export_requested = true;
-            }
+            self.apply_share(&state);
+        }
+        if let Some(spec) = cli.view {
+            self.apply_view_spec(&spec);
+        }
+        if cli.de {
+            self.de_coloring = true;
+        }
+        if cli.buddhabrot {
+            self.buddhabrot = true;
+        }
+        if let Some(p) = cli.buddha_palette {
+            self.buddha_palette = p.min(BUDDHA_PALETTE_NAMES.len() as u32 - 1);
+        }
+        self.export_path = cli.export_path;
+        if cli.export {
+            self.export_requested = true;
         }
-
-        app
     }
 
     /// Apply a view spec "re,im,half_height[,iterations]" (re/im are decimal,
@@ -728,6 +772,14 @@ impl FractalApp {
         self.generation = self.generation.wrapping_add(1);
     }
 
+    /// The current reference orbit, as uploaded to the GPU. Used by headless
+    /// rendering to build its own `ExportRender` without going through
+    /// `egui_wgpu`'s callback machinery.
+    #[cfg(not(target_arch = "wasm32"))]
+    pub(crate) fn reference_points(&self) -> &[[f32; 2]] {
+        &self.reference
+    }
+
     /// Recompute the reference orbit when needed. Native: dispatch to a worker
     /// thread and pick up completed results. Web: compute inline.
     fn ensure_reference(&mut self) {
@@ -806,7 +858,63 @@ impl FractalApp {
         }
     }
 
-    fn make_uniforms(&self, aspect: f64) -> Uniforms {
+    /// Compute the reference orbit for the current view synchronously, on the
+    /// calling thread — unlike `ensure_reference`, which dispatches to the
+    /// native worker (or, on wasm, computes inline but still runs once per
+    /// frame poll). Used by headless rendering, which has no frame loop to
+    /// poll a background result on and only ever needs one reference.
+    #[cfg(not(target_arch = "wasm32"))]
+    pub(crate) fn compute_reference_blocking(&mut self) {
+        if self.auto_iterations {
+            self.max_iterations = self.auto_iteration_count();
+        }
+        let mut key = self.current_key();
+        let precision = self.view.precision_bits();
+        let max_iter = key.iter.min(MAX_REF_POINTS as u32 - 1);
+
+        // Lambda in Set mode has a static fractal centered at origin.
+        if key.kind == FractalKind::Lambda && !key.julia {
+            key.center_re = big_from_f64(0.0, precision);
+            key.center_im = big_from_f64(0.0, precision);
+        }
+
+        let points = if key.julia {
+            let jr = big_from_f64(key.julia_c.0, precision);
+            let ji = big_from_f64(key.julia_c.1, precision);
+            compute_reference(
+                &key.center_re,
+                &key.center_im,
+                &jr,
+                &ji,
+                max_iter,
+                precision,
+                key.kind,
+                key.power,
+                key.phoenix_p,
+                key.lambda_l,
+            )
+        } else {
+            compute_set_reference(
+                &key.center_re,
+                &key.center_im,
+                max_iter,
+                precision,
+                key.kind,
+                key.power,
+                key.phoenix_p,
+                key.lambda_l,
+            )
+        };
+        self.apply_reference(
+            points,
+            key.center_re.clone(),
+            key.center_im.clone(),
+            key.half_height,
+        );
+        self.last_request = Some(key);
+    }
+
+    pub(crate) fn make_uniforms(&self, aspect: f64) -> Uniforms {
         let (span_x, span_y) = self.view.span(aspect);
         Uniforms {
             span: [span_x as f32, span_y as f32],
@@ -907,13 +1015,12 @@ impl FractalApp {
         self.status = None;
         self.export = Some(Arc::clone(&shared));
 
