extract frame from a video for detection with opencv - c++

Question

i'm developing a program to detect object in video or image.
It works with the image, but now i want to use it with video. I use a specific folder to pick the image so i wanted to save frames from video in that folder before the detection.
The variable video and salvataggio are alredy setted.
In the following code i navigate throw the folder to analize the video:
DIR *dir;
dir = opendir(video.c_str());
string vidName;
struct dirent *ent;
if (dir != NULL) {
while ((ent = readdir (dir)) != NULL) {
vidName= ent->d_name;
if(vidName.compare(".")!= 0 && vidName.compare("..")!= 0)
{
//string vidPath(neg + vidName);
estraiframe(video, vidName, salvataggio);
}
}
closedir (dir);
}
else {
cout<<"directory "<< video << " not present"<<endl;
}
}
The function estraiframe save the frame in the output folder.
void estraiframe(string path, string vidName, string output){
string vidPath(path + vidName);
VideoCapture cap(vidPath);
if( !cap.isOpened()){
cout << "Cannot open the video file" << endl;
return;
}
double count = cap.get(CV_CAP_PROP_FRAME_COUNT);
double rate = cap.get(CV_CAP_PROP_FPS);
int counter = 0;
for (int i=1; i< count; i+=rate*5)
{
cap.set(CV_CAP_PROP_POS_FRAMES,i);
Mat frame;
cap.read(frame);
counter++;
string nomeframe = to_string(counter) + "-frame_from"+vidName+".jpg";
string percorso (output+nomeframe);
cout << percorso;
imwrite(percorso,frame);
}
}
Apparently it works but after the last frame it gave me the following error:
Assertion stream_index < ogg->nstreams failed at libavformat/oggdec.c:898
I locked for it but i didin't find where is the error

Answer 1

Your video contains various stream indexes, e.g. 3 audios, and 1 video stream would result in 4 stream indexes. I recommend that you check the amount of streams your video contains and print ogg->nstreams to check if this corresponds. Looking at the source code in oggdec.c at line 898
static int ogg_read_seek(AVFormatContext *s, int stream_index,
int64_t timestamp, int flags)
{
struct ogg *ogg = s->priv_data;
struct ogg_stream *os = ogg->streams + stream_index;
int ret;
av_assert0(stream_index < ogg->nstreams); /* Line 898 */
You're clearly going out of bounds here.

Answer 2

Apparently I manage to solve it, I modified the function that extract the frame.
void estraiframe(string path, string vidName, string output){
string vidPath(path + vidName);
VideoCapture cap(vidPath);
if( !cap.isOpened()){
cout << "Cannot open the video file" << endl;
return;
}
double rate = cap.get(CV_CAP_PROP_FPS);
int counter = 0;
Mat frame;
int i=1;
while(1)
{
cap.read ( frame);
if( frame.empty()) break;
counter++;
if (counter == rate*5*i){
i++;
string nomeframe = to_string(counter) + "-frame_from"+vidName+".jpg";
string percorso (output+nomeframe);
imwrite(percorso, frame);
}
char key = waitKey(10);
if ( key == 27) break;
}
}