cvCaptureFromCAM program creates segmentation faults only some of the time - c++

Question

I've been kicking around with OpenCV 2.4.3 and a Logitech C920 camera hoping to get a primitive sort of facial recognition scheme going. Very simple, not very sophisticated.
#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/calib3d/calib3d.hpp"
#include "opencv2/nonfree/features2d.hpp"
#include <iostream>
#include <stdio.h>
using namespace std;
using namespace cv;
/** Function Headers */
void grabcurrentuser();
void capturecurrentuser( Mat vsrc );
/** Global Variables **/
string face_cascade_name = "haarcascade_frontalface_alt.xml";
CascadeClassifier face_cascade;
int main( void ){//[main]
grabcurrentuser();
}//]/main]
void grabcurrentuser(){//[grabcurrentuser]
CvCapture* videofeed;
Mat videoframe;
//Load face cascade
if( !face_cascade.load( face_cascade_name ) ){
printf("Can't load haarcascade_frontalface_alt.xml\n");
}
//Read from source video feed for current user
videofeed = cvCaptureFromCAM( 1 );
if( videofeed ){
for(int i=0; i<10;i++){//Change depending on platform
videoframe = cvQueryFrame( videofeed );
//Debug source videofeed with display
if( !videoframe.empty() ){
imshow( "Source Video Feed", videoframe );
//Perform face detection on videoframe
capturecurrentuser( videoframe );
}else{
printf("Videoframe is empty or error!!!"); break;
}
int c = waitKey(33);//change to increase or decrease delay between frames
if( (char)c == 'q' ) { break; }
}
}
}//[/grabcurrentuser]
void capturecurrentuser( Mat vsrc ){//[capturecurrentuser]
std::vector<Rect> faces;
Mat vsrc_gray;
Mat currentuserface;
//Preprocess frame for face detection
cvtColor( vsrc, vsrc_gray, CV_BGR2GRAY );
equalizeHist( vsrc_gray, vsrc_gray );
//Find face
face_cascade.detectMultiScale( vsrc_gray, faces, 1.1, 2, 0|CV_HAAR_SCALE_IMAGE, Size(30,30) );
//Take face and crop out into a Mat
currentuserface = vsrc_gray( faces[0] );
//Save the mat into a jpg file on disk
imwrite( "currentuser.jpg", currentuserface );
//Show saved image in a window
imshow( "Current User Face", currentuserface );
}//[/capturecurrentuser]
The above code is the first component of this system. It's job is to accept the video feed, take 10 frames or so (hence the for loop) and run a haar cascade on the frames to obtain a face. Once a face is acquired, it cuts that face out into a Mat and saves it as a jpg in the working directory.
It's worked so far, but seems to be a very tempermental piece of code. It's giving me the desired output most of the time (I don't intend to ask here how I can make things more accurate or precise - but feel free to tell me :D) but other times it ends in a segmentation fault. The following is an example of normal output (i've looked around and seen that the VIDIOC invalid argument is something that can be ignored - again, if its an easy fix feel free to tell me) with the segmentation fault.
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
VIDIOC_QUERYMENU: Invalid argument
init done
opengl support available
Segmentation fault (core dumped)
Can anyone tell me why sometimes with concurrent runs of this program I run into a single or series of segmentation fault results like above, and other times not? This program is designed to create an output thats shunted off to another program I wrote, so I can't have it seizing up on me like this.
Much appreciated!

Answer 1

Your problem is in the following line:
currentuserface = vsrc_gray( faces[0] );
From my experience segmentation faults arise when you are accessing stuff that does not exist.
The program works fine if a face is detected because faces[0] contains data. However, when no face is detected (cover the camera), no rectangle is stored in faces[0]. Thus the error occurs.
Try initialising like this, so that imshow and imwrite works when nothing is detected:
cv::Mat currentuserface = cv::Mat::zeros(vsrc.size(), CV_8UC1);
and then check if faces is empty before you initialize currentuserface with it:
if( !faces.empty() )
currentuserface = vsrc_gray( faces[0] );