-        // Progress budget: rendering fills [0, RENDER_END], encoding the rest.
-        const RENDER_END: f32 = 0.6;
-
         #[cfg(not(target_arch = "wasm32"))]
         {
-            let name = std::env::var("MANDEL_EXPORT_PATH")
-                .unwrap_or_else(|_| format!("fractal-{}.png", unix_timestamp()));
+            let name = self
+                .export_path
+                .clone()
+                .unwrap_or_else(|| format!("fractal-{}.png", unix_timestamp()));
             std::thread::spawn(move || {
                 let er = ExportRender::new(
                     &device,
@@ -926,51 +1033,11 @@ impl FractalApp {
                     uniforms,
                     reference.as_slice(),
                 );
-
-                // Render the image tile by tile, waiting for each so progress
-                // reflects real GPU work.
-                for t in 0..er.tiles {
-                    er.render_tile(&device, &queue, t);
-                    let _ = device.poll(wgpu::PollType::Wait {
-                        submission_index: None,
-                        timeout: None,
+                let sh = Arc::clone(&shared);
+                let png =
+                    crate::fractal::export_to_png_blocking(&device, &queue, &er, |phase, f| {
+                        set_progress(&sh, phase, f)
                     });
-                    let done = (t + 1) as f32 / er.tiles as f32;
-                    set_progress(&shared, "Rendering", RENDER_END * done);
-                }
-                er.copy_to_readback(&device, &queue);
-
-                // Wait for the copy, then read the mapped bytes.
-                let (tx, rx) = std::sync::mpsc::channel();
-                er.readback()
-                    .slice(..)
-                    .map_async(wgpu::MapMode::Read, move |res| {
-                        let _ = tx.send(res);
-                    });
-                let _ = device.poll(wgpu::PollType::Wait {
-                    submission_index: None,
-                    timeout: None,
-                });
-                let _ = rx.recv();
-
-                set_progress(&shared, "Encoding", RENDER_END);
-                let png = {
-                    let data = er
-                        .readback()
-                        .slice(..)
-                        .get_mapped_range()
-                        .expect("map readback buffer");
-                    let sh = Arc::clone(&shared);
-                    crate::fractal::encode_png_with_progress(
-                        &data,
-                        er.width,
-                        er.height,
-                        er.padded_bpr,
-                        er.swap_rb,
-                        |f| set_progress(&sh, "Encoding", RENDER_END + (0.97 - RENDER_END) * f),
-                    )
-                };
-                er.readback().unmap();
 
                 set_progress(&shared, "Saving", 0.98);
                 let result = std::fs::write(&name, &png)
@@ -981,6 +1048,8 @@ impl FractalApp {
         }
         #[cfg(target_arch = "wasm32")]
         {
+            // Progress budget: rendering fills [0, RENDER_END], encoding the rest.
+            const RENDER_END: f32 = 0.6;
             wasm_bindgen_futures::spawn_local(async move {
                 let er = ExportRender::new(
                     &device,
@@ -1093,6 +1162,13 @@ impl FractalApp {
                                 self.fullscreen = !self.fullscreen;
                                 self.apply_fullscreen(ui.ctx());
                             }
+                            if ui
+                                .button("Help")
+                                .on_hover_text("About this app, and mouse/touch controls")
+                                .clicked()
+                            {
+                                self.help_open = !self.help_open;
+                            }
                             // FPS readout. Monospace + fixed width so the number
                             // changing doesn't jitter the button row.
                             ui.add(
@@ -1111,6 +1187,206 @@ impl FractalApp {
             });
     }
 
