feat(render-parity): add deterministic frame comparison tool

- Introduced `render-parity` crate for comparing rendered frames against reference images.
- Added command-line options for specifying manifest and output directory.
- Implemented image comparison metrics: mean absolute difference, maximum absolute difference, and changed pixel ratio.
- Created a configuration file `cases.toml` for defining test cases with global defaults and specific parameters.
- Added functionality to capture frames from `render-demo` and save diff images on discrepancies.
- Updated documentation to include usage instructions and CI model for automated testing.

4 files changed, 642 insertions, 0 deletions

diff --git a/crates/render-parity/Cargo.toml b/crates/render-parity/Cargo.toml
new file mode 100644
index 0000000..70c7ac3
--- /dev/null
+++ b/crates/render-parity/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "render-parity"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+image = { version = "0.25", default-features = false, features = ["png"] }
+serde = { version = "1", features = ["derive"] }
+toml = "0.8"
diff --git a/crates/render-parity/README.md b/crates/render-parity/README.md
new file mode 100644
index 0000000..a94520e
--- /dev/null
+++ b/crates/render-parity/README.md
@@ -0,0 +1,16 @@
+# render-parity
+
+Deterministic frame-diff runner for `parkan-render-demo`.
+
+Usage:
+
+```bash
+cargo run -p render-parity -- \
+  --manifest parity/cases.toml \
+  --output-dir target/render-parity/current
+```
+
+Options:
+
+- `--demo-bin <path>`: use prebuilt `parkan-render-demo` binary instead of `cargo run`.
+- `--keep-going`: continue all cases even after failures.
diff --git a/crates/render-parity/src/lib.rs b/crates/render-parity/src/lib.rs
new file mode 100644
index 0000000..cb412e9
--- /dev/null
+++ b/crates/render-parity/src/lib.rs
@@ -0,0 +1,212 @@
+use image::{ImageBuffer, Rgba, RgbaImage};
+use serde::Deserialize;
+
+#[derive(Debug, Clone, Deserialize, Default)]
+pub struct ManifestMeta {
+    pub width: Option<u32>,
+    pub height: Option<u32>,
+    pub lod: Option<usize>,
+    pub group: Option<usize>,
+    pub angle: Option<f32>,
+    pub diff_threshold: Option<u8>,
+    pub max_mean_abs: Option<f32>,
+    pub max_changed_ratio: Option<f32>,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct CaseSpec {
+    pub id: String,
+    pub archive: String,
+    pub model: Option<String>,
+    pub reference: String,
+    pub width: Option<u32>,
+    pub height: Option<u32>,
+    pub lod: Option<usize>,
+    pub group: Option<usize>,
+    pub angle: Option<f32>,
+    pub diff_threshold: Option<u8>,
+    pub max_mean_abs: Option<f32>,
+    pub max_changed_ratio: Option<f32>,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+pub struct ParityManifest {
+    #[serde(default)]
+    pub meta: ManifestMeta,
+    #[serde(rename = "case", default)]
+    pub cases: Vec<CaseSpec>,
+}
+
+#[derive(Debug, Clone)]
+pub struct DiffMetrics {
+    pub width: u32,
+    pub height: u32,
+    pub mean_abs: f32,
+    pub max_abs: u8,
+    pub changed_pixels: u64,
+    pub changed_ratio: f32,
+}
+
+pub fn compare_images(
+    reference: &RgbaImage,
+    actual: &RgbaImage,
+    diff_threshold: u8,
+) -> Result<DiffMetrics, String> {
+    let (rw, rh) = reference.dimensions();
+    let (aw, ah) = actual.dimensions();
+    if rw != aw || rh != ah {
+        return Err(format!(
+            "image size mismatch: reference={}x{}, actual={}x{}",
+            rw, rh, aw, ah
+        ));
+    }
+
+    let mut diff_sum = 0u64;
+    let mut max_abs = 0u8;
+    let mut changed_pixels = 0u64;
+    let pixel_count = u64::from(rw).saturating_mul(u64::from(rh));
+
+    for (ref_px, act_px) in reference.pixels().zip(actual.pixels()) {
+        let mut pixel_changed = false;
+        for chan in 0..3 {
+            let a = i16::from(ref_px[chan]);
+            let b = i16::from(act_px[chan]);
+            let diff = (a - b).unsigned_abs() as u8;
+            diff_sum = diff_sum.saturating_add(u64::from(diff));
+            if diff > max_abs {
+                max_abs = diff;
+            }
