Refactor skinned mesh weight data structure (#16655)

irr/include/WeightBuffer.h

@@ -0,0 +1,67 @@
+// Copyright (C) 2025 Lars Müller
+// For conditions of distribution and use, see copyright notice in irrlicht.h
+
+#pragma once
+
+#include "vector3d.h"
+#include "matrix4.h"
+#include "IVertexBuffer.h"
+
+#include <cassert>
+#include <memory>
+#include <optional>
+
+namespace scene
+{
+
+struct WeightBuffer
+{
+	constexpr static u16 MAX_WEIGHTS_PER_VERTEX = 4;
+	// ID-weight pairs for a joint
+	struct VertexWeights {
+		std::array<u16, MAX_WEIGHTS_PER_VERTEX> joint_ids = {};
+		std::array<f32, MAX_WEIGHTS_PER_VERTEX> weights = {};
+		void addWeight(u16 joint_id, f32 weight);
+		/// Transform given position and normal with these weights
+		void skinVertex(core::vector3df &pos, core::vector3df &normal,
+				const std::vector<core::matrix4> &joint_transforms) const;
+	};
+	std::vector<VertexWeights> weights;
+
+	std::optional<std::vector<u32>> animated_vertices;
+
+	// A bit of a hack for now: Store static positions here so we can use them for skinning.
+	// Ideally we might want a design where we do not mutate the original vertex buffer at all.
+	std::unique_ptr<core::vector3df[]> static_positions;
+	std::unique_ptr<core::vector3df[]> static_normals;
+
+	WeightBuffer(size_t n_verts) : weights(n_verts) {}
+
+	const std::array<u16, MAX_WEIGHTS_PER_VERTEX> &getJointIds(u32 vertex_id) const
+	{ return weights[vertex_id].joint_ids; }
+
+	const std::array<f32, MAX_WEIGHTS_PER_VERTEX> &getWeights(u32 vertex_id) const
+	{ return weights[vertex_id].weights; }
+
+	size_t size() const
+	{ return weights.size(); }
+
+	void addWeight(u32 vertex_id, u16 joint_id, f32 weight);
+
+	/// Transform position and normal using the weights of the given vertex
+	void skinVertex(u32 vertex_id, core::vector3df &pos, core::vector3df &normal,
+			const std::vector<core::matrix4> &joint_transforms) const;
+
+	/// @note src and dst can be the same buffer
+	void skin(IVertexBuffer *dst,
+			const std::vector<core::matrix4> &joint_transforms) const;
+
+	/// Prepares this buffer for use in skinning.
+	void finalize();
+
+	void updateStaticPose(const IVertexBuffer *vbuf);
+
+	void resetToStatic(IVertexBuffer *vbuf) const;
+};
+
+} // end namespace scene