Question

I’m trying to implement a circular buffer for use as a frame buffer with OpenCV (using C).

I’ve shamelessly stolen the circular buffer implementation from this post to save reinventing the wheel:

EDIT: OK, so I’ve redefined a few things. Namely I implemented my own circular buffer. Now I’m getting errors which don’t make sense.

Here is the circular buffer implementation I’m using:

#define BUFFER_SIZE 100

typedef struct
{
    IplImage* queue[BUFFER_SIZE];
    IplImage *in;
    IplImage *out;
    int num_frames;
    int in_ctr;
    int out_ctr;
    int update_flag;
} frame_buffer;

Here is the get function:

IplImage* buff_get()
{
    IplImage* nextfr;
    if(frbuff.num_frames == 0)
    {
        return NULL;
    }
    nextfr = frbuff.out++;
    if(++frbuff.out_ctr == BUFFER_SIZE)
    {
        frbuff.out = &frbuff.queue[0];
        frbuff.out_ctr = 0;
    }
    --frbuff.num_frames;
    return nextfr;
}

Here is the put function:

int buff_put(IplImage* nextfr)
{
    if(++frbuff.num_frames > BUFFER_SIZE)
    {
       return 0;
    }
    frbuff.in++; 
    frbuff.in = nextfr;
    if(++frbuff.in_ctr == BUFFER_SIZE)
    {
        frbuff.in = &frbuff.queue[0];
        frbuff.in_ctr = 0;
    }

    return 1;
}

Everything seems to go OK. Frames appear on the buffer, which I know because I can print the size out. But it all goes bad when I try to show the image that’s on the buffer.

If I then try to do this:

IplImage* curr_frame = cvCreateImage(cvSize(640,480),8,3);
cvNamedWindow("proc_window",CV_WINDOW_AUTOSIZE);
cvShowImage("proc_window",curr_frame);

while(1)
{     
   if(buff_size() > 0) 
   {    
        if(buff_flag_check()) curr_frame = buff_get();
        if(curr_frame != NULL)
        {
            cvShowImage("proc_window",curr_frame);
        }

}

I recieve the following error upon calling cvShowImage():

OpenCV Error: Bad flag (parameter or structure field) (Unrecognized or unsupported array type) in cvGetMat, file /home/fagg/src/OpenCV-2.3.1/modules/core/src/array.cpp, line 2482
terminate called after throwing an instance of 'cv::Exception'
what():  /home/fagg/src/OpenCV-2.3.1/modules/core/src/array.cpp:2482: error: (-206)     Unrecognized or unsupported array type in function cvGetMat

I’m quite confused as to what’s going on here. Hopefully someone with fresher eyes than myself can see what’s going on…

Answer 1

The snippet of code you provided won’t compile as cb_init() takes a pointer to a circular_buffer as first argument.

and if you get a segfault in cb_init() it is because malloc() fails to return the requested size and the implementation in the link does not handle errors.

void cb_init(circular_buffer *cb, size_t capacity, size_t sz)
{
    cb->buffer = malloc(capacity * sz);
    if(cb->buffer == NULL)
        // handle error
    cb->buffer_end = (char *)cb->buffer + capacity * sz;//segfault when using cb->buffer which is null in case of malloc() failure
    cb->capacity = capacity;
    cb->count = 0;
    cb->sz = sz;
    cb->head = cb->buffer;
    cb->tail = cb->buffer;
}