How to use cv::imdecode, if the contents of an image file are in a char array?

Question

I have a jpeg image in buffer jpegBuffer. I'm trying to pass it to cv::imdecode function:

Mat matrixJprg = imdecode(Mat(jpegBuffer), 1);

I get this error:

/home/richard/Desktop/richard/client/src/main.cc:108: error: no matching function for call to ‘cv::Mat::Mat(char*&)’

This is how I fill jpegBuffer:

FILE* pFile;
long lSize;
char * jpegBuffer;
pFile = fopen ("img.jpg", "rb");
if (pFile == NULL)
{
    exit (1);
}

// obtain file size.
fseek (pFile , 0 , SEEK_END);
lSize = ftell (pFile);
rewind (pFile);

// allocate memory to contain the whole file.
jpegBuffer = (char*) malloc (lSize);
if (jpegBuffer == NULL)
{
    exit (2);
}

// copy the file into the buffer.
fread (jpegBuffer, 1, lSize, pFile);

// terminate
fclose (pFile);

Answer 1

I had to do the-same thing and my image data was already in char array format and was arriving from a network and a plugin source. The current answer shows how to do this but it requires copying the data into a vector first which is a waste of time and resources.

This is possible to do directly without creating a copy of it. You were so close with your imdecode(Mat(jpegBuffer), 1); code in your question.

You need to use the constructor overload for the Mat class below:

Mat(int rows, int cols, int type, void* data, size_t step=AUTO_STEP);

To create this Mat, pass 1 to the rows, the size of the array to the cols, CV_8UC1 to the type and the char array itself to the data param. Pass this Mat to the cv::imdecode function with the mat as the first param and CV_LOAD_IMAGE_UNCHANGED as the second param.

Basic example:

char *buffer = dataFromNetwork;
int bufferLength = sizeOfDataFromNetwork;

cv::Mat matImg;
matImg = cv::imdecode(cv::Mat(1, bufferLength, CV_8UC1, buffer), CV_LOAD_IMAGE_UNCHANGED);

---

Complete Example (Reads file named "test.jpg" into char array and uses imdecode to decode the data from the char array then displays it):

#include <iostream>
#include <fstream>
#include <opencv2/highgui.hpp>
#include <string>

using namespace std;

int main()
{
  // Open image file to read from
  char imgPath[] = "./test.jpg";
  ifstream fileImg(imgPath, ios::binary);
  fileImg.seekg(0, std::ios::end);
  int bufferLength = fileImg.tellg();
  fileImg.seekg(0, std::ios::beg);

  if (fileImg.fail())
  {
    cout << "Failed to read image" << endl;
    cin.get();
    return -1;
  }

  // Read image data into char array
  char* buffer = new char[bufferLength];
  fileImg.read(buffer, bufferLength);

  // Decode data into Mat
  cv::Mat matImg;
  matImg = cv::imdecode(cv::Mat(1, bufferLength, CV_8UC1, buffer), cv::IMREAD_UNCHANGED);

  // Create Window and display it
  namedWindow("Image from Char Array", cv::WINDOW_AUTOSIZE);
  if (!(matImg.empty()))
  {
    imshow("Image from Char Array", matImg);
  }
  cv::waitKey(0);

  delete[] buffer;

  return 0;
}

Answer 2

Mat has no constructor that takes a char* argument. Try this instead:

std::ifstream file("img.jpg");
std::vector<char> data;

file >> std::noskipws;
std::copy(std::istream_iterator<char>(file), std::istream_iterator<char>(), std::back_inserter(data));

Mat matrixJprg = imdecode(Mat(data), 1);

EDIT:

You should also take a look at LoadImageM.

If you have your data already in a char* buffer one way is to copy the data into an std::vector.

std::vector<char> data(buf, buf + size);