From ce8547c8d36c06fe82aa2340ef4b13f3572dd27c Mon Sep 17 00:00:00 2001
From: lisyarus <lisyarus@gmail.com>
Date: Tue, 15 Oct 2024 21:42:00 +0300
Subject: [PATCH] Preprocess glTF rotation animations to force shortest-arc
 interpolation

---
 .../gfx/include/psemek/gfx/gltf_animation.hpp | 28 ++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/libs/gfx/include/psemek/gfx/gltf_animation.hpp b/libs/gfx/include/psemek/gfx/gltf_animation.hpp
index a50826e1..dcdb1a63 100644
--- a/libs/gfx/include/psemek/gfx/gltf_animation.hpp
+++ b/libs/gfx/include/psemek/gfx/gltf_animation.hpp
@@ -98,7 +98,33 @@ namespace psemek::gfx
 			: input_(std::move(input))
 			, output_(std::move(output))
 			, interpolation_(interpolation)
-		{}
+		{
+			if constexpr (path == gltf_asset::animation::channel::rotation)
+			{
+				if (interpolation == geom::easing_type::linear)
+				{
+					for (std::size_t i = 1; i < input_.size(); ++i)
+					{
+						if (geom::dot(output_[i - 1].coords, output_[i].coords) < 0.f)
+						{
+							output_[i] = - output_[i];
+						}
+					}
+				}
+				else if (interpolation == geom::easing_type::cubic)
+				{
+					for (std::size_t i = 1; i < input_.size(); ++i)
+					{
+						if (geom::dot(output_[3 * (i - 1) + 1].coords, output_[3 * i + 1].coords) < 0.f)
+						{
+							output_[3 * i + 0] = - output_[3 * i + 0];
+							output_[3 * i + 1] = - output_[3 * i + 1];
+							output_[3 * i + 2] = - output_[3 * i + 2];
+						}
+					}
+				}
+			}
+		}
 
 		gltf_animation_channel(gltf_animation_channel &&) = default;