Convert blob images to Mat images - c++

Question

I am using mysql database for my face recognition project.Here I store images into the table and the images are stored as blob images.when select these images back, i need to write these blob images to separate image files using a file pointer and it will be stored into a folder in which the program resides.And for using those images, i need to read it again from that folder.Then only i can use that images to another function. But i need to use those blob images directly from the database(when we select it from the DB, i need to pass it to another function). So that i can reduce operations like reading it again from the folder.How can i convert bl .So for passing DB images directly to another function, i think we need to convert it's type or something.
void SelectImage()
{
MYSQL *conn;
MYSQL_RES *result;
MYSQL_ROW row;
char temp[900];
char filename[50];
unsigned long *lengths;
FILE *fp;
my_ulonglong numRows;
unsigned int numFields;
int mysqlStatus = 0;
MYSQL_RES *mysqlResult = NULL;
conn = mysql_init(NULL);
mysql_real_connect(conn, "localhost", "root", "athira#iot", "Athira", 0, NULL, 0);
int state=mysql_query(conn, "SELECT * FROM ima1");
mysqlResult=mysql_store_result(conn);
if(mysqlResult)
{
numRows=mysql_num_rows(mysqlResult);
}
cout<<"rows:"<<numRows<<endl;
for(int i=1;i<=numRows;i++){
sprintf(temp,"SELECT data FROM ima1 WHERE id=%d",i);
sprintf(filename,"Gen_Image%d.jpeg",i);
fp = fopen(filename, "wb");// open a file for writing.
cout<<temp<<" to "<<filename<<endl;
mysql_query(conn, temp);//select an image with id
result = mysql_store_result(conn);
row = mysql_fetch_row(result);//row contains row data
lengths = mysql_fetch_lengths(result);//this is the length of th image
fwrite(row[0], lengths[0], 1, fp);//writing image to a file
cout<<"selected..."<<endl;
img.create(100,100,CV_16UC1);
memcpy(img.data,row.data,lengths);
mysql_free_result(result);
fclose(fp);
}
mysql_close(conn);
}
I tried to convert it to Mat type but it's showing error..
error is this,
error: request for member ‘data’ in ‘row’, which is of non-class type ‘MYSQL_ROW {aka char**}’

Answer 1

If your images are stored using some image format (JPEG, PNG, BMP, ...) and not as raw pixel data, then you should be looking at the imdecode function in OpenCV.
If your data is stored as raw pixels, then you'd have to make sure that the memory layout of your BLOB data can be expressed using the step/stride functionality OpenCV uses to layout its matrices.