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gl_rasterizer: Fix issue with interpolation of opposite quaternions.
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@ -158,12 +158,34 @@ void RasterizerOpenGL::Reset() {
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res_cache.InvalidateAll();
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}
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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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*
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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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*
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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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*/
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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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Math::Vec4f b{ qb.x.ToFloat32(), qb.y.ToFloat32(), qb.z.ToFloat32(), qb.w.ToFloat32() };
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return (Math::Dot(a, b) < 0.f);
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}
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void RasterizerOpenGL::AddTriangle(const Pica::Shader::OutputVertex& v0,
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const Pica::Shader::OutputVertex& v1,
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const Pica::Shader::OutputVertex& v2) {
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vertex_batch.emplace_back(v0);
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vertex_batch.emplace_back(v1);
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vertex_batch.emplace_back(v2);
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vertex_batch.emplace_back(v0, false);
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vertex_batch.emplace_back(v1, AreQuaternionsOpposite(v0.quat, v1.quat));
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vertex_batch.emplace_back(v2, AreQuaternionsOpposite(v0.quat, v2.quat));
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}
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void RasterizerOpenGL::DrawTriangles() {
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@ -248,7 +248,7 @@ private:
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/// Structure that the hardware rendered vertices are composed of
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struct HardwareVertex {
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HardwareVertex(const Pica::Shader::OutputVertex& v) {
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HardwareVertex(const Pica::Shader::OutputVertex& v, bool flip_quaternion) {
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position[0] = v.pos.x.ToFloat32();
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position[1] = v.pos.y.ToFloat32();
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position[2] = v.pos.z.ToFloat32();
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@ -270,6 +270,12 @@ private:
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view[0] = v.view.x.ToFloat32();
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view[1] = v.view.y.ToFloat32();
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view[2] = v.view.z.ToFloat32();
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if (flip_quaternion) {
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for (float& x : normquat) {
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x = -x;
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}
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}
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}
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GLfloat position[4];
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