[Qt3d] 导出QtEntity为Obj格式（遍历QtEntity）

原文链接：https://www.yuque.com/softdev/qt/txv1lx

class Qui3DView{
private:
  struct date_struct{
    QPointer<Qt3DWindow> m_windowOn3D = new Qt3DWindow();
    QPointer<QtEntity> m_rootEntity = new QtEntity();
    QPointer<QtMaterial> m_material = new QtMaterial();
  }db;
}

//没有找到相关资料
//Qt3d还是比较复杂的，很难考虑全面
//这里简单使用一个
bool Qui3DView::saveObj(const QString& filePath)
{
    using QAttribute = Qt3DRender::QAttribute;
    
    QFile file(filePath);
    
    if (!file.open(QIODevice::WriteOnly | QIODevice::Text))
    {
        return false;
    }
    
    QTextStream stream(&file);
    int filePntSize = 0; //文件中已经存的点个数
    
    auto comps = db.m_rootEntity->components();
    
    for (QComponent* comp : comps)
    {
        QMesh* mesh = dynamic_cast<QMesh*>(comp);
        uint pntSize = 0; //此mesh的点个数
        
        if (!mesh)
        {
            continue;
        }
        
        QGeometry* geom = mesh->geometry();
        
        auto attris = geom->attributes();
        
        for (QAttribute* attri : attris)
        {
            auto type = attri->attributeType();
            
            switch (type)
            {
                case Qt3DRender::QAttribute::VertexAttribute:
                    {
                        pntSize = attri->count();
                        
                        auto buf = attri->buffer();
                        auto bufferBytes = buf->data();
                        const quint32 stride = bufferBytes.size() / pntSize;
                        const quint32 elementSize = stride / sizeof(float);
                        float* fptr = reinterpret_cast<float*>(bufferBytes.data());
                        
                        for (decltype(pntSize) i = 0; i < pntSize; ++i)
                        {
                            auto x = *fptr;
                            auto y = *(fptr + 1);
                            auto z = *(fptr + 2);
                            
                            stream << "v " << x << " " << y << " " << z << endl;
                            
                            fptr += elementSize;
                        }
                        
                        stream.flush();
                        break;
                    }
                case Qt3DRender::QAttribute::IndexAttribute:
                    {
                        auto buf = attri->buffer();
                        auto indexBytes = buf->data();
                        auto faceSize = indexBytes.size() / sizeof(quint16) / 3;
                        quint16* usptr = reinterpret_cast<quint16*>(indexBytes.data());
                        
                        for (decltype(faceSize) i = 0; i < faceSize; ++i)
                        {
                            auto idx1 = *usptr++;
                            auto idx2 = *usptr++;
                            auto idx3 = *usptr++;
                            
                            stream << "f " << idx1 + filePntSize + 1 << " " << idx2 + filePntSize + 1 << " " << idx3 + filePntSize + 1  << endl;
                            //+1是因为obj里的索引是从1开始的
                        }
                        
                        stream.flush();
                        break;
                    }
                case Qt3DRender::QAttribute::DrawIndirectAttribute:
                    break;
                default:
                    break;
            }
            
        }
        
        filePntSize += pntSize; //偏移量
    }
    
    stream.flush();
    file.close();
    return true;
}