+            if diff > diff_threshold {
+                pixel_changed = true;
+            }
+        }
+        if pixel_changed {
+            changed_pixels = changed_pixels.saturating_add(1);
+        }
+    }
+
+    let channels = pixel_count.saturating_mul(3);
+    let mean_abs = if channels == 0 {
+        0.0
+    } else {
+        diff_sum as f32 / channels as f32
+    };
+    let changed_ratio = if pixel_count == 0 {
+        0.0
+    } else {
+        changed_pixels as f32 / pixel_count as f32
+    };
+
+    Ok(DiffMetrics {
+        width: rw,
+        height: rh,
+        mean_abs,
+        max_abs,
+        changed_pixels,
+        changed_ratio,
+    })
+}
+
+pub fn build_diff_image(reference: &RgbaImage, actual: &RgbaImage) -> Result<RgbaImage, String> {
+    let (rw, rh) = reference.dimensions();
+    let (aw, ah) = actual.dimensions();
+    if rw != aw || rh != ah {
+        return Err(format!(
+            "image size mismatch: reference={}x{}, actual={}x{}",
+            rw, rh, aw, ah
+        ));
+    }
+
+    let mut out: ImageBuffer<Rgba<u8>, Vec<u8>> = ImageBuffer::new(rw, rh);
+    for (dst, (ref_px, act_px)) in out
+        .pixels_mut()
+        .zip(reference.pixels().zip(actual.pixels()))
+    {
+        let dr = (i16::from(ref_px[0]) - i16::from(act_px[0])).unsigned_abs() as u8;
+        let dg = (i16::from(ref_px[1]) - i16::from(act_px[1])).unsigned_abs() as u8;
+        let db = (i16::from(ref_px[2]) - i16::from(act_px[2])).unsigned_abs() as u8;
+        *dst = Rgba([dr, dg, db, 255]);
+    }
+    Ok(out)
+}
+
+pub fn evaluate_metrics(
+    metrics: &DiffMetrics,
+    max_mean_abs: f32,
+    max_changed_ratio: f32,
+) -> Vec<String> {
+    let mut violations = Vec::new();
+    if metrics.mean_abs > max_mean_abs {
+        violations.push(format!(
+            "mean_abs {:.4} > allowed {:.4}",
+            metrics.mean_abs, max_mean_abs
+        ));
+    }
+    if metrics.changed_ratio > max_changed_ratio {
+        violations.push(format!(
+            "changed_ratio {:.4}% > allowed {:.4}%",
+            metrics.changed_ratio * 100.0,
+            max_changed_ratio * 100.0
+        ));
+    }
+    violations
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn solid(w: u32, h: u32, r: u8, g: u8, b: u8) -> RgbaImage {
+        let mut img = RgbaImage::new(w, h);
+        for px in img.pixels_mut() {
+            *px = Rgba([r, g, b, 255]);
+        }
+        img
+    }
+
+    #[test]
+    fn compare_identical_images() {
+        let ref_img = solid(4, 3, 10, 20, 30);
+        let act_img = solid(4, 3, 10, 20, 30);
+        let metrics = compare_images(&ref_img, &act_img, 2).expect("comparison must succeed");
+        assert_eq!(metrics.width, 4);
+        assert_eq!(metrics.height, 3);
+        assert_eq!(metrics.max_abs, 0);
+        assert_eq!(metrics.changed_pixels, 0);
+        assert_eq!(metrics.mean_abs, 0.0);
+        assert_eq!(metrics.changed_ratio, 0.0);
+    }
+
+    #[test]
+    fn compare_detects_changes_and_thresholds() {
+        let mut ref_img = solid(2, 2, 100, 100, 100);
+        let mut act_img = solid(2, 2, 100, 100, 100);
+        ref_img.put_pixel(1, 1, Rgba([120, 100, 100, 255]));
+        act_img.put_pixel(1, 1, Rgba([100, 100, 100, 255]));
+
+        let metrics = compare_images(&ref_img, &act_img, 5).expect("comparison must succeed");
+        assert_eq!(metrics.max_abs, 20);
+        assert_eq!(metrics.changed_pixels, 1);
+        assert!((metrics.changed_ratio - 0.25).abs() < 1e-6);
+        assert!(metrics.mean_abs > 0.0);
+
+        let violations = evaluate_metrics(&metrics, 2.0, 0.20);
+        assert_eq!(violations.len(), 1);
+        assert!(violations[0].contains("changed_ratio"));
+    }
+
+    #[test]
+    fn build_diff_image_returns_per_channel_abs_diff() {
+        let mut ref_img = solid(1, 1, 100, 150, 200);
+        let mut act_img = solid(1, 1, 90, 180, 170);
+        ref_img.put_pixel(0, 0, Rgba([100, 150, 200, 255]));
+        act_img.put_pixel(0, 0, Rgba([90, 180, 170, 255]));
+
+        let diff = build_diff_image(&ref_img, &act_img).expect("diff image must build");
+        let px = diff.get_pixel(0, 0);
