perf: Use multithreading for headless animation export

src/app.rs

@@ -147,8 +147,61 @@ const SET_PRESETS: [&[SetPreset]; FractalKind::ComplexMultibrot as usize + 1] =
     &[],
 ];
 
+/// A reference-orbit computation detached from the app (see
+/// `FractalApp::reference_job`), so it can run on any thread.
+#[cfg(not(target_arch = "wasm32"))]
+#[derive(Clone)]
+pub(crate) struct RefJob {
+    key: RequestKey,
+    precision: usize,
+    /// The frame's iteration count (auto-iterations resolved).
+    max_iterations: u32,
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+impl RefJob {
+    /// Iterate the reference orbit at full precision (the expensive part).
+    pub(crate) fn compute(&self) -> Vec<[f32; 2]> {
+        let key = &self.key;
+        let precision = self.precision;
+        let morph = key.morph.map(|(k, w)| (k, w as f64));
+        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,
+                key.iter,
+                precision,
+                key.kind,
+                key.power,
+                key.phoenix_p,
+                key.lambda_l,
+                key.complex_power,
+                morph,
+            )
+        } else {
+            compute_set_reference(
+                &key.center_re,
+                &key.center_im,
+                key.iter,
+                precision,
+                key.kind,
+                key.power,
+                key.phoenix_p,
+                key.lambda_l,
+                key.complex_power,
+                morph,
+            )
+        }
+    }
+}
+
 /// Parameters a reference orbit was (or will be) computed for. Used to decide
 /// when the current reference is stale enough to recompute.
+#[derive(Clone)]
 struct RequestKey {
     center_re: Big,
     center_im: Big,
@@ -1233,6 +1286,17 @@ impl FractalApp {
     /// poll a background result on and only ever needs one reference.
     #[cfg(not(target_arch = "wasm32"))]
     pub(crate) fn compute_reference_blocking(&mut self) {
+        let job = self.reference_job();
+        let points = job.compute();
+        self.finish_reference(job, points);
+    }
+
+    /// Snapshot everything the reference orbit for the current view depends
+    /// on, as a self-contained job that can be computed on another thread
+    /// (headless animation computes many frames' orbits in parallel). Also
+    /// applies auto-iterations, like `compute_reference_blocking`.
+    #[cfg(not(target_arch = "wasm32"))]
+    pub(crate) fn reference_job(&mut self) -> RefJob {
         if self.auto_iterations {
             self.max_iterations = self.auto_iteration_count();
         }
@@ -1240,45 +1304,25 @@ impl FractalApp {
         // One-shot render: no later frames for iteration headroom to serve.
         key.iter = self.max_iterations.min(MAX_REF_POINTS as u32 - 1);
         let precision = self.view.precision_bits();
-        let max_iter = key.iter;
 
         // Lambda in Set mode has a static fractal centered at origin.
         if key.kind == FractalKind::Lambda && !key.julia && key.morph.is_none() {
             key.center_re = big_from_f64(0.0, precision);
             key.center_im = big_from_f64(0.0, precision);
         }
+        RefJob {
+            key,
+            precision,
+            max_iterations: self.max_iterations,
+        }
+    }
 
-        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,
-                key.complex_power,
-                key.morph.map(|(k, w)| (k, w as f64)),
-            )
-        } else {
-            compute_set_reference(
-                &key.center_re,
-                &key.center_im,
-                max_iter,
-                precision,
-                key.kind,
-                key.power,
-                key.phoenix_p,
-                key.lambda_l,
-                key.complex_power,
-                key.morph.map(|(k, w)| (k, w as f64)),
-            )
-        };
+    /// Install the orbit computed for `job` (from `reference_job`) as the
+    /// current reference, along with the iteration count it was made for.
+    #[cfg(not(target_arch = "wasm32"))]
+    pub(crate) fn finish_reference(&mut self, job: RefJob, points: Vec<[f32; 2]>) {
+        self.max_iterations = job.max_iterations;
+        let key = job.key;
         self.apply_reference(
             points,
             key.center_re.clone(),

src/fractal/mod.rs

@@ -14,7 +14,7 @@ pub use reference::{compute_reference, compute_set_reference};
 pub use renderer::PipelineKey;
 #[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};
+#[cfg(not(target_arch = "wasm32"))]
+pub use renderer::{encode_png, export_to_png_blocking, render_readback_blocking};
 pub use share::ShareState;