+    /// Floating bottom-left overlay: a single button that toggles the
+    /// "Fractal Info" window. Kept separate from `overlay_buttons` (top-left)
+    /// so it stays out of the way of the panel toggle / fullscreen controls,
+    /// but is still reachable even when the controls panel is collapsed.
+    fn info_button(&mut self, ui: &mut egui::Ui) {
+        egui::Area::new(egui::Id::new("info_button"))
+            .anchor(egui::Align2::LEFT_BOTTOM, egui::vec2(8.0, -8.0))
+            .show(ui.ctx(), |ui| {
+                egui::Frame::popup(ui.style())
+                    .shadow(egui::Shadow::NONE)
+                    .show(ui, |ui| {
+                        if ui
+                            .button("ⓘ Info")
+                            .on_hover_text("Show details about the current fractal")
+                            .clicked()
+                        {
+                            self.info_open = !self.info_open;
+                        }
+                    });
+            });
+    }
+
+    /// Window with details about what's currently on screen: formula, active
+    /// per-kind constants, zoom depth, iteration count. Reads live state, so
+    /// it stays correct as the user pans/zooms/switches kinds.
+    fn info_window(&mut self, ctx: &egui::Context) {
+        let mut open = self.info_open;
+        egui::Window::new("Fractal Info")
+            .id(egui::Id::new("info_window"))
+            .open(&mut open)
+            .collapsible(false)
+            .resizable(false)
+            .anchor(egui::Align2::LEFT_BOTTOM, egui::vec2(8.0, -44.0))
+            .show(ctx, |ui| {
+                ui.label(
+                    egui::RichText::new(kind_label(self.kind))
+                        .strong()
+                        .heading(),
+                );
+                let mode_label = match self.mode {
+                    FractalMode::Mandelbrot => {
+                        "Mandelbrot mode — parameter space (c varies per pixel, z₀ = 0)"
+                    }
+                    FractalMode::Julia => {
+                        "Julia mode — dynamical plane for a fixed c (z₀ varies per pixel)"
+                    }
+                };
+                ui.label(mode_label);
+                ui.separator();
+
+                ui.label(format!("formula: {}", kind_formula(self.kind, self.power)));
+                if self.mode == FractalMode::Julia {
+                    ui.label(format!("c = {:.6} {:+.6}i", self.julia_c.0, self.julia_c.1));
+                }
+                if self.kind == FractalKind::Phoenix {
+                    ui.label(format!(
+                        "p = {:.6} {:+.6}i",
+                        self.phoenix_p.0, self.phoenix_p.1
+                    ));
+                }
+                if self.kind == FractalKind::Lambda {
+                    ui.label(format!(
+                        "λ = {:.6} {:+.6}i",
+                        self.lambda_l.0, self.lambda_l.1
+                    ));
+                }
+                ui.separator();
+
+                if self.buddhabrot {
+                    ui.label("rendering: Buddhabrot (Monte-Carlo orbit density)");
+                    ui.label(format!(
+                        "sample caps: R {} / G {} / B {}",
+                        self.buddha_r_cap, self.buddha_g_cap, self.buddha_b_cap
+                    ));
+                } else {
+                    ui.label(format!(
+                        "magnification: {}×",
+                        format_magnification(self.view.magnification())
+                    ));
+                    ui.label(format!(
+                        "iterations: {}{}",
+                        self.max_iterations,
+                        if self.auto_iterations { " (auto)" } else { "" }
+                    ));
+                    ui.label(format!("precision: {} bits", self.view.precision_bits()));
+                    ui.label(format!(
+                        "center re: {}",
+                        truncate_digits(&self.center_re_edit, 24)
+                    ));
+                    ui.label(format!(
+                        "center im: {}",
+                        truncate_digits(&self.center_im_edit, 24)
+                    ));
+                }
+            });
+        self.info_open = open;
+    }
+
+    /// Help window: what the app does, plus a reference for mouse/touch and
+    /// keyboard controls.
+    fn help_window(&mut self, ctx: &egui::Context) {
+        let mut open = self.help_open;
+        egui::Window::new("Help")
+            .id(egui::Id::new("help_window"))
+            .open(&mut open)
+            .collapsible(false)
+            .default_width(360.0)
+            .show(ctx, |ui| {
+                egui::ScrollArea::vertical()
+                    .max_height(480.0)
+                    .show(ui, |ui| {
+                        ui.heading("About");
+                        ui.label(
+                            "A deep-zoom fractal explorer. It renders the Mandelbrot set \
