OpenCV to use in memory buffers or file pointers


The two functions in openCV cvLoadImage and cvSaveImage accept file path's as arguments.

For example, when saving a image it's cvSaveImage("/tmp/output.jpg", dstIpl) and it writes on the disk.

Is there any way to feed this a buffer already in memory? So instead of a disk write, the output image will be in memory.

I would also like to know this for both cvSaveImage and cvLoadImage (read and write to memory buffers). Thanks!

My goal is to store the Encoded (jpeg) version of the file in Memory. Same goes to cvLoadImage, I want to load a jpeg that's in memory in to the IplImage format.

5/7/2009 1:40:43 AM

Accepted Answer

There are a couple of undocumented functions in the SVN version of the libary:

CV_IMPL CvMat* cvEncodeImage( const char* ext, 
                              const CvArr* arr, const int* _params )

CV_IMPL IplImage* cvDecodeImage( const CvMat* _buf, int iscolor )

Latest check in message states that they are for native encoding/decoding for bmp, png, ppm and tiff (encoding only).

Alternatively you could use a standard image encoding library (e.g. libjpeg) and manipulate the data in the IplImage to match the input structure of the encoding library.

5/7/2009 1:16:58 AM

This worked for me

// decode jpg (or other image from a pointer)
// imageBuf contains the jpg image
    cv::Mat imgbuf = cv::Mat(480, 640, CV_8U, imageBuf);
    cv::Mat imgMat = cv::imdecode(imgbuf, CV_LOAD_IMAGE_COLOR);
// imgMat is the decoded image

// encode image into jpg
    cv::vector<uchar> buf;
    cv::imencode(".jpg", imgMat, buf, std::vector<int>() );
// encoded image is now in buf (a vector)
    imageBuf = (unsigned char *) realloc(imageBuf, buf.size());
    memcpy(imageBuf, &buf[0], buf.size());
//  size of imageBuf is buf.size();

I was asked about a C version instead of C++:

#include <opencv/cv.h>
#include <opencv/highgui.h>

main(int argc, char **argv)
    char *cvwin = "camimg";

    cvNamedWindow(cvwin, CV_WINDOW_AUTOSIZE);

    // setup code, initialization, etc ...
    [ ... ]

    while (1) {      
        // getImage was my routine for getting a jpeg from a camera
        char *img = getImage(fp);
        CvMat mat;

   // substitute 640/480 with your image width, height 
        cvInitMatHeader(&mat, 640, 480, CV_8UC3, img, 0);
        IplImage *cvImg = cvDecodeImage(&mat, CV_LOAD_IMAGE_COLOR);
        cvShowImage(cvwin, cvImg);
        if (27 == cvWaitKey(1))         // exit when user hits 'ESC' key


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