opencv凸包求取源码解析

CV_IMPL CvSeq* cvConvexityDefects( const CvArr* array,
                                   const CvArr* hullarray,
                                   CvMemStorage* storage )
{
    CvSeq* defects = 0;

    int i, index;
    CvPoint* hull_cur;

    /* is orientation of hull different from contour one */
    int rev_orientation;

    CvContour contour_header;
    union { CvContour c; CvSeq s; } hull_header;
    CvSeqBlock block, hullblock;
    CvSeq *ptseq = (CvSeq*)array, *hull = (CvSeq*)hullarray;

    CvSeqReader hull_reader;
    CvSeqReader ptseq_reader;                                                                    //读序列，应该是读轮廓序列
    CvSeqWriter writer;
    int is_index;

    if( CV_IS_SEQ( ptseq ))
    {
        if( !CV_IS_SEQ_POINT_SET( ptseq ))
            CV_Error( CV_StsUnsupportedFormat,
                "Input sequence is not a sequence of points" );
        if( !storage )
            storage = ptseq->storage;
    }
    else
    {
        ptseq = cvPointSeqFromMat( CV_SEQ_KIND_GENERIC, array, &contour_header, &block );
    }

    if( CV_SEQ_ELTYPE( ptseq ) != CV_32SC2 )
        CV_Error( CV_StsUnsupportedFormat, "Floating-point coordinates are not supported here" );

    if( CV_IS_SEQ( hull ))
    {
        int hulltype = CV_SEQ_ELTYPE( hull );
        if( hulltype != CV_SEQ_ELTYPE_PPOINT && hulltype != CV_SEQ_ELTYPE_INDEX )
            CV_Error( CV_StsUnsupportedFormat,
                "Convex hull must represented as a sequence "
                "of indices or sequence of pointers" );
        if( !storage )
            storage = hull->storage;
    }
    else
    {
        CvMat* mat = (CvMat*)hull;

        if( !CV_IS_MAT( hull ))
            CV_Error(CV_StsBadArg, "Convex hull is neither sequence nor matrix");

        if( (mat->cols != 1 && mat->rows != 1) ||
            !CV_IS_MAT_CONT(mat->type) || CV_MAT_TYPE(mat->type) != CV_32SC1 )
            CV_Error( CV_StsBadArg,
            "The matrix should be 1-dimensional and continuous array of int's" );

        if( mat->cols + mat->rows - 1 > ptseq->total )
            CV_Error( CV_StsBadSize, "Convex hull is larger than the point sequence" );

        hull = cvMakeSeqHeaderForArray(
            CV_SEQ_KIND_CURVE|CV_MAT_TYPE(mat->type)|CV_SEQ_FLAG_CLOSED,
            sizeof(CvContour), CV_ELEM_SIZE(mat->type), mat->data.ptr,
            mat->cols + mat->rows - 1, &hull_header.s, &hullblock );
    }

    is_index = CV_SEQ_ELTYPE(hull) == CV_SEQ_ELTYPE_INDEX;

    if( !storage )
        CV_Error( CV_StsNullPtr, "NULL storage pointer" );

    defects = cvCreateSeq( CV_SEQ_KIND_GENERIC, sizeof(CvSeq), sizeof(CvConvexityDefect), storage );

    if( ptseq->total < 4 || hull->total < 3)
    {
        //CV_ERROR( CV_StsBadSize,
        //    "point seq size must be >= 4, convex hull size must be >= 3" );
        return defects;
    }