+                             and several related fractals (Burning Ship, Tricorn, \
+                             Multibrot, Celtic, Perpendicular, Buffalo, Phoenix, Lambda), \
+                             and can zoom in far beyond what ordinary floating-point math \
+                             allows by using perturbation theory: one very precise \
+                             reference orbit is computed once, and every pixel on screen \
+                             is then rendered cheaply as a small offset from that shared \
+                             orbit.",
+                        );
+                        ui.add_space(4.0);
+                        ui.label(
+                            "Mandelbrot mode plots one point per pixel in parameter space \
+                             (does c escape?). Julia mode fixes c and instead varies the \
+                             starting point z₀ across the plane. Buddhabrot mode switches \
+                             to a different, Monte-Carlo rendering of orbit density \
+                             instead of the ordinary escape-time set.",
+                        );
+                        ui.separator();
+
+                        ui.heading("Mouse & touch");
+                        egui::Grid::new("help_mouse_grid")
+                            .num_columns(2)
+                            .spacing([12.0, 6.0])
+                            .show(ui, |ui| {
+                                ui.label("Drag");
+                                ui.label("Pan the view");
+                                ui.end_row();
+                                ui.label("Scroll / trackpad");
+                                ui.label("Zoom toward the cursor");
+                                ui.end_row();
+                                ui.label("Pinch (touch)");
+                                ui.label("Zoom toward the gesture center");
+                                ui.end_row();
+                                ui.label("Two-finger drag (touch)");
+                                ui.label("Pan the view");
+                                ui.end_row();
+                            });
+                        ui.separator();
+
+                        ui.heading("Keyboard");
+                        egui::Grid::new("help_keyboard_grid")
+                            .num_columns(2)
+                            .spacing([12.0, 6.0])
+                            .show(ui, |ui| {
+                                ui.label("Arrow keys");
+                                ui.label("Pan the view");
+                                ui.end_row();
+                                ui.label("Z / S");
+                                ui.label("Zoom in / out toward the center");
+                                ui.end_row();
+                                ui.label("+ / -");
+                                ui.label("Increase / decrease iterations");
+                                ui.end_row();
+                                ui.label("R");
+                                ui.label("Reset to the default view");
+                                ui.end_row();
+                                ui.label("H");
+                                ui.label("Toggle this Help window");
+                                ui.end_row();
+                                ui.label("I");
+                                ui.label("Toggle the Info window");
+                                ui.end_row();
+                                ui.label("A");
+                                ui.label("Toggle antialiasing (2×2)");
+                                ui.end_row();
+                            });
+                        ui.label(
+                            "Shortcuts are ignored while a text field (e.g. the center/zoom \
+                             edit boxes) has focus. Esc / F11 toggle fullscreen via the \
+                             browser or OS, as usual.",
+                        );
+                        ui.separator();
+
+                        ui.heading("Tips");
+                        ui.label(
+                            "• The controls panel (top-left \"Menu\" button) holds the \
+                             fractal formula, coloring, animation, and export options.\n\
+                             • The bottom-left \"Info\" button shows details about what's \
+                             currently on screen.\n\
+                             • \"Copy link\" (in the panel) encodes the exact view so it \
+                             can be reopened later or sent to someone else.",
+                        );
+                    });
+            });
+        self.help_open = open;
+    }
+
     /// Push the desired fullscreen state to the platform.
     #[cfg(not(target_arch = "wasm32"))]
     fn apply_fullscreen(&mut self, ctx: &egui::Context) {
@@ -1726,6 +2002,84 @@ impl FractalApp {
             ui.ctx().request_repaint();
         }
 
