問題描述:

由於需要，把一張圖片載入到cv::Mat物件中(image), 並呼叫演算法計算出一個特徵向量(feature) 和一些關鍵點(std::vector<cv::KeyPoint> keyPoints )，把以上這三個物件寫入Sqlite3資料庫(BLOB型別)， 之後如果需要又重新從資料庫中讀出來，並還原成對應的型別。

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

部分程式碼：

1. 寫入：

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<cv::KeyPoint> 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;
}<span style="font-weight: bold;">
</span>
 

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(_("特徵模版儲存資料為空，無法建立物件！"));
				} 
				
				cv::Mat&& img = MakeMat(f1);
				cv::Mat&& feature = MakeMat(f2);

				//- Parse keypoints data.
				uchar* data = (uchar*)f3.GetData();
				int len = f3.GetDataLen();
				std::vector<cv::KeyPoint> 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]; 
<span style="white-space:pre">	</span>int _type = p[2]; int offset = sizeof(int) * 3;
	return cv::Mat(h, w, _type, (uchar*)data + offset);
}
 

Sqlite3 存取二進位制的文章很多，不記錄了。wxSqlite3對blob的操作在wxSqlite3Statement類中，有類似繫結資料(Bind...)和提交操作(ExecuteUpdate...)的函式，具體檢視相應的文件或者看原始碼有註釋的。關於KeyPoint的儲存，由於cv::KeyPoint這個類的所有資料成員全部都是實體資料，比如int, float 這樣的資料，那麼直接可以把每個cv::KeyPoint物件的地址加上長度(  int len = sizeof(cv::KeyPoint) )存入即可，讀資料庫還原的時候只需 獲取當該片記憶體，強制轉換成 cv::KeyPoint* 就行了，但是如果某些情況，比如物件的成員變數裡面有指標指向了另外的地址上的資料，那麼這樣行不通，強轉後的這些指標因為環境變化而無效，如果操作程式會崩潰掉的。