Opencv Cv::Mat to Std::Ifstream for Base64 Encoding

OpenCV cv::Mat to std::ifstream for base64 encoding

In order to be able to send an image via HTTP, you also need to encode its width, height and type. You need to serialize the Mat into a stream and encode that stream with libb64. On the other side you need to decode that stream and deserialize the image to retrieve it.

I implemented a small test program that does this serialization and deserialization using std::stringstream as a buffer. I chose it because it extends both std::istream and std::ostream which libb64 uses.

The serialize function serializes a cv::Mat into a std::stringstream. In it, I write the image width, height, type, size of the buffer and the buffer itself.

The deserialize function does the reverse. It reads the width, height, type, size of the buffer and the buffer. It's not as efficient as it could be because it needs to allocate a temporary buffer to read the data from the stringstream. Also, it needs to clone the image so that it does not rely on the temporary buffer and it will handle its own memory allocation. I'm sure that with some tinkering it can be made more efficient.

The main function loads an image, serializes it, encodes it using libb64, then decodes it, deserializes it and displays it in a window. This should simulate what you are trying to do .

// Serialize a cv::Mat to a stringstream
stringstream serialize(Mat input)
{
    // We will need to also serialize the width, height, type and size of the matrix
    int width = input.cols;
    int height = input.rows;
    int type = input.type();
    size_t size = input.total() * input.elemSize();

    // Initialize a stringstream and write the data
    stringstream ss;
    ss.write((char*)(&width), sizeof(int));
    ss.write((char*)(&height), sizeof(int));
    ss.write((char*)(&type), sizeof(int));
    ss.write((char*)(&size), sizeof(size_t));

    // Write the whole image data
    ss.write((char*)input.data, size);

    return ss;
}

// Deserialize a Mat from a stringstream
Mat deserialize(stringstream& input)
{
    // The data we need to deserialize
    int width = 0;
    int height = 0;
    int type = 0;
    size_t size = 0;

    // Read the width, height, type and size of the buffer
    input.read((char*)(&width), sizeof(int));
    input.read((char*)(&height), sizeof(int));
    input.read((char*)(&type), sizeof(int));
    input.read((char*)(&size), sizeof(size_t));

    // Allocate a buffer for the pixels
    char* data = new char[size];
    // Read the pixels from the stringstream
    input.read(data, size);

    // Construct the image (clone it so that it won't need our buffer anymore)
    Mat m = Mat(height, width, type, data).clone();

    // Delete our buffer
    delete[]data;

    // Return the matrix
    return m;
}

void main()
{
    // Read a test image
    Mat input = imread("D:\\test\\test.jpg");

    // Serialize the input image to a stringstream
    stringstream serializedStream = serialize(input);

    // Base64 encode the stringstream
    base64::encoder E;
    stringstream encoded;
    E.encode(serializedStream, encoded);

    // Base64 decode the stringstream
    base64::decoder D;
    stringstream decoded;
    D.decode(encoded, decoded);

    // Deserialize the image from the decoded stringstream
    Mat deserialized = deserialize(decoded);

    // Show the retrieved image
    imshow("Retrieved image", deserialized);
    waitKey(0);
}

Convert the Base64(String or Byte array) to mat(image) in c++(opencv)

You need to use imdecode on your decoded string.

Have a look at this code (base64 decoding is from here):

#include <opencv2\opencv.hpp>
#include <vector>
#include <string>
using namespace std;
using namespace cv;

// Code from: http://www.adp-gmbh.ch/cpp/common/base64.html

static const std::string base64_chars =
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz"
"0123456789+/";

static inline bool is_base64(unsigned char c) {
    return (isalnum(c) || (c == '+') || (c == '/'));
}

std::string base64_decode(std::string const& encoded_string) {
    int in_len = encoded_string.size();
    int i = 0;
    int j = 0;
    int in_ = 0;
    unsigned char char_array_4[4], char_array_3[3];
    std::string ret;

    while (in_len-- && (encoded_string[in_] != '=') && is_base64(encoded_string[in_])) {
        char_array_4[i++] = encoded_string[in_]; in_++;
        if (i == 4) {
            for (i = 0; i < 4; i++)
                char_array_4[i] = base64_chars.find(char_array_4[i]);

            char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
            char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
            char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

            for (i = 0; (i < 3); i++)
                ret += char_array_3[i];
            i = 0;
        }
    }

    if (i) {
        for (j = i; j < 4; j++)
            char_array_4[j] = 0;

        for (j = 0; j < 4; j++)
            char_array_4[j] = base64_chars.find(char_array_4[j]);

        char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
        char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
        char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

        for (j = 0; (j < i - 1); j++) ret += char_array_3[j];
    }

    return ret;
}

int main()
{
    string encoded_string = "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";

    string decoded_string = base64_decode(encoded_string);
    vector<uchar> data(decoded_string.begin(), decoded_string.end());

    Mat img = imdecode(data, IMREAD_UNCHANGED);

    imshow("Image", img);
    waitKey();

    return 0;
}

How to restore base64 encoded opencv java matrix to C++ cv::Mat?

