gl_rasterizer: Fix issue with interpolation of opposite quaternions.

src/video_core/renderer_opengl/gl_rasterizer.cpp

@@ -158,12 +158,34 @@ void RasterizerOpenGL::Reset() {
     res_cache.InvalidateAll();
 }
 
+/**
+ * This is a helper function to resolve an issue with opposite quaternions being interpolated by
+ * OpenGL. See below for a detailed description of this issue (yuriks):
+ *
+ * For any rotation, there are two quaternions Q, and -Q, that represent the same rotation. If you
+ * interpolate two quaternions that are opposite, instead of going from one rotation to another
+ * using the shortest path, you'll go around the longest path. You can test if two quaternions are
+ * opposite by checking if Dot(Q1, W2) < 0. In that case, you can flip either of them, therefore
+ * making Dot(-Q1, W2) positive.
+ *
+ * NOTE: This solution corrects this issue per-vertex before passing the quaternions to OpenGL. This
+ * should be correct for nearly all cases, however a more correct implementation (but less trivial
+ * and perhaps unnecessary) would be to handle this per-fragment, by interpolating the quaternions
+ * manually using two Lerps, and doing this correction before each Lerp.
+ */
+static bool AreQuaternionsOpposite(Math::Vec4<Pica::float24> qa, Math::Vec4<Pica::float24> qb) {
+    Math::Vec4f a{ qa.x.ToFloat32(), qa.y.ToFloat32(), qa.z.ToFloat32(), qa.w.ToFloat32() };
+    Math::Vec4f b{ qb.x.ToFloat32(), qb.y.ToFloat32(), qb.z.ToFloat32(), qb.w.ToFloat32() };
+
+    return (Math::Dot(a, b) < 0.f);
+}
+
 void RasterizerOpenGL::AddTriangle(const Pica::Shader::OutputVertex& v0,
                                    const Pica::Shader::OutputVertex& v1,
                                    const Pica::Shader::OutputVertex& v2) {
-    vertex_batch.emplace_back(v0);
-    vertex_batch.emplace_back(v1);
-    vertex_batch.emplace_back(v2);
+    vertex_batch.emplace_back(v0, false);
+    vertex_batch.emplace_back(v1, AreQuaternionsOpposite(v0.quat, v1.quat));
+    vertex_batch.emplace_back(v2, AreQuaternionsOpposite(v0.quat, v2.quat));
 }
 
 void RasterizerOpenGL::DrawTriangles() {

src/video_core/renderer_opengl/gl_rasterizer.h

@@ -248,7 +248,7 @@ private:
 
     /// Structure that the hardware rendered vertices are composed of
     struct HardwareVertex {
-        HardwareVertex(const Pica::Shader::OutputVertex& v) {
+        HardwareVertex(const Pica::Shader::OutputVertex& v, bool flip_quaternion) {
             position[0] = v.pos.x.ToFloat32();
             position[1] = v.pos.y.ToFloat32();
             position[2] = v.pos.z.ToFloat32();
@@ -270,6 +270,12 @@ private:
             view[0] = v.view.x.ToFloat32();
             view[1] = v.view.y.ToFloat32();
             view[2] = v.view.z.ToFloat32();
+
+            if (flip_quaternion) {
+                for (float& x : normquat) {
+                    x = -x;
+                }
+            }
         }
 
         GLfloat position[4];