Qt Quick 3D - Simple Skinning Example

 // Copyright (C) 2020 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

 #include "skingeometry.h"

 #include <qmath.h>

 struct Vertex {
     QVector3D position;
     qint32 joints[4];
     float weights[4];

     float pad;
 };

 static const int s_vertexSize = sizeof(Vertex);
 Q_STATIC_ASSERT(s_vertexSize == 48);

 SkinGeometry::SkinGeometry(QQuick3DObject *parent)
     : QQuick3DGeometry(parent)
 {
     addAttribute(QQuick3DGeometry::Attribute::PositionSemantic, 0,
                  QQuick3DGeometry::Attribute::ComponentType::F32Type);
     addAttribute(QQuick3DGeometry::Attribute::IndexSemantic, 0,
                  QQuick3DGeometry::Attribute::ComponentType::U32Type);
     addAttribute(QQuick3DGeometry::Attribute::JointSemantic, offsetof(Vertex, joints[0]),
                  QQuick3DGeometry::Attribute::ComponentType::I32Type);
     addAttribute(QQuick3DGeometry::Attribute::WeightSemantic, offsetof(Vertex, weights[0]),
                  QQuick3DGeometry::Attribute::ComponentType::F32Type);
 }

 QList<QVector3D> SkinGeometry::positions() const
 {
     return m_positions;
 }

 QList<qint32> SkinGeometry::joints() const
 {
     return m_joints;
 }

 QList<float> SkinGeometry::weights() const
 {
     return m_weights;
 }

 QList<quint32> SkinGeometry::indexes() const
 {
     return m_indexes;
 }

 void SkinGeometry::setPositions(const QList<QVector3D> &positions)
 {
     if (positions == m_positions)
         return;
     m_positions = positions;
     emit positionsChanged();
     m_vertexDirty = true;
 }

 void SkinGeometry::setJoints(const QList<qint32> &joints)
 {
     if (joints == m_joints)
         return;
     m_joints = joints;
     emit jointsChanged();
     m_vertexDirty = true;
 }

 void SkinGeometry::setWeights(const QList<float> &weights)
 {
     if (weights == m_weights)
         return;
     m_weights = weights;
     emit weightsChanged();
     m_vertexDirty = true;
 }

 void SkinGeometry::setIndexes(const QList<quint32> &indexes)
 {
     if (indexes == m_indexes)
         return;
     m_indexes = indexes;
     emit indexesChanged();
     m_indexDirty = true;
 }

 QSSGRenderGraphObject *SkinGeometry::updateSpatialNode(QSSGRenderGraphObject *node)
 {
     if (m_vertexDirty) {
         m_vertexDirty = false;

         constexpr float maxFloat = std::numeric_limits<float>::max();
         auto boundsMin = QVector3D(maxFloat, maxFloat, maxFloat);
         auto boundsMax = QVector3D(-maxFloat, -maxFloat, -maxFloat);

         const int numVertexes = m_positions.size();
         m_vertexBuffer.resize(numVertexes * s_vertexSize);
         Vertex *vert = reinterpret_cast<Vertex *>(m_vertexBuffer.data());

         for (int i = 0; i < numVertexes; ++i) {
             Vertex &v = vert[i];
             v.position = m_positions[i];
             if (m_joints.size() >= 4 * (i + 1))
                 memcpy(v.joints, m_joints.constData() + 4 * i, 4 * sizeof(qint32));
             else
                 v.joints[0] = v.joints[1] = v.joints[2] = v.joints[3] = 0;
             if (m_weights.size() >= 4 * (i + 1))
                 memcpy(v.weights, m_weights.constData() + 4 * i, 4 * sizeof(float));
             else
                 v.weights[0] = v.weights[1] = v.weights[2] = v.weights[3] = 0.0f;

             boundsMin.setX(std::min(boundsMin.x(), v.position.x()));
             boundsMin.setY(std::min(boundsMin.y(), v.position.y()));
             boundsMin.setZ(std::min(boundsMin.z(), v.position.z()));

             boundsMax.setX(std::max(boundsMax.x(), v.position.x()));
             boundsMax.setY(std::max(boundsMax.y(), v.position.y()));
             boundsMax.setZ(std::max(boundsMax.z(), v.position.z()));
         }

         setStride(s_vertexSize);
         setVertexData(m_vertexBuffer);
         setPrimitiveType(QQuick3DGeometry::PrimitiveType::Triangles);
         setBounds(boundsMin, boundsMax);
     }

     if (m_indexDirty) {
         m_indexDirty = false;
         m_indexBuffer = QByteArray(reinterpret_cast<char *>(m_indexes.data()), m_indexes.size() * sizeof(quint32));
         setIndexData(m_indexBuffer);
     }

     node = QQuick3DGeometry::updateSpatialNode(node);
     return node;
 }