Assuming that you read the data from the JSON, you can create the Mat like this:

int rows = 26;
int cols = 32;
int type = 0;
string data = "aSbt5TQabz...";
string decoded_data = base64_decode(data);  
Mat m(rows, cols, type, (void*)decoded_data.data());

Code for base64_decode is from here

NOTE

I needed to manually convert the characted \u003D to = to make it work. You can avoid to escape HTML characters using Gson gson = new GsonBuilder().disableHtmlEscaping().create();. See here.

Code:

#include <opencv2\opencv.hpp>
#include <vector>
#include <string>
using namespace std;
using namespace cv;

// Code from: http://www.adp-gmbh.ch/cpp/common/base64.html

static const std::string base64_chars =
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz"
"0123456789+/";

static inline bool is_base64(unsigned char c) {
    return (isalnum(c) || (c == '+') || (c == '/'));
}

std::string base64_decode(std::string const& encoded_string) {
    int in_len = encoded_string.size();
    int i = 0;
    int j = 0;
    int in_ = 0;
    unsigned char char_array_4[4], char_array_3[3];
    std::string ret;

    while (in_len-- && (encoded_string[in_] != '=') && is_base64(encoded_string[in_])) {
        char_array_4[i++] = encoded_string[in_]; in_++;
        if (i == 4) {
            for (i = 0; i < 4; i++)
                char_array_4[i] = base64_chars.find(char_array_4[i]);

            char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
            char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
            char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

            for (i = 0; (i < 3); i++)
                ret += char_array_3[i];
            i = 0;
        }
    }

    if (i) {
        for (j = i; j < 4; j++)
            char_array_4[j] = 0;

        for (j = 0; j < 4; j++)
            char_array_4[j] = base64_chars.find(char_array_4[j]);

        char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
        char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
        char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];

        for (j = 0; (j < i - 1); j++) ret += char_array_3[j];
    }

    return ret;
}

int main()
{
    // Read your JSON here...

    int rows = 26;
    int cols = 32;
    int type = 0;
    //string data(L"aSbt5TQabzJ9qv7s3ymQchrEfSyp8OWO5v8nkG6oUiFAMLFkAExBEGCgJggSF0AyAFRoCAlgsADJ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\nwij1oBl6rKxJm/IRoVi300g4gYpsHDtr2jPZqzR42MDCIA\u003d\u003d\n");
    string data = "aSbt5TQabzJ9qv7s3ymQchrEfSyp8OWO5v8nkG6oUiFAMLFkAExBEGCgJggSF0AyAFRoCAlgsADJ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\nwij1oBl6rKxJm/IRoVi300g4gYpsHDtr2jPZqzR42MDCIA==\n";

    string decoded_data = base64_decode(data);

    Mat m(rows, cols, type, (void*)decoded_data.data());

    cout << m << endl;

    return 0;
}

stringstream to base64 encode and send it to the server ios

You can encode the string obtained by the stringstream like:

Mat m;
...
std::string s = serialize(m).str();
std::string encoded = base64_encode(s.c_str());
// send "encoded" to server

The code for base64_encode can be found, for example, here.

Encoding multiple layers of information with OpenCV

I think you can use bits to encode this information. For example, suppose you are using 8U image (1 byte per pixel). Then you can use something like

0x01 : leaf
0x02 : tree
0x04 : forest

to represent these features. Here you are using a bit mask for each feature, so you can OR these values if a pixel belongs to more than one class.

So,

0x03 would be leaf, tree

0x06 would be tree, forest

For confidence levels you will have to use separate channels. If you are okay with losing some precision, you can use 8U channels for these instead of 32F channels. So, rather than storing 0.6578 in a float, you can store it as 66 in a byte.

If the matrix created this way contains lot of zeros, you can further reduce the memory usage by storing it as a sparse matrix.

Using Mat::at(i,j) in opencv for a 2-D Mat object

Img_mat is a 3 channeled image. Each channel consists of pixel values uchar in data type.
So with split(Img_mat, BGR) the Img_mat is split into 3 planes of blue, green and red which are collectively stored in a vector BGR. So BGR[0] is the first (blue) plane with uchar data type pixels...hence it will be

int dataB = (int)BGR[0].at<uchar>(i,j);
int dataG = (int)BGR[1].at<uchar>(i,j);

so on...

Accessing elements in a multi-channel OpenCV Mat

The first element of a cv::Mat is always at (0, 0), so just correct cv::Vec3f elem = rgb_m.at<cv::Vec3f>(1, 1); by cv::Vec3f elem = rgb_m.at<cv::Vec3f>(0, 0); and cv::Vec3f elem2 = lab_m.at<cv::Vec3f>(1, 1); by cv::Vec3f elem2 = lab_m.at<cv::Vec3f>(0, 0);

Different base64 encoding for same Image

On the line:

vector<uchar> data(decoded_string.begin(), decoded_string.end());

you are presumably holding the JPEG-encoded representation of one image in data. So you may as well just write this to a binary file, rather than use cv::imwrite() which is for writing a Mat to a file.

If, for some inexplicable reason, you want to use cv::imwrite(), you need to pass it a Mat. So you would end up decoding the JPEG representation to a Mat and then encoding to JPEG and writing - which seems silly:

cv::Mat img = cv::imdecode(data, cv::IMREAD_COLOR);
cv::imwrite('result.jpg',img);

TLDR;

What I'm saying is that your data is already JPEG-encoded, you read it from a JPEG file.

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