Function icvFetchContour

deps/opencv/modules/imgproc/src/contours.cpp:500–609

marks domain border with +/- and stores the contour into CvSeq. method: <0 - chain ==0 - direct >0 - simple approximation */

            >0  - simple approximation
*/
static void
icvFetchContour( schar                  *ptr,
                 int                    step,
                 CvPoint                pt,
                 CvSeq*                 contour,
                 int    _method )
{
    const schar     nbd = 2;
    int             deltas[16];
    CvSeqWriter     writer;
    schar           *i0 = ptr, *i1, *i3, *i4 = 0;
    int             prev_s = -1, s, s_end;
    int             method = _method - 1;

    assert( (unsigned) _method <= CV_CHAIN_APPROX_SIMPLE );

    /* initialize local state */
    CV_INIT_3X3_DELTAS( deltas, step, 1 );
    memcpy( deltas + 8, deltas, 8 * sizeof( deltas[0] ));

    /* initialize writer */
    cvStartAppendToSeq( contour, &writer );

    if( method < 0 )
        ((CvChain *) contour)->origin = pt;

    s_end = s = CV_IS_SEQ_HOLE( contour ) ? 0 : 4;

    do
    {
        s = (s - 1) & 7;
        i1 = i0 + deltas[s];
        if( *i1 != 0 )
            break;
    }
    while( s != s_end );

    if( s == s_end )            /* single pixel domain */
    {
        *i0 = (schar) (nbd | -128);
        if( method >= 0 )
        {
            CV_WRITE_SEQ_ELEM( pt, writer );
        }
    }
    else
    {
        i3 = i0;
        prev_s = s ^ 4;

        /* follow border */
        for( ;; )
        {
            s_end = s;

            for( ;; )
            {
                i4 = i3 + deltas[++s];