将cv :: Mat存储在字节数组中，以便将数据传输到服务器

Question

I need to read an image with OpenCV, get its size and send it to a server so it processes the image and give it back to me the extracted features.

I have been thinking of using a vector<byte> , but I don't understand how to copy the data to a cv::Mat. I wan't it to be fast so I am trying to access the data with a pointer but I have a runtime exception. I have something like this.

Mat image = imread((path + "name.jpg"), 0);
vector<byte> v_char;
for(int i = 0; i < image.rows; i++)
    {   
        for(int j = 0; j < image.cols; j++)
        {
            v_char.push_back(*(uchar*)(image.data+ i + j));             

        }           
    }

• Which is the best approach for this task?

Answer 1

Direct access is a good idea as it is the fastest for OpenCV, but you are missing the step and that is probably the reason why your program breaks. The next line is wrong:

v_char.push_back(*(uchar*)(image.data+ i + j)); 

You don't have to increment i , you have to increment i + image.step . It will be this way:

Mat image = imread((path + "name.jpg"), 0);
vector<byte> v_char;
for(int i = 0; i < image.rows; i++)
    {   
        for(int j = 0; j < image.cols; j++)
        {
            v_char.push_back(*(uchar*)(image.data+ i*image.step + j));             

        }           
    }

Answer 2

Improving on Jav_Rock's answer here's how I would do it.

Mat image = ...;
vector<byte> v_char(image.rows * image.cols);

for (int i = 0; i < image.rows; i++)
  memcpy(&v_char[i * image.cols], image.data + i * image.step, image.cols);

EDIT: Initialization by constructor will allocate enough space to avoid extra reallocation, but it will also set all items in the vector to default value (0). The following code avoids this extra initialization.

Mat image = ...;
vector<byte> v_char;
v_char.reserve(image.rows * image.cols);

for (int i = 0; i < image.rows; i++)
{
  int segment_start = image.data + i * image.step;
  v_char.insert(v_char.end(), segment_start, segment_start + image.cols);
}

Answer 3

You have received great answers so far, but this is not your main problem. What you probably want to do before sending an image to a server is to compress it.

So, take a look at cv::imencode() on how to compress it, and cv::imdecode() to transform it back to an OpenCV matrix in the server. just push the imencode ouptut to a socket and you're done.

Answer 4

I don't understand completely why you need to use a vector, but if it's really necessary I recommend you to do a simple memcpy:

vector<byte> v_char(image.width * image.height); //Allocating the vector with the same size of the matrix
memcpy(v_char.data(), image.data, v_char.size() * sizeof(byte));