From e446a0a0360c408f183c08e7680d439157f71b6a Mon Sep 17 00:00:00 2001
From: t-yrka <78016060+t-yrka@users.noreply.github.com>
Date: Tue, 17 Dec 2024 01:39:49 +0100
Subject: [PATCH] Return the target Quat when the angle is small (#591)

Returning self is misleading as it skips the rotation completely
---
 codegen/templates/quat.rs.tera | 2 +-
 src/f32/coresimd/quat.rs       | 2 +-
 src/f32/neon/quat.rs           | 2 +-
 src/f32/scalar/quat.rs         | 2 +-
 src/f32/sse2/quat.rs           | 2 +-
 src/f32/wasm32/quat.rs         | 2 +-
 src/f64/dquat.rs               | 2 +-
 tests/quat.rs                  | 5 +++++
 8 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/codegen/templates/quat.rs.tera b/codegen/templates/quat.rs.tera
index 017096d2..7f9ff9b3 100644
--- a/codegen/templates/quat.rs.tera
+++ b/codegen/templates/quat.rs.tera
@@ -729,7 +729,7 @@ impl {{ self_t }} {
         glam_assert!(self.is_normalized() && rhs.is_normalized());
         let angle = self.angle_between(rhs);
         if angle <= 1e-4 {
-            return *self;
+            return rhs;
         }
         let s = (max_angle / angle).clamp(-1.0, 1.0);
         self.slerp(rhs, s)
diff --git a/src/f32/coresimd/quat.rs b/src/f32/coresimd/quat.rs
index 7bcb0b54..f0479a24 100644
--- a/src/f32/coresimd/quat.rs
+++ b/src/f32/coresimd/quat.rs
@@ -585,7 +585,7 @@ impl Quat {
         glam_assert!(self.is_normalized() && rhs.is_normalized());
         let angle = self.angle_between(rhs);
         if angle <= 1e-4 {
-            return *self;
+            return rhs;
         }
         let s = (max_angle / angle).clamp(-1.0, 1.0);
         self.slerp(rhs, s)
diff --git a/src/f32/neon/quat.rs b/src/f32/neon/quat.rs
index 76664ad5..8390f2e6 100644
--- a/src/f32/neon/quat.rs
+++ b/src/f32/neon/quat.rs
@@ -590,7 +590,7 @@ impl Quat {
         glam_assert!(self.is_normalized() && rhs.is_normalized());
         let angle = self.angle_between(rhs);
         if angle <= 1e-4 {
-            return *self;
+            return rhs;
         }
         let s = (max_angle / angle).clamp(-1.0, 1.0);
         self.slerp(rhs, s)
diff --git a/src/f32/scalar/quat.rs b/src/f32/scalar/quat.rs
index 2cbeb4c6..a55c0ece 100644
--- a/src/f32/scalar/quat.rs
+++ b/src/f32/scalar/quat.rs
@@ -597,7 +597,7 @@ impl Quat {
         glam_assert!(self.is_normalized() && rhs.is_normalized());
         let angle = self.angle_between(rhs);
         if angle <= 1e-4 {
-            return *self;
+            return rhs;
         }
         let s = (max_angle / angle).clamp(-1.0, 1.0);
         self.slerp(rhs, s)
diff --git a/src/f32/sse2/quat.rs b/src/f32/sse2/quat.rs
index 634f7803..cc477ba7 100644
--- a/src/f32/sse2/quat.rs
+++ b/src/f32/sse2/quat.rs
@@ -593,7 +593,7 @@ impl Quat {
         glam_assert!(self.is_normalized() && rhs.is_normalized());
         let angle = self.angle_between(rhs);
         if angle <= 1e-4 {
-            return *self;
+            return rhs;
         }
         let s = (max_angle / angle).clamp(-1.0, 1.0);
         self.slerp(rhs, s)
diff --git a/src/f32/wasm32/quat.rs b/src/f32/wasm32/quat.rs
index 8d0e19f2..b7307ba0 100644
--- a/src/f32/wasm32/quat.rs
+++ b/src/f32/wasm32/quat.rs
@@ -585,7 +585,7 @@ impl Quat {
         glam_assert!(self.is_normalized() && rhs.is_normalized());
         let angle = self.angle_between(rhs);
         if angle <= 1e-4 {
-            return *self;
+            return rhs;
         }
         let s = (max_angle / angle).clamp(-1.0, 1.0);
         self.slerp(rhs, s)
diff --git a/src/f64/dquat.rs b/src/f64/dquat.rs
index 52b67e5d..b6c2d84b 100644
--- a/src/f64/dquat.rs
+++ b/src/f64/dquat.rs
@@ -579,7 +579,7 @@ impl DQuat {
         glam_assert!(self.is_normalized() && rhs.is_normalized());
         let angle = self.angle_between(rhs);
         if angle <= 1e-4 {
-            return *self;
+            return rhs;
         }
         let s = (max_angle / angle).clamp(-1.0, 1.0);
         self.slerp(rhs, s)
diff --git a/tests/quat.rs b/tests/quat.rs
index 3c8a090a..eebc1279 100644
--- a/tests/quat.rs
+++ b/tests/quat.rs
@@ -356,6 +356,11 @@ macro_rules! impl_quat_tests {
             );
             assert_approx_eq!(q2, q0.rotate_towards(q1, -FRAC_PI_2), eps);
             assert_approx_eq!(q2, q0.rotate_towards(q1, -FRAC_PI_2 * 1.5), eps);
+
+            // Small angles
+            let q0 = $quat::from_euler(EulerRot::YXZ, 0.0, 0.0, 0.0);
+            let q1 = $quat::from_euler(EulerRot::YXZ, 1e-4, 0.0, 0.0);
+            assert_eq!(q1, q0.rotate_towards(q1, FRAC_PI_2))
         });
 
         glam_test!(test_fmt, {