How to convert Opencv Mat to JPEG char data

Question

Recently, i am having trouble with converting a Mat frame captured from my webcam by OpenCV to a normal JPEG unsigned char array. I've tried one or two way on Google but the result seems not the correct jpeg uchar array. Here is a piece of my code:

 VideoCapture cap(0);
 if(!cap.isOpened())
    return -1;
 Mat frame;
 cap >> frame;
 if( frame.empty()) 
    return -1;
 int size = frame.total() * frame.elemSize();
 unsigned char* buffer = new unsigned char[size];
 memcpy(buffer, frame.data, size * sizeof(unsigned char));

Then i used fwrite to write that buffer into a file.jpg (it looks silly but it does work if the buffer is correct),but the file cannot be openned or be determined as a jpeg image.

Can anyone help me figure this out?

Answer 1

Check out the OpenCV function imencode() . It will fill a buffer with data encoded as the correct image type (based on the file type argument) so that it can be written to a file and other programs will know what to do with it.

The problem with your current approach is that you are attempting to write raw image data as a JPEG, but JPEG is a compressed data format so programs won't know what to do with the data you've written. It would be equivalent of taking a binary file and just saving it as a JPEG, the file won't have the right headers to be decoded as an image and the data otherwise likely won't match up with the JPEG format anyways.