+        // Keyboard: arrows pan, z/s zoom in/out, +/- adjust iterations, R
+        // resets the view, H/I toggle the Help/Info windows. Skipped while a
+        // text field (e.g. the center/zoom edit boxes) has focus.
+        if !ui.ctx().egui_wants_keyboard_input() {
+            let dt = ui.input(|i| i.stable_dt as f64).clamp(0.0, 0.1);
+            let (left, right, up, down, zoom_in, zoom_out) = ui.input(|i| {
+                (
+                    i.key_down(egui::Key::ArrowLeft),
+                    i.key_down(egui::Key::ArrowRight),
+                    i.key_down(egui::Key::ArrowUp),
+                    i.key_down(egui::Key::ArrowDown),
+                    i.key_down(egui::Key::Z),
+                    i.key_down(egui::Key::S),
+                )
+            });
+
+            // Pixels/sec pan speed — matches a brisk mouse drag regardless of
+            // frame rate. See `pan_pixels`'s screen-space (+x right, +y down)
+            // convention: Right/Down pan the *camera* right/down, which is
+            // the opposite delta sign from a drag that would show the same
+            // content (a drag grabs the canvas; these keys move the camera).
+            const PAN_SPEED_PX: f64 = 700.0;
+            let mut dx = 0.0;
+            let mut dy = 0.0;
+            if left {
+                dx += PAN_SPEED_PX * dt;
+            }
+            if right {
+                dx -= PAN_SPEED_PX * dt;
+            }
+            if down {
+                dy += PAN_SPEED_PX * dt;
+            }
+            if up {
+                dy -= PAN_SPEED_PX * dt;
+            }
+            if dx != 0.0 || dy != 0.0 {
+                self.view.pan_pixels(dx, dy, height_px);
+                interacted = true;
+            }
+
+            // e-folds/sec, same scale as the auto-zoom animation.
+            const ZOOM_SPEED: f64 = 1.0;
+            if zoom_in != zoom_out {
+                let rate = if zoom_in { ZOOM_SPEED } else { -ZOOM_SPEED };
+                let factor = (-rate * dt).exp();
+                self.view.zoom_at_pixel(0.0, 0.0, height_px, factor);
+                interacted = true;
+            }
+            if left || right || up || down || zoom_in || zoom_out {
+                ui.ctx().request_repaint();
+            }
+
+            if ui.input(|i| i.key_pressed(egui::Key::R)) {
+                self.view = Self::default_view_for(self.mode, self.kind);
+                interacted = true;
+            }
+            if ui.input(|i| i.key_pressed(egui::Key::H)) {
+                self.help_open = !self.help_open;
+            }
+            if ui.input(|i| i.key_pressed(egui::Key::I)) {
+                self.info_open = !self.info_open;
+            }
+            if ui.input(|i| i.key_pressed(egui::Key::A)) {
+                self.antialias = !self.antialias;
+            }
+            if ui.input(|i| i.key_pressed(egui::Key::Plus) || i.key_pressed(egui::Key::Equals)) {
+                self.auto_iterations = false;
+                self.max_iterations = ((self.max_iterations as f64 * 1.25).round() as u32)
+                    .clamp(32, MAX_REF_POINTS as u32 - 1);
+            }
+            if ui.input(|i| i.key_pressed(egui::Key::Minus)) {
+                self.auto_iterations = false;
+                self.max_iterations = ((self.max_iterations as f64 / 1.25).round() as u32)
+                    .clamp(32, MAX_REF_POINTS as u32 - 1);
+            }
+        }
+
         if self.buddhabrot {
             // No reference orbit / perturbation machinery: iterate directly in
             // f32 from the live view. Progressive accumulation means this
@@ -1844,6 +2198,9 @@ impl eframe::App for FractalApp {
         // Floating overlay, always reachable (even when the panel is collapsed):
         // toggle the panel and toggle fullscreen. Essential on a phone.
         self.overlay_buttons(ui);
+        self.info_button(ui);
+        self.info_window(ui.ctx());
+        self.help_window(ui.ctx());
 
