QA: frame scores refuse NaN samples and mismatched buffers (#427 follow-up)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QiAJeFttZp83Fh7YuzAR5f

crates/effects/src/fidelity.rs

@@ -2,17 +2,21 @@
 
 use effectcraft_raster::Image;
 
+/// How far a rendered frame is from its reference.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct FrameScore {
+    /// Pixels compared (the frames' width × height).
     pub compared_pixels: usize,
+    /// Mean absolute difference over every channel sample.
     pub mean_absolute_error: f64,
+    /// Largest absolute difference of any channel sample.
     pub max_error: f32,
 }
 
 /// Compare two premultiplied rendered frames. A size mismatch is reported as an error instead of
 /// silently producing a misleading score.
 pub fn compare_frames(actual: &Image, reference: &Image) -> Result<FrameScore, String> {
-    if actual.width != reference.width || actual.height != reference.height {
+    if actual.width != reference.width || actual.height != reference.height || actual.data.len() != reference.data.len() {
         return Err(format!("frame size mismatch: {}x{} vs {}x{}", actual.width, actual.height, reference.width, reference.height));
     }
     let mut sum = 0.0f64;
@@ -20,6 +24,10 @@ pub fn compare_frames(actual: &Image, reference: &Image) -> Result<FrameScore, S
     for (a, b) in actual.data.iter().zip(&reference.data) {
         for (&x, &y) in a.iter().zip(b) {
             let error = (x - y).abs();
+            // A NaN would make the mean NaN, which a `mean > tolerance` check reads as a pass.
+            if !error.is_finite() {
+                return Err("frame has a non-finite sample".into());
+            }
             sum += f64::from(error);
             max = max.max(error);
         }
@@ -43,5 +51,7 @@ mod tests {
         assert!((score.mean_absolute_error - 0.0625).abs() < f64::EPSILON);
         assert_eq!(score.max_error, 0.5);
         assert!(compare_frames(&actual, &Image::new(1, 1)).is_err());
+        actual.data[0][1] = f32::NAN;
+        assert!(compare_frames(&actual, &reference).is_err(), "NaN is an error, not a pass");
     }
 }

crates/effects/src/lib.rs

@@ -21,12 +21,12 @@ mod color2;
 mod color3;
 mod color_fx;
 pub mod controls;
-pub mod fidelity;
 mod distort;
 mod distort2;
 mod distort3;
 pub mod distort4;
 pub mod face_track;
+pub mod fidelity;
 mod generate;
 mod generate2;
 mod generate3;