+        assert_eq!(px[0], 10);
+        assert_eq!(px[1], 30);
+        assert_eq!(px[2], 30);
+        assert_eq!(px[3], 255);
+    }
+}
diff --git a/crates/render-parity/src/main.rs b/crates/render-parity/src/main.rs
new file mode 100644
index 0000000..22795bc
--- /dev/null
+++ b/crates/render-parity/src/main.rs
@@ -0,0 +1,405 @@
+use image::RgbaImage;
+use render_parity::{
+    build_diff_image, compare_images, evaluate_metrics, CaseSpec, ManifestMeta, ParityManifest,
+};
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+const DEFAULT_MANIFEST: &str = "parity/cases.toml";
+const DEFAULT_OUTPUT_DIR: &str = "target/render-parity/current";
+const DEFAULT_WIDTH: u32 = 1280;
+const DEFAULT_HEIGHT: u32 = 720;
+const DEFAULT_LOD: usize = 0;
+const DEFAULT_GROUP: usize = 0;
+const DEFAULT_ANGLE: f32 = 0.0;
+const DEFAULT_DIFF_THRESHOLD: u8 = 8;
+const DEFAULT_MAX_MEAN_ABS: f32 = 2.0;
+const DEFAULT_MAX_CHANGED_RATIO: f32 = 0.01;
+
+struct Args {
+    manifest: PathBuf,
+    output_dir: PathBuf,
+    demo_bin: Option<PathBuf>,
+    keep_going: bool,
+}
+
+#[derive(Debug, Clone)]
+struct EffectiveCase {
+    id: String,
+    archive: PathBuf,
+    model: Option<String>,
+    reference: PathBuf,
+    width: u32,
+    height: u32,
+    lod: usize,
+    group: usize,
+    angle: f32,
+    diff_threshold: u8,
+    max_mean_abs: f32,
+    max_changed_ratio: f32,
+}
+
+fn main() {
+    let args = match parse_args() {
+        Ok(v) => v,
+        Err(err) => {
+            eprintln!("{err}");
+            print_help();
+            std::process::exit(2);
+        }
+    };
+
+    if let Err(err) = run(args) {
+        eprintln!("{err}");
+        std::process::exit(1);
+    }
+}
+
+fn parse_args() -> Result<Args, String> {
+    let mut manifest = PathBuf::from(DEFAULT_MANIFEST);
+    let mut output_dir = PathBuf::from(DEFAULT_OUTPUT_DIR);
+    let mut demo_bin = None;
+    let mut keep_going = false;
+
+    let mut it = std::env::args().skip(1);
+    while let Some(arg) = it.next() {
+        match arg.as_str() {
+            "--manifest" => {
+                let value = it
+                    .next()
+                    .ok_or_else(|| String::from("missing value for --manifest"))?;
+                manifest = PathBuf::from(value);
+            }
+            "--output-dir" => {
+                let value = it
+                    .next()
+                    .ok_or_else(|| String::from("missing value for --output-dir"))?;
+                output_dir = PathBuf::from(value);
+            }
+            "--demo-bin" => {
+                let value = it
+                    .next()
+                    .ok_or_else(|| String::from("missing value for --demo-bin"))?;
+                demo_bin = Some(PathBuf::from(value));
+            }
+            "--keep-going" => {
+                keep_going = true;
+            }
+            "--help" | "-h" => {
+                print_help();
+                std::process::exit(0);
+            }
+            other => {
+                return Err(format!("unknown argument: {other}"));
+            }
+        }
+    }
+
+    Ok(Args {
+        manifest,
+        output_dir,
+        demo_bin,
+        keep_going,
+    })
+}
+
+fn print_help() {
+    eprintln!(
+        "render-parity [--manifest <cases.toml>] [--output-dir <dir>] [--demo-bin <path>] [--keep-going]"
+    );
+    eprintln!("  --manifest    path to parity manifest (default: {DEFAULT_MANIFEST})");
+    eprintln!("  --output-dir  where current renders and diff images are written");
+    eprintln!("  --demo-bin    prebuilt parkan-render-demo binary path");
+    eprintln!("  --keep-going  continue all cases even after failures");
+}
+
+fn run(args: Args) -> Result<(), String> {
+    let workspace = workspace_root()?;
+    let manifest_path = resolve_path(&workspace, &args.manifest);
+    let output_dir = resolve_path(&workspace, &args.output_dir);
+    let demo_bin = args
+        .demo_bin
+        .as_ref()
+        .map(|path| resolve_path(&workspace, path));
+
+    let manifest_raw = fs::read_to_string(&manifest_path)
+        .map_err(|err| format!("failed to read manifest {}: {err}", manifest_path.display()))?;