         if std::mem::take(&mut self.export_requested) {
             self.do_export(frame);
@@ -1872,7 +2229,7 @@ fn finish_export(shared: &Arc<Mutex<ExportShared>>, result: Result<String, Strin
 }
 
 #[cfg(not(target_arch = "wasm32"))]
-fn unix_timestamp() -> u64 {
+pub(crate) fn unix_timestamp() -> u64 {
     std::time::SystemTime::now()
         .duration_since(std::time::UNIX_EPOCH)
         .map(|d| d.as_secs())

src/cli.rs

@@ -0,0 +1,98 @@
+// Native command-line arguments. Currently mirrors the old `MANDEL_*` debug
+// env vars one-for-one; this is the foundation a future headless (no-window,
+// render-to-file) mode will build on.
+
+use clap::{Parser, ValueEnum};
+
+use crate::fractal::FractalKind;
+
+#[derive(Parser, Debug, Default)]
+#[command(name = "mandelbrot", about = "Deep-zoom fractal explorer", version)]
+pub struct Cli {
+    /// Fractal formula to render.
+    #[arg(long, value_enum)]
+    pub kind: Option<KindArg>,
+
+    /// Exponent for the Multibrot kind (z -> z^power + c), clamped to [2, 8].
+    #[arg(long)]
+    pub power: Option<u32>,
+
+    /// Start in Julia mode with this seed constant.
+    #[arg(long, value_name = "RE,IM")]
+    pub julia: Option<String>,
+
+    /// Restore a view from a share-link fragment (the part after '#').
+    #[arg(long, value_name = "FRAGMENT")]
+    pub share: Option<String>,
+
+    /// Jump to a view on startup.
+    #[arg(long, value_name = "RE,IM,HALF_HEIGHT[,ITERATIONS]")]
+    pub view: Option<String>,
+
+    /// Enable distance-estimation shading.
+    #[arg(long)]
+    pub de: bool,
+
+    /// Switch to the Buddhabrot renderer.
+    #[arg(long)]
+    pub buddhabrot: bool,
+
+    /// Buddhabrot tonemap palette index.
+    #[arg(long, value_name = "INDEX")]
+    pub buddha_palette: Option<u32>,
+
+    /// Render a PNG export on startup.
+    #[arg(long)]
+    pub export: bool,
+
+    /// Output path for --export/--headless (default: fractal-<timestamp>.png).
+    #[arg(long, value_name = "PATH")]
+    pub export_path: Option<String>,
+
+    /// Run without opening a window: render the current view to a PNG and
+    /// exit. Combine with --kind/--julia/--share/--view etc. to pick what to
+    /// render. Not yet supported with --buddhabrot.
+    #[arg(long)]
+    pub headless: bool,
+
+    /// Output image width in pixels (--headless only).
+    #[arg(long, value_name = "PX", default_value_t = 1920)]
+    pub width: u32,
+
+    /// Output image height in pixels (--headless only).
+    #[arg(long, value_name = "PX", default_value_t = 1080)]
+    pub height: u32,
+}
+
+#[derive(Copy, Clone, Debug, ValueEnum)]
+pub enum KindArg {
+    Mandelbrot,
+    #[value(alias = "ship")]
+    BurningShip,
+    #[value(alias = "mandelbar")]
+    Tricorn,
+    #[value(alias = "multi")]
+    Multibrot,
+    Celtic,
+    #[value(alias = "perp")]
+    Perpendicular,
+    Buffalo,
+    Phoenix,
+    Lambda,
+}
+
+impl From<KindArg> for FractalKind {
+    fn from(k: KindArg) -> Self {
+        match k {
+            KindArg::Mandelbrot => FractalKind::Mandelbrot,
+            KindArg::BurningShip => FractalKind::BurningShip,
+            KindArg::Tricorn => FractalKind::Tricorn,
+            KindArg::Multibrot => FractalKind::Multibrot,
+            KindArg::Celtic => FractalKind::Celtic,
+            KindArg::Perpendicular => FractalKind::Perpendicular,
+            KindArg::Buffalo => FractalKind::Buffalo,
+            KindArg::Phoenix => FractalKind::Phoenix,
+            KindArg::Lambda => FractalKind::Lambda,
+        }
+    }
+}