src/fractal/renderer.rs

@@ -1223,6 +1223,84 @@ pub fn export_to_png_blocking(
     png
 }
 
+/// Render every tile of `er` in one go (no per-tile GPU stall, unlike
+/// [`export_to_png_blocking`]), read it back, and return a copy of the padded
+/// readback bytes (`er.padded_bpr` per row) for [`encode_png`]. Used by the
+/// headless animation pipeline, which encodes on other threads.
+#[cfg(not(target_arch = "wasm32"))]
+pub fn render_readback_blocking(
+    device: &wgpu::Device,
+    queue: &wgpu::Queue,
+    er: &ExportRender,
+) -> Vec<u8> {
+    for t in 0..er.tiles {
+        er.render_tile(device, queue, t);
+    }
+    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();
+
+    let bytes = er
+        .readback()
+        .slice(..)
+        .get_mapped_range()
+        .expect("map readback buffer")
+        .to_vec();
+    er.readback().unmap();
+    bytes
+}
+
+/// Like [`encode_png_with_progress`], but encodes the whole image at once
+/// (no progress) at the given compression level. Non-streaming, so the fast
+/// `fdeflate` levels don't pay the streaming-mode size penalty.
+#[cfg(not(target_arch = "wasm32"))]
+pub fn encode_png(
+    padded: &[u8],
+    width: u32,
+    height: u32,
+    padded_bpr: u32,
+    swap_rb: bool,
+    compression: png::Compression,
+) -> Vec<u8> {
+    let row = (width * 4) as usize;
+    let mut pixels = Vec::with_capacity(row * height as usize);
+    for y in 0..height as usize {
+        let src_off = y * padded_bpr as usize;
+        let src = &padded[src_off..src_off + row];
+        if swap_rb {
+            pixels.extend(
+                src.as_chunks::<4>()
+                    .0
+                    .iter()
+                    .flat_map(|&[b, g, r, a]| [r, g, b, a]),
+            );
+        } else {
+            pixels.extend_from_slice(src);
+        }
+    }
+
+    let mut out = Vec::new();
+    {
+        let mut encoder = png::Encoder::new(&mut out, width, height);
+        encoder.set_color(png::ColorType::Rgba);
+        encoder.set_depth(png::BitDepth::Eight);
+        encoder.set_compression(compression);
+        let mut writer = encoder.write_header().expect("png header");
+        writer.write_image_data(&pixels).expect("png data");
+    }
+    out
+}
+
 /// 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

@@ -5,12 +5,17 @@
 // once, and renders through the same `ExportRender` path the "Export PNG"
 // button uses.
 
+use std::collections::HashMap;
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use std::sync::{Mutex, mpsc};
+
 use eframe::egui_wgpu::wgpu;
 
-use crate::app::{FractalApp, parse_complex_pair, unix_timestamp};
+use crate::app::{FractalApp, RefJob, parse_complex_pair, unix_timestamp};
 use crate::cli::Cli;
 use crate::fractal::{
-    ExportRender, FractalKind, FractalRenderer, PipelineKey, ShareState, export_to_png_blocking,
+    ExportRender, FractalKind, FractalRenderer, PipelineKey, ShareState, encode_png,
+    export_to_png_blocking, render_readback_blocking,
 };
 use crate::view::{
     ViewState, big_from_decimal_str, interpolate_f64, interpolate_view, parse_view_spec,
@@ -197,14 +202,9 @@ fn run_animation(
     let out_dir = export_path.unwrap_or_else(|| format!("frames-{}", unix_timestamp()));
     std::fs::create_dir_all(&out_dir).map_err(|e| format!("failed to create {out_dir}: {e}"))?;
 
-    let (device, queue) = pollster::block_on(request_device())?;
-    let format = wgpu::TextureFormat::Bgra8Unorm;
-    let renderer = FractalRenderer::new(&device, format);
-    // The shader specialization (kind, Julia, DE, morph) can change between
-    // frames during a kind morph; rebuild the pipeline only when it does.
-    let mut pipeline_cache: Option<(PipelineKey, _)> = None;
-
-    for i in 0..frames {
+    // Everything about frame `i` is a pure function of its `t`, so the app can
+    // be put into any frame's state at any time, in any order.
+    let apply_frame = |app: &mut FractalApp, i: u32| {
         let raw_t = i as f64 / (frames - 1) as f64;
         let t = if targets.linear {
             raw_t
@@ -228,38 +228,126 @@ fn run_animation(
             interpolate_f64(yaw0 as f64, yaw1 as f64, t) as f32,
             interpolate_f64(pitch0 as f64, pitch1 as f64, t) as f32,
         );
+    };
 
-        eprintln!("[{:>4}/{frames}] computing reference orbit…", i + 1);
-        app.compute_reference_blocking();
+    // Snapshot every frame's reference-orbit job up front (cheap: just the
+    // parameters), so the orbits themselves can be computed in parallel.
+    let jobs: Vec<RefJob> = (0..frames)
+        .map(|i| {
+            apply_frame(&mut app, i);
+            app.reference_job()
+        })
+        .collect();
 