+    let manifest: ParityManifest = toml::from_str(&manifest_raw).map_err(|err| {
+        format!(
+            "failed to parse manifest {}: {err}",
+            manifest_path.display()
+        )
+    })?;
+
+    if manifest.cases.is_empty() {
+        println!(
+            "render-parity: no cases in {} (nothing to validate)",
+            manifest_path.display()
+        );
+        return Ok(());
+    }
+
+    fs::create_dir_all(&output_dir).map_err(|err| {
+        format!(
+            "failed to create output directory {}: {err}",
+            output_dir.display()
+        )
+    })?;
+
+    let manifest_dir = manifest_path
+        .parent()
+        .map(Path::to_path_buf)
+        .unwrap_or_else(|| workspace.clone());
+
+    let mut failed_cases = 0usize;
+    for case in &manifest.cases {
+        let effective = make_effective_case(&manifest.meta, case, &manifest_dir)?;
+        let case_file = output_dir.join(format!("{}.png", sanitize_case_id(&effective.id)));
+        let diff_file = output_dir
+            .join("diff")
+            .join(format!("{}.png", sanitize_case_id(&effective.id)));
+
+        let run_res = run_single_case(
+            &workspace, // ensure `cargo run` executes from workspace root
+            demo_bin.as_deref(),
+            &effective,
+            &case_file,
+            &diff_file,
+        );
+
+        match run_res {
+            Ok(()) => {}
+            Err(err) => {
+                failed_cases = failed_cases.saturating_add(1);
+                eprintln!("[FAIL] {}: {}", effective.id, err);
+                if !args.keep_going {
+                    break;
+                }
+            }
+        }
+    }
+
+    if failed_cases > 0 {
+        return Err(format!(
+            "render-parity failed: {} case(s) did not match reference frames",
+            failed_cases
+        ));
+    }
+
+    println!("render-parity: all cases passed");
+    Ok(())
+}
+
+fn run_single_case(
+    workspace: &Path,
+    demo_bin: Option<&Path>,
+    case: &EffectiveCase,
+    case_file: &Path,
+    diff_file: &Path,
+) -> Result<(), String> {
+    run_render_capture(workspace, demo_bin, case, case_file)?;
+
+    let reference = load_rgba(&case.reference)?;
+    let actual = load_rgba(case_file)?;
+    let metrics = compare_images(&reference, &actual, case.diff_threshold)?;
+    let violations = evaluate_metrics(&metrics, case.max_mean_abs, case.max_changed_ratio);
+
+    if violations.is_empty() {
+        println!(
+            "[OK] {} mean_abs={:.4} changed={:.4}% max_abs={} ({}x{})",
+            case.id,
+            metrics.mean_abs,
+            metrics.changed_ratio * 100.0,
+            metrics.max_abs,
+            metrics.width,
+            metrics.height
+        );
+        return Ok(());
+    }
+
+    if let Some(parent) = diff_file.parent() {
+        fs::create_dir_all(parent).map_err(|err| {
+            format!(
+                "failed to create diff output directory {}: {err}",
+                parent.display()
+            )
+        })?;
+    }
+    let diff = build_diff_image(&reference, &actual)?;
+    diff.save(diff_file)
+        .map_err(|err| format!("failed to save diff image {}: {err}", diff_file.display()))?;
+
+    let mut details = String::new();
+    for item in violations {
+        if !details.is_empty() {
+            details.push_str("; ");
+        }
+        details.push_str(&item);
+    }
+    Err(format!(
+        "{} | diff={} | mean_abs={:.4}, changed={:.4}% ({} px), max_abs={}",
+        details,
+        diff_file.display(),
+        metrics.mean_abs,
+        metrics.changed_ratio * 100.0,
+        metrics.changed_pixels,
+        metrics.max_abs
+    ))
+}
+
+fn run_render_capture(
+    workspace: &Path,
+    demo_bin: Option<&Path>,
+    case: &EffectiveCase,
+    out_path: &Path,
+) -> Result<(), String> {
+    if let Some(parent) = out_path.parent() {
+        fs::create_dir_all(parent).map_err(|err| {
+            format!(
+                "failed to create capture directory {}: {err}",
+                parent.display()
+            )
+        })?;
+    }
+
