opencv binary data jpg image to cv::Mat - c++

Question

I want to load an image in c++ opencv that comes from a postgresql database.
The image, jpg extension, is stored as a binary data (bytea type) in the base, that I can access thanks to libpqxx.
The problem is that I do not know how to convert the data into a cv::Mat instance. With a regular image I could use imread('myImage.jpg', ...), but in this case I cannot even load the database image in the data attribute of Mat because it is jpeg and not bmp.
Any idea ? Is there some opencv method I could use that could understand directly the binary data and convert it to the appropriate structure ? the imdecode() functions seems to be used for bitmap datas.
edit : Berak, using a vector the imdecode function returns null Matrice What happens "If the buffer is too short or contains invalid data, the empty matrix/image is returned." Here is the code :
pqxx::result r=bdd::requete("SELECT image FROM lrad.img WHERE id=3",1);//returns the bytea image in r[0]["image"]
const char* buffer=r[0]["image"].c_str();
vector<uchar>::size_type size = strlen((const char*)buffer);
vector<uchar> jpgbytes(buffer, buffer+size);
Mat img = imdecode(jpgbytes, CV_LOAD_IMAGE_COLOR);
//jpgbytes.size()=1416562 img.size()=[0 x 0]
What am I missing ?

Answer 1

still, use imdecode . it can handle png,jpg,bmp,ppm,webp,jp2,exr, but no gif.
vector<uchar> jpgbytes; // from your db
Mat img = imdecode(jpgbytes);
(you should do the same for bmp or any other supported formats, don't mess with Mat's raw data pointers!)

Answer 2

Ok I have the process to convert a bytea data to a cv::Mat, here is the code.
inline int dec(uchar x){ //convert uchar to int
if (x>='0'&&x<='9') return (x-'0');
else if (x>='a'&&x<='f') return (x-'a'+10);
else if (x>='A'&&x<='F') return (x-'A'+10);
return 0;
}
cv::Mat bytea2Mat(const pqxx::result::field& f){
const char* buffer=f.c_str();
vector<uchar>::size_type size = strlen((const char*)buffer);
vector<uchar> jpgbytes(size/2-1);
for (size_t i=0; i!=size/2-1;i++) {
jpgbytes[i]=(dec(buffer[2*(i+1)])<<4)+dec(buffer[2*(i+1)+1]);
}
cout <<size/2<<";"<<jpgbytes.size()<<endl;
return imdecode(jpgbytes, CV_LOAD_IMAGE_COLOR);
}
The bytea output is encrypted as a char* looking like "\x41204230", for an original input string of "a b0" in hexa form. (the \x may not be present according to the data input)
to get the original data you have to calculate the original input from the two char, ('4','1'= 0x41=65). The vector is half the size of the char*.