    /* recognize co-orientation of ptseq and its hull */                                                                                              
    {
        int sign = 0;
        int index1, index2, index3;

        if( !is_index )
        {
            CvPoint* pos = *CV_SEQ_ELEM( hull, CvPoint*, 0 );
            index1 = cvSeqElemIdx( ptseq, pos );

            pos = *CV_SEQ_ELEM( hull, CvPoint*, 1 );
            index2 = cvSeqElemIdx( ptseq, pos );

            pos = *CV_SEQ_ELEM( hull, CvPoint*, 2 );
            index3 = cvSeqElemIdx( ptseq, pos );
        }
        else
        {
            index1 = *CV_SEQ_ELEM( hull, int, 0 );
            index2 = *CV_SEQ_ELEM( hull, int, 1 );
            index3 = *CV_SEQ_ELEM( hull, int, 2 );
        }

        sign += (index2 > index1) ? 1 : 0;
        sign += (index3 > index2) ? 1 : 0;
        sign += (index1 > index3) ? 1 : 0;

        rev_orientation = (sign == 2) ? 0 : 1;
    }

    cvStartReadSeq( ptseq, &ptseq_reader, 0 );
    cvStartReadSeq( hull, &hull_reader, rev_orientation );

    if( !is_index )
    {
        hull_cur = *(CvPoint**)hull_reader.prev_elem;
        index = cvSeqElemIdx( ptseq, (char*)hull_cur, 0 );                                              //返回序列中元素的索引
    }
    else
    {
        index = *(int*)hull_reader.prev_elem;                                                                 //prev_elem为指向上一个元素的指针
        hull_cur = CV_GET_SEQ_ELEM( CvPoint, ptseq, index );                                      //从序列中取出所给元素的地址，返回的是指针
    }
    cvSetSeqReaderPos( &ptseq_reader, index );
    cvStartAppendToSeq( defects, &writer );

    /* cycle through ptseq and hull with computing defects */                                //开始计算凸缺陷             
    for( i = 0; i < hull->total; i++ )
    {
        CvConvexityDefect defect;
        int is_defect = 0;
        double dx0, dy0;
        double depth = 0, scale;
        CvPoint* hull_next;

        if( !is_index )
            hull_next = *(CvPoint**)hull_reader.ptr;
        else
        {
            int t = *(int*)hull_reader.ptr;
            hull_next = CV_GET_SEQ_ELEM( CvPoint, ptseq, t );
        }

        dx0 = (double)hull_next->x - (double)hull_cur->x;
        dy0 = (double)hull_next->y - (double)hull_cur->y;
        assert( dx0 != 0 || dy0 != 0 );
        scale = 1./sqrt(dx0*dx0 + dy0*dy0);

        defect.start = hull_cur;                                                                                                //凸缺陷的开始点
        defect.end = hull_next;                                                                                               //凸缺陷的结束点

        for(;;)                                                                                                                           //这个循环就是用来计算每个凸缺陷的最远距离
        {
            /* go through ptseq to achieve next hull point */
            CV_NEXT_SEQ_ELEM( sizeof(CvPoint), ptseq_reader );

            if( ptseq_reader.ptr == (schar*)hull_next )
                break;
            else
            {
                CvPoint* cur = (CvPoint*)ptseq_reader.ptr;

                /* compute distance from current point to hull edge */
                double dx = (double)cur->x - (double)hull_cur->x;                                              //cur应该是轮廓上的某个点，hull_cur是什么？
                double dy = (double)cur->y - (double)hull_cur->y;

                /* compute depth */
                double dist = fabs(-dy0*dx + dx0*dy) * scale;                                                          //计算距离

                if( dist > depth )                                                                                                     //该凸缺陷下所有距离排序，得出最大距离
                {
                    depth = dist;
                    defect.depth_point = cur;
                    defect.depth = (float)depth;
                    is_defect = 1;
                }
            }
        }
        if( is_defect )
        {
            CV_WRITE_SEQ_ELEM( defect, writer );
        }

        hull_cur = hull_next;
        if( rev_orientation )
        {
            CV_PREV_SEQ_ELEM( hull->elem_size, hull_reader );
        }
        else
        {
            CV_NEXT_SEQ_ELEM( hull->elem_size, hull_reader );
        }
    }

    return cvEndWriteSeq( &writer );                       //终止写序列
}


该程序中的depth求取办法就是现根据凸缺陷的初始点和终止点求得凸包线方程，然后遍历凸缺陷下的轮廓（除去凸包线），逐个按点到直线的距离公式求取距离，最后排序得到最远处的点。