SQlite3 二进制数据(BLOB)的存取【基于wxSqlite3的封装的操作】

问题描述:

由于需要，把一张图片加载到cv::Mat对象中(image), 并调用算法计算出一个特征向量(feature) 和一些关键点(std::vector keyPoints )，把以上这三个对象写入Sqlite3数据库(BLOB类型)， 之后如果需要又重新从数据库中读出来，并还原成对应的类型。

cv::Mat image = imread(...);
cv::Mat feature = algorithm.Calc(...);
std::vector ps = algorithm.Calc(...);

部分代码：

1. 写入：

int MTemplateFactory::New(const MTemplateFeature& feature)
{
	MDatabaseArchive* archive = MDatabaseManager::getSingleton().GetArchive();
	if (archive)
	{
		MString sql;
		if (feature.GetColor() <= 0)
			sql = MString::Format("insert into tabTemplate (name, branch_id, feature, image, keypoints) values ('%s', %d, ?, ?, ?)", feature.GetName(), feature.GetBranchId());
		else
			sql = MString::Format("insert into tabTemplate (name, branch_id, color, feature, image, keypoints) values ('%s', %d, %d, ?, ?, ?)", feature.GetName(), feature.GetBranchId(), feature.GetColor());

		try
		{ 
			//- Prepare data
			cv::Mat img = feature.GetImage();
			cv::Mat f = feature.GetFeature();
			std::vector ps = feature.GetKeyPoints();

			//- Image.
			wxMemoryBuffer buf1;
			buf1.AppendData(&img.rows, 4); int type1 = img.type();
			buf1.AppendData(&img.cols, 4); buf1.AppendData(&type1, 4);
			buf1.AppendData(img.data, img.cols * img.rows * img.elemSize());
			//- Feature.
			wxMemoryBuffer buf2;
			buf2.AppendData(&f.rows, 4); int type2 = img.type();
			buf2.AppendData(&f.cols, 4); buf2.AppendData(&type2, 4);
			buf2.AppendData(f.data, f.cols * f.rows * f.eleSize());
			//- keyPoints.
			wxMemoryBuffer buf3;
			int eleSize = sizeof(cv::KeyPoint);
			for (auto point : ps)
			{
				buf3.AppendData(&point, eleSize);
			} 

			wxSQLite3Statement stmt = archive->PrepareStatement(sql);   
			stmt.Bind(1, buf2);
			stmt.Bind(2, buf1);
			stmt.Bind(3, buf3);
			if (stmt.ExecuteUpdate() > 0)
				return archive->ExecuteScalar("select MAX(id) from tabTemplate");
		}
		catch (wxSQLite3Exception& e)
		{
			throw MException(e.GetMessage());
		}
		catch (cv::Exception& e)
		{
			throw MException(e.msg);
		} 
	}
	return 0;
}

2. 读取:

MTemplateArray MTemplateFactory::List()
{
	MTemplateArray arr;
	MDatabaseArchive* archive = MDatabaseManager::getSingleton().GetArchive();
	if (archive)
	{ 
		try
		{
			wxSQLite3ResultSet res = archive->ExecuteQuery("select * from tabTemplate");
			while (res.NextRow())
			{
				int id = res.GetInt(0);
				MString name = res.GetString(1);
				int branch_id = res.GetInt(2);
				int colorId = res.GetInt(3);

				wxMemoryBuffer f1, f2, f3;
				res.GetBlob("image", f1);
				res.GetBlob("feature", f2);
				res.GetBlob("keypoints", f3);

				if (f1.IsEmpty() == true || f2.IsEmpty() == true || f3.IsEmpty() == true)
				{
					throw MException(_("特征模版保存数据为空，无法创建对象！"));
				} 
				
				cv::Mat&& img = MakeMat(f1);
				cv::Mat&& feature = MakeMat(f2);

				//- Parse keypoints data.
				uchar* data = (uchar*)f3.GetData();
				int len = f3.GetDataLen();
				std::vector ps;
				int eleSize = sizeof(cv::KeyPoint);
				int cnt = len / eleSize;
				for (int i = 0; i < cnt; i++)
				{
					cv::KeyPoint* p = (cv::KeyPoint*)(data + i * eleSize);
					ps.push_back(cv::KeyPoint(*p));
				}

				//- Construct template feature object...
				arr.push_back(MTemplateFeature(id, name, branch_id, colorId, img.clone(), feature.clone(), ps));
			}

		}
		catch (cv::Exception &e)
		{
			throw MException(e.msg);
		}
		catch (wxSQLite3Exception& e)
		{
			throw MException(e.GetMessage());
		}
	}
	return arr;
}

cv::Mat MTemplateFactory::MakeMat(const wxMemoryBuffer& buf )
{
	assert(buf.IsEmpty() == false);
	void* data = buf.GetData();
	int len = buf.GetDataLen();

	int* p = (int*)data;
	int h = p[0];
	int w = p[1]; 
	int _type = p[2]; int offset = sizeof(int) * 3;
	return cv::Mat(h, w, _type, (uchar*)data + offset);
}

说明几点注意的地方:

Sqlite3 存取二进制的文章很多，不记录了。wxSqlite3对blob的操作在wxSqlite3Statement类中，有类似绑定数据(Bind...)和提交操作(ExecuteUpdate...)的函数，具体查看相应的文档或者看源码有注释的。关于KeyPoint的保存，由于cv::KeyPoint这个类的所有数据成员全部都是实体数据，比如int, float 这样的数据，那么直接可以把每个cv::KeyPoint对象的地址加上长度(  int len = sizeof(cv::KeyPoint) )存入即可，读数据库还原的时候只需 获取当该片内存，强制转换成 cv::KeyPoint* 就行了，但是如果某些情况，比如对象的成员变量里面有指针指向了另外的地址上的数据，那么这样行不通，强转后的这些指针因为环境变化而无效，如果操作程序会崩溃掉的。