+    let mut cmd = if let Some(bin) = demo_bin {
+        Command::new(bin)
+    } else {
+        let mut command = Command::new("cargo");
+        command.args(["run", "-p", "render-demo", "--features", "demo", "--"]);
+        command
+    };
+
+    cmd.current_dir(workspace)
+        .arg("--archive")
+        .arg(&case.archive)
+        .arg("--lod")
+        .arg(case.lod.to_string())
+        .arg("--group")
+        .arg(case.group.to_string())
+        .arg("--width")
+        .arg(case.width.to_string())
+        .arg("--height")
+        .arg(case.height.to_string())
+        .arg("--angle")
+        .arg(case.angle.to_string())
+        .arg("--capture")
+        .arg(out_path);
+
+    if let Some(model) = case.model.as_deref() {
+        cmd.arg("--model").arg(model);
+    }
+
+    let output = cmd.output().map_err(|err| {
+        let mode = if demo_bin.is_some() {
+            "parkan-render-demo"
+        } else {
+            "cargo run -p render-demo"
+        };
+        format!("failed to execute {} for case {}: {err}", mode, case.id)
+    })?;
+    if !output.status.success() {
+        let stdout = String::from_utf8_lossy(&output.stdout);
+        let stderr = String::from_utf8_lossy(&output.stderr);
+        return Err(format!(
+            "render command exited with status {:?}\nstdout:\n{}\nstderr:\n{}",
+            output.status.code(),
+            stdout,
+            stderr
+        ));
+    }
+
+    Ok(())
+}
+
+fn load_rgba(path: &Path) -> Result<RgbaImage, String> {
+    image::open(path)
+        .map_err(|err| format!("failed to load image {}: {err}", path.display()))
+        .map(|img| img.to_rgba8())
+}
+
+fn make_effective_case(
+    meta: &ManifestMeta,
+    case: &CaseSpec,
+    manifest_dir: &Path,
+) -> Result<EffectiveCase, String> {
+    let width = case.width.or(meta.width).unwrap_or(DEFAULT_WIDTH);
+    let height = case.height.or(meta.height).unwrap_or(DEFAULT_HEIGHT);
+    if width == 0 || height == 0 {
+        return Err(format!(
+            "case '{}' has invalid dimensions {}x{}",
+            case.id, width, height
+        ));
+    }
+
+    let archive = resolve_path(manifest_dir, Path::new(&case.archive));
+    let reference = resolve_path(manifest_dir, Path::new(&case.reference));
+    if !archive.is_file() {
+        return Err(format!(
+            "case '{}' archive not found: {}",
+            case.id,
+            archive.display()
+        ));
+    }
+    if !reference.is_file() {
+        return Err(format!(
+            "case '{}' reference frame not found: {}",
+            case.id,
+            reference.display()
+        ));
+    }
+
+    Ok(EffectiveCase {
+        id: case.id.clone(),
+        archive,
+        model: case.model.clone(),
+        reference,
+        width,
+        height,
+        lod: case.lod.or(meta.lod).unwrap_or(DEFAULT_LOD),
+        group: case.group.or(meta.group).unwrap_or(DEFAULT_GROUP),
+        angle: case.angle.or(meta.angle).unwrap_or(DEFAULT_ANGLE),
+        diff_threshold: case
+            .diff_threshold
+            .or(meta.diff_threshold)
+            .unwrap_or(DEFAULT_DIFF_THRESHOLD),
+        max_mean_abs: case
+            .max_mean_abs
+            .or(meta.max_mean_abs)
+            .unwrap_or(DEFAULT_MAX_MEAN_ABS),
+        max_changed_ratio: case
+            .max_changed_ratio
+            .or(meta.max_changed_ratio)
+            .unwrap_or(DEFAULT_MAX_CHANGED_RATIO),
+    })
+}
+
+fn sanitize_case_id(id: &str) -> String {
+    id.chars()
+        .map(|c| {
+            if c.is_ascii_alphanumeric() || c == '-' || c == '_' {
+                c
+            } else {
+                '_'
+            }
+        })
+        .collect()
+}
+
+fn workspace_root() -> Result<PathBuf, String> {
+    let root = Path::new(env!("CARGO_MANIFEST_DIR"))
+        .join("..")
+        .join("..")
+        .canonicalize()
+        .map_err(|err| format!("failed to resolve workspace root: {err}"))?;
+    Ok(root)
+}
+
+fn resolve_path(base: &Path, path: &Path) -> PathBuf {
+    if path.is_absolute() {
+        path.to_path_buf()
+    } else {
+        base.join(path)
+    }
+}