Merge #820: fix(geom): decompose of a flattened transform reports 0 shear instead of NaN

crates/geom/src/decompose.rs

@@ -24,7 +24,13 @@ pub fn decompose(m: Affine) -> Decomposed {
     let det = a * d - b * c;
     let rotation = (-b).atan2(a).to_degrees();
     let rotation = if rotation <= -180.0 + 1e-9 { 180.0 } else { rotation };
-    Decomposed { scale_x: sx, scale_y: det / sx, rotation, shear: ((a * c + b * d) / det).atan().to_degrees() }
+    // A flattened transform (e.g. scale_y = 0, so `det` and the shear numerator are both 0) has
+    // no recoverable shear: 0 / 0 would be NaN, and a NaN here poisons the object when the
+    // caller feeds the decomposed values back into `compose` (transform.set does exactly that).
+    // Report 0 instead; a non-zero numerator over a zero determinant still limits to ±90°.
+    let shear = ((a * c + b * d) / det).atan().to_degrees();
+    let shear = if shear.is_nan() { 0.0 } else { shear };
+    Decomposed { scale_x: sx, scale_y: det / sx, rotation, shear }
 }
 
 /// The linear transform with these values (no translation); `decompose` inverts it.
@@ -50,4 +56,29 @@ mod tests {
         // transform.rotate's convention: rotate(-θ) is θ counter-clockwise.
         assert!((decompose(Affine::rotate(-0.5)).rotation - 0.5f64.to_degrees()).abs() < 1e-9);
     }
+
+    #[test]
+    fn flattened_transform_has_no_nan_shear() {
+        // Regression: an object scaled to zero height (scale_y = 0) is singular — its
+        // determinant and shear numerator are both 0, and decompose used to report NaN shear,
+        // which compose then turned into a NaN transform. Shear is unrecoverable there, so
+        // it reports 0 and every other field stays finite and exact.
+        for v in [
+            Decomposed { scale_x: 1.0, scale_y: 0.0, rotation: 0.0, shear: 20.0 },
+            Decomposed { scale_x: 2.0, scale_y: 0.0, rotation: 30.0, shear: -45.0 },
+        ] {
+            let back = decompose(compose(&v));
+            assert!(back.scale_x.is_finite() && back.scale_y.is_finite() && back.rotation.is_finite() && back.shear.is_finite(), "{v:?} → {back:?}");
+            assert_eq!(back.shear, 0.0, "{v:?} → {back:?}");
+            assert!((back.scale_x - v.scale_x).abs() < 1e-9, "{v:?} → {back:?}");
+            assert_eq!(back.scale_y, 0.0, "{v:?} → {back:?}");
+            assert!((back.rotation - v.rotation).abs() < 1e-9, "{v:?} → {back:?}");
+            // Feeding the decomposed values back into compose stays finite too.
+            assert!(compose(&back).as_coeffs().iter().all(|c| c.is_finite()), "{v:?} → {back:?}");
+        }
+        // Directly singular matrices, including one whose shear numerator is non-zero
+        // (that case limits to ±90°, not NaN).
+        assert_eq!(decompose(Affine::new([1.0, 0.0, 0.0, 0.0, 0.0, 0.0])).shear, 0.0);
+        assert!((decompose(Affine::new([1.0, 0.0, 1.0, 0.0, 0.0, 0.0])).shear.abs() - 90.0).abs() < 1e-9);
+    }
 }