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Merge pull request #2767 from yuriks/quaternion-flip-comment
OpenGL: Update comment on AreQuaternionsOpposite with new information
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@ -182,19 +182,22 @@ RasterizerOpenGL::RasterizerOpenGL() : shader_dirty(true) {
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RasterizerOpenGL::~RasterizerOpenGL() {}
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/**
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* This is a helper function to resolve an issue with opposite quaternions being interpolated by
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* OpenGL. See below for a detailed description of this issue (yuriks):
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* This is a helper function to resolve an issue when interpolating opposite quaternions. See below
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* for a detailed description of this issue (yuriks):
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*
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* For any rotation, there are two quaternions Q, and -Q, that represent the same rotation. If you
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* interpolate two quaternions that are opposite, instead of going from one rotation to another
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* using the shortest path, you'll go around the longest path. You can test if two quaternions are
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* opposite by checking if Dot(Q1, W2) < 0. In that case, you can flip either of them, therefore
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* making Dot(-Q1, W2) positive.
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* opposite by checking if Dot(Q1, Q2) < 0. In that case, you can flip either of them, therefore
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* making Dot(Q1, -Q2) positive.
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*
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* NOTE: This solution corrects this issue per-vertex before passing the quaternions to OpenGL. This
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* should be correct for nearly all cases, however a more correct implementation (but less trivial
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* and perhaps unnecessary) would be to handle this per-fragment, by interpolating the quaternions
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* manually using two Lerps, and doing this correction before each Lerp.
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* This solution corrects this issue per-vertex before passing the quaternions to OpenGL. This is
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* correct for most cases but can still rotate around the long way sometimes. An implementation
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* which did `lerp(lerp(Q1, Q2), Q3)` (with proper weighting), applying the dot product check
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* between each step would work for those cases at the cost of being more complex to implement.
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*
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* Fortunately however, the 3DS hardware happens to also use this exact same logic to work around
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* these issues, making this basic implementation actually more accurate to the hardware.
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*/
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static bool AreQuaternionsOpposite(Math::Vec4<Pica::float24> qa, Math::Vec4<Pica::float24> qb) {
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Math::Vec4f a{qa.x.ToFloat32(), qa.y.ToFloat32(), qa.z.ToFloat32(), qa.w.ToFloat32()};
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