-        let uniforms = app.make_uniforms(width as f64 / height as f64);
-        let key = PipelineKey::from_uniforms(&uniforms);
-        if pipeline_cache.as_ref().is_none_or(|(k, _)| *k != key) {
-            pipeline_cache = Some((key, renderer.export_handles(&device, &uniforms)));
+    let (device, queue) = pollster::block_on(request_device())?;
+    let format = wgpu::TextureFormat::Bgra8Unorm;
+    let renderer = FractalRenderer::new(&device, format);
+    let aspect = width as f64 / height as f64;
+
+    // Three-stage pipeline, connected by bounded channels (which also cap
+    // memory): `threads` workers compute reference orbits (CPU, the expensive
+    // part at deep zoom) → this thread renders each frame on the GPU → `threads`
+    // workers PNG-encode and write frames. Frames flow through out of order
+    // (at most ~`threads` apart); each is written under its own index.
+    let threads = std::thread::available_parallelism().map_or(4, |n| n.get());
+    let next_job = AtomicUsize::new(0);
+    let saved = AtomicUsize::new(0);
+    let failed = AtomicBool::new(false);
+    let error: Mutex<Option<String>> = Mutex::new(None);
+    let fail = |e: String| {
+        failed.store(true, Ordering::Relaxed);
+        error.lock().unwrap().get_or_insert(e);
+    };
+
+    eprintln!("rendering {frames} frames ({width}×{height}) on {threads} threads…");
+    let (png_tx, png_rx) = mpsc::sync_channel::<(usize, Vec<u8>, u32, bool)>(threads * 2);
+    let png_rx = Mutex::new(png_rx);
+    std::thread::scope(|scope| {
+        let (ref_tx, ref_rx) = mpsc::sync_channel::<(usize, Vec<[f32; 2]>)>(threads * 2);
+        for _ in 0..threads {
+            let ref_tx = ref_tx.clone();
+            let (jobs, next_job, failed) = (&jobs, &next_job, &failed);
+            scope.spawn(move || {
+                loop {
+                    let i = next_job.fetch_add(1, Ordering::Relaxed);
+                    if i >= jobs.len() || failed.load(Ordering::Relaxed) {
+                        break;
+                    }
+                    if ref_tx.send((i, jobs[i].compute())).is_err() {
+                        break;
+                    }
+                }
+            });
         }
-        let (_, handles) = pipeline_cache.as_ref().unwrap();
-        let er = ExportRender::new(
-            &device,
-            &queue,
-            handles,
-            width,
-            height,
-            uniforms,
-            app.reference_points(),
-            app.lights(),
-        );
+        drop(ref_tx);
 
-        let png = export_to_png_blocking(&device, &queue, &er, |phase, fraction| {
-            eprint!(
-                "\r[{:>4}/{frames}] {phase} {:>3.0}%",
-                i + 1,
-                fraction * 100.0
+        for _ in 0..threads {
+            let (png_rx, out_dir, saved, failed, fail) =
+                (&png_rx, &out_dir, &saved, &failed, &fail);
+            scope.spawn(move || {
+                loop {
+                    // Hold the lock only for the receive, not the encode.
+                    let Ok((i, padded, bpr, swap_rb)) = png_rx.lock().unwrap().recv() else {
+                        break;
+                    };
+                    if failed.load(Ordering::Relaxed) {
+                        break;
+                    }
+                    let png =
+                        encode_png(&padded, width, height, bpr, swap_rb, png::Compression::Fast);
+                    let path = format!("{out_dir}/frame-{:05}.png", i + 1);
+                    if let Err(e) = std::fs::write(&path, &png) {
+                        fail(format!("save failed: {e}"));
+                        break;
+                    }
+                    let done = saved.fetch_add(1, Ordering::Relaxed) + 1;
+                    eprint!("\r[{done:>4}/{frames}] saved");
+                }
+            });
+        }
+
+        // GPU stage, on this thread (it owns the app and the device). The
+        // shader specialization (kind, Julia, DE, morph) can change between
+        // frames during a kind morph; build each pipeline once.
+        let mut pipelines = HashMap::new();
+        for (i, points) in ref_rx.iter() {
+            if failed.load(Ordering::Relaxed) {
+                break;
+            }
+            apply_frame(&mut app, i as u32);
+            app.finish_reference(jobs[i].clone(), points);
+
+            let uniforms = app.make_uniforms(aspect);
+            let handles = pipelines
+                .entry(PipelineKey::from_uniforms(&uniforms))
+                .or_insert_with(|| renderer.export_handles(&device, &uniforms));
+            let er = ExportRender::new(
+                &device,
+                &queue,
+                handles,
+                width,
+                height,
+                uniforms,
+                app.reference_points(),
+                app.lights(),
             );
-        });
-        eprintln!();
+            let padded = render_readback_blocking(&device, &queue, &er);
+            if png_tx.send((i, padded, er.padded_bpr, er.swap_rb)).is_err() {
+                break;
+            }
+        }
+        // Dropping the channel ends lets the workers drain and exit.
+        drop(png_tx);
+        drop(ref_rx);
+    });
+    eprintln!();
 
-        let path = format!("{out_dir}/frame-{:05}.png", i + 1);
-        std::fs::write(&path, &png).map_err(|e| format!("save failed: {e}"))?;
+    if let Some(e) = error.into_inner().unwrap() {
+        return Err(e);
+    }
+    let saved = saved.into_inner();
+    if saved != frames as usize {
+        return Err(format!("only {saved} of {frames} frames were rendered"));
     }
 
     println!("saved {frames} frames to {out_dir}/ ({width}×{height})");