Replace committed PNG renders with small JPEG previews

Full-size PNG renders are now ignored; downscaled 1280x720 JPEG
previews live in renders/ and are shown in a README gallery. The
renderer writes JPEG when the output name ends in .jpg/.jpeg.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,2 @@
+# Full-size renders stay local; commit downscaled JPEG previews to renders/ instead.
+*.png

README.md

@@ -83,7 +83,9 @@ cargo build --release
 ./target/release/pathtracer --list-scenes
 ```
 
-It uses all cores via rayon, shows a progress bar and (for the `cornell` scene) the region brightness table, but does not write `preview.png` or `render_metrics.png`. Without an output name it writes `<scene>.png`.
+It uses all cores via rayon, shows a progress bar and (for the `cornell` scene) the region brightness table, but does not write `preview.png` or `render_metrics.png`. Without an output name it writes `<scene>.png`; an output name ending in `.jpg` or `.jpeg` writes a JPEG (quality 90) instead, which for noisy renders is several times smaller.
+
+PNG files are ignored by git (see `.gitignore`), so full-size renders stay local. To add a render to the repo, commit a downscaled JPEG preview to [`renders/`](renders/) (the gallery below uses 1280x720, quality 85, about 100 KB each).
 
 ### Scenes
 
@@ -100,6 +102,16 @@ It uses all cores via rayon, shows a progress bar and (for the `cornell` scene)
 | `haze` | The `city` under a layer of atmospheric haze that fades the horizon and glows around the sun |
 | `city` | Massive scale: a procedural 400x400-lot city of ~124,000 objects (buildings with stepped tiers, mirror-glass towers, parks with trees) under a low sun. Renders 800x600 at 64 spp in ~1 s; without the BVH it is ~1000x slower |
 
+### Gallery
+
+| | |
+|---|---|
+| ![beam](renders/beam.jpg) `beam` | ![fog](renders/fog.jpg) `fog` |
+| ![caustics](renders/caustics.jpg) `caustics` | ![media](renders/media.jpg) `media` |
+| ![city](renders/city.jpg) `city` | ![haze](renders/haze.jpg) `haze` |
+| ![spheres](renders/spheres.jpg) `spheres` | ![night](renders/night.jpg) `night` |
+| ![classic](renders/classic.jpg) `classic` | ![cornell (Python)](renders/cornell_python.jpg) `cornell`, rendered by `raytracer.py` |
+
 Scenes are defined in [`rust/src/scenes.rs`](rust/src/scenes.rs) with a small builder (`plane`, `sphere`, `cuboid`, `box_light`, `sphere_light`, `rect_light`, `spot_light`, `fog`, `caustic_photons`, `media_photons`). Every emitter must be added with a `*_light` method so light sampling knows about it.
 
 ### Volumetrics

rust/Cargo.lock

@@ -169,6 +169,8 @@ dependencies = [
  "moxcms",
  "num-traits",
  "png",
+ "zune-core",
+ "zune-jpeg",
 ]
 
 [[package]]
@@ -581,3 +583,18 @@ name = "zlib-rs"
 version = "0.6.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b268e58e7c693d7c271f93ffc4ba3b380412554231c85bf61ca7af91042a4112"
+
+[[package]]
+name = "zune-core"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d56377fd46368984a170bc5aac5567e52ca5da874caa60bea39fcbca78fb658b"
+
+[[package]]
+name = "zune-jpeg"
+version = "0.5.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "27bc9d5b815bc103f142aa054f561d9187d191692ec7c2d1e2b4737f8dbd7296"
+dependencies = [
+ "zune-core",
+]

rust/Cargo.toml

@@ -8,7 +8,7 @@ description = "Cornell Box path tracer (Rust port of raytracer.py)"
 rand = { version = "0.8", features = ["small_rng"] }
 rayon = "1"
 indicatif = "0.17"
-image = { version = "0.25", default-features = false, features = ["png"] }
+image = { version = "0.25", default-features = false, features = ["png", "jpeg"] }
 
 [profile.release]
 opt-level = 3

rust/src/main.rs

@@ -419,13 +419,25 @@ fn render(scene: &Scene, s: &Settings, caustics: Option<&CausticMaps>) -> (Vec<V
     (image, total_samples)
 }
 
-fn save_png(image: &[Vec3], width: usize, height: usize, path: &str) -> image::ImageResult<()> {
+/// JPEG quality for .jpg/.jpeg output: much smaller files than PNG for
+/// noisy renders, with no visible loss at this setting.
+const JPEG_QUALITY: u8 = 90;
+
+/// Saves as JPEG if `path` ends in .jpg/.jpeg, otherwise as PNG.
+fn save_image(image: &[Vec3], width: usize, height: usize, path: &str) -> image::ImageResult<()> {
     let bytes: Vec<u8> = image
         .iter()
         .flat_map(|c| [c.x, c.y, c.z])
         .map(|c| (c.clamp(0.0, 1.0) * 255.0) as u8)
         .collect();
+    let lower = path.to_ascii_lowercase();
+    if lower.ends_with(".jpg") || lower.ends_with(".jpeg") {
+        let file = std::io::BufWriter::new(std::fs::File::create(path)?);
+        let mut encoder = image::codecs::jpeg::JpegEncoder::new_with_quality(file, JPEG_QUALITY);
+        encoder.encode(&bytes, width as u32, height as u32, image::ExtendedColorType::Rgb8)
+    } else {
         image::save_buffer(path, &bytes, width as u32, height as u32, image::ColorType::Rgb8)
+    }
 }
 
 fn region_mean(image: &[Vec3], width: usize, rows: std::ops::Range<usize>, cols: std::ops::Range<usize>) -> f64 {
@@ -582,7 +594,7 @@ fn main() {
     let elapsed = start.elapsed().as_secs_f64();
     let pixels = (settings.width * settings.height) as f64;
 
-    if let Err(e) = save_png(&image, settings.width, settings.height, &settings.output) {
+    if let Err(e) = save_image(&image, settings.width, settings.height, &settings.output) {
         eprintln!("failed to write {}: {e}", settings.output);
         std::process::exit(1);
     }