src/fractal/mod.rs

@@ -8,8 +8,9 @@ pub mod share;
 
 pub use buddhabrot::{BuddhabrotCallback, BuddhabrotRenderer, BuddhabrotUniforms};
 pub use reference::{FractalKind, compute_reference, compute_set_reference};
-pub use renderer::{
-    ExportRender, FractalCallback, FractalRenderer, MAX_REF_POINTS, Uniforms,
-    encode_png_with_progress,
-};
+#[cfg(target_arch = "wasm32")]
+pub use renderer::encode_png_with_progress;
+#[cfg(not(target_arch = "wasm32"))]
+pub use renderer::export_to_png_blocking;
+pub use renderer::{ExportRender, FractalCallback, FractalRenderer, MAX_REF_POINTS, Uniforms};
 pub use share::ShareState;

src/fractal/renderer.rs

@@ -757,6 +757,59 @@ impl ExportRender {
     }
 }
 
+/// Render `er` tile by tile (blocking on the GPU after each tile so progress
+/// reflects real work), read it back, and encode the result as PNG bytes.
+/// Blocks the calling thread throughout, so it's only for native targets:
+/// the UI export path runs it on a background thread, headless rendering
+/// runs it directly since it has no frame loop to share a thread with.
+#[cfg(not(target_arch = "wasm32"))]
+pub fn export_to_png_blocking(
+    device: &wgpu::Device,
+    queue: &wgpu::Queue,
+    er: &ExportRender,
+    mut on_progress: impl FnMut(&'static str, f32),
+) -> Vec<u8> {
+    // Progress budget: rendering fills [0, RENDER_END], encoding the rest.
+    const RENDER_END: f32 = 0.6;
+
+    for t in 0..er.tiles {
+        er.render_tile(device, queue, t);
+        let _ = device.poll(wgpu::PollType::Wait {
+            submission_index: None,
+            timeout: None,
+        });
+        let done = (t + 1) as f32 / er.tiles as f32;
+        on_progress("Rendering", RENDER_END * done);
+    }
+    er.copy_to_readback(device, queue);
+
+    let (tx, rx) = std::sync::mpsc::channel();
+    er.readback()
+        .slice(..)
+        .map_async(wgpu::MapMode::Read, move |res| {
+            let _ = tx.send(res);
+        });
+    let _ = device.poll(wgpu::PollType::Wait {
+        submission_index: None,
+        timeout: None,
+    });
+    let _ = rx.recv();
+
+    on_progress("Encoding", RENDER_END);
+    let png = {
+        let data = er
+            .readback()
+            .slice(..)
+            .get_mapped_range()
+            .expect("map readback buffer");
+        encode_png_with_progress(&data, er.width, er.height, er.padded_bpr, er.swap_rb, |f| {
+            on_progress("Encoding", RENDER_END + (0.97 - RENDER_END) * f)
+        })
+    };
+    er.readback().unmap();
+    png
+}
+
 /// Convert a padded BGRA/RGBA readback into tightly-packed RGBA8 and encode it
 /// as PNG bytes, reporting progress in `[0, 1]` via `on_progress` as rows are
 /// streamed to the compressor (encoding is the slow, subdividable phase).

