c, opencv - accessing camera JPG image over ip

Question

I have read many, many threads about streaming images over IP in OpenCV 2.3.1, but I still cannot get my program to work.

I downloaded IP Webcam for Android from https://market.android.com/details?id=com.pas.webcam&hl=en, and recently learned OpenCV to retrieve images from my Android phone camera.

Its built-in manual said that the image from the phone camera can be located at http://the.phone.ip.address:8080/shot.jpg. I have opened it from browser several times and it always looks fine. I also built OpenCV manually, with FFmpeg support.

So far I've tried

CvCapture* webcam = cvCaptureFromFile("http://192.168.1.220:8080/shot.jpg");

but it returns NULL and outputs

[image2 @ 0xd701e0]Could not find codec parameters (Video: mjpeg, yuv420p)

I also tried replacing http with rtsp, but it still doesn't work. I also tried to replace the url with some other image url (one direct link to random image from Google Images, and one from localhost) and it always kills with a segfault.

Here's my full source

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <cv.h>
#include <highgui.h>

int main(int argc, char* argv[])
{ 
  CvCapture* webcam = cvCaptureFromFile("http://192.168.1.220:8080/shot.jpg");
  if(!webcam)
    {
      fprintf(stderr, "cannot open webcam\n");
      return 1;
    }

  IplImage* img = cvQueryFrame(webcam);
  if(!img)
    {
      fprintf(stderr, "cannot get image\n");
      return 1;
    }

  cvNamedWindow("test", CV_WINDOW_AUTOSIZE);
  cvShowImage("test", img);
  cvWaitKey(0);
  cvReleaseImage(&img);
  /**/ cvReleaseCapture(&webcam); /**/
  cvDestroyWindow("test");
  return 0;
}

Can OpenCV really read images over IP, or am I missing something?

Answer 1

I'm not specifically familiar with openCV, but having spent a minute looking at the docs, two things jump out at me:-

First off, your're not dealing with a true video stream here: your android app just makes the current JPEG capture available and you have to continually re-sample it. Because it's an Image, not a Video, you should use cvLoadImage() instead.

Second, you're passing a URL, not a filename. You'll need a way to use HTTP to fetch the image to a local file before you try opening it with openCV.

I'd suggest you save a snapshot of the JPEG file locally from your browser, and then try getting your code working with that. Once you have it working from a local file, try adding the HTTP fetching stuff.