src/headless.rs

@@ -0,0 +1,82 @@
+// Headless PNG rendering: parse the CLI, build the exact same view/state the
+// windowed app would from it, then render straight to a file. No window, no
+// event loop, no worker-thread debounce (nothing to debounce for a one-shot
+// render); it just creates its own wgpu device, computes the reference orbit
+// once, and renders through the same `ExportRender` path the "Export PNG"
+// button uses.
+
+use eframe::egui_wgpu::wgpu;
+
+use crate::app::{FractalApp, unix_timestamp};
+use crate::cli::Cli;
+use crate::fractal::{ExportRender, FractalRenderer, export_to_png_blocking};
+
+/// Cap on the output image dimension (px), to stay within GPU texture limits.
+const MAX_DIM: u32 = 8192 * 16;
+
+pub fn run(cli: Cli) -> Result<(), String> {
+    if cli.buddhabrot {
+        return Err("headless mode doesn't support --buddhabrot yet".into());
+    }
+
+    let width = cli.width.clamp(16, MAX_DIM);
+    let height = cli.height.clamp(16, MAX_DIM);
+    let export_path = cli
+        .export_path
+        .clone()
+        .unwrap_or_else(|| format!("fractal-{}.png", unix_timestamp()));
+
+    let mut app = FractalApp::default_state();
+    app.apply_cli(cli);
+
+    eprintln!("computing reference orbit…");
+    app.compute_reference_blocking();
+
+    let (device, queue) = pollster::block_on(request_device())?;
+    let format = wgpu::TextureFormat::Rgba8UnormSrgb;
+    let renderer = FractalRenderer::new(&device, format);
+    let (pipeline, bind_group_layout, format) = renderer.export_handles();
+
+    let uniforms = app.make_uniforms(width as f64 / height as f64);
+    let er = ExportRender::new(
+        &device,
+        &queue,
+        pipeline,
+        &bind_group_layout,
+        format,
+        width,
+        height,
+        uniforms,
+        app.reference_points(),
+    );
+
+    eprintln!("rendering {width}×{height}…");
+    let png = export_to_png_blocking(&device, &queue, &er, |phase, fraction| {
+        eprint!("\r{phase} {:>3.0}%", fraction * 100.0);
+    });
+    eprintln!();
+
+    std::fs::write(&export_path, &png).map_err(|e| format!("save failed: {e}"))?;
+    println!("saved {export_path} ({width}×{height})");
+    Ok(())
+}
+
+/// Set up a wgpu device with no surface/window attached, matching the limits
+/// `main::wgpu_options` requests for the windowed app (the fractal fragment
+/// shader needs storage buffers, which downlevel/WebGL-style limits disallow).
+async fn request_device() -> Result<(wgpu::Device, wgpu::Queue), String> {
+    let instance = wgpu::Instance::default();
+    let adapter = instance
+        .request_adapter(&wgpu::RequestAdapterOptions::default())
+        .await
+        .map_err(|e| format!("no compatible GPU adapter: {e}"))?;
+    adapter
+        .request_device(&wgpu::DeviceDescriptor {
+            label: Some("headless fractal device"),
+            required_features: wgpu::Features::empty(),
+            required_limits: adapter.limits(),
+            ..Default::default()
+        })
+        .await
+        .map_err(|e| format!("failed to create device: {e}"))
+}

src/main.rs

@@ -9,10 +9,14 @@
 // and calls the wasm `main`, which boots eframe onto the page's <canvas>.
 
 mod app;
-mod lights;
 mod fractal;
+mod lights;
 mod view;
 
+#[cfg(not(target_arch = "wasm32"))]
+mod cli;
+#[cfg(not(target_arch = "wasm32"))]
+mod headless;
 #[cfg(not(target_arch = "wasm32"))]
 mod worker;
 
@@ -47,11 +51,24 @@ fn wgpu_options() -> eframe::egui_wgpu::WgpuConfiguration {
 
 #[cfg(not(target_arch = "wasm32"))]
 fn main() -> eframe::Result {
+    use clap::Parser as _;
+
     env_logger::builder()
         .filter_level(log::LevelFilter::Info)
         .parse_default_env()
         .init();
 
+    let cli = cli::Cli::parse();
+    if cli.headless {
+        return match headless::run(cli) {
+            Ok(()) => Ok(()),
+            Err(e) => {
+                eprintln!("error: {e}");
+                std::process::exit(1);
+            }
+        };
+    }
+
     let native_options = eframe::NativeOptions {
         renderer: eframe::Renderer::Wgpu,
         wgpu_options: wgpu_options(),