flann中关于indexParam的使用说明

今天研究了一下opencv中的FLANN库，踩坑一下午，遇到问题：OpenCV Error: Unsupported format or combination of formats (type=5)

在stackoverflow下也没人解决，所以特意记录下。

在库中使用Index类，完成knn的查找。其构造函数为：

 Index(InputArray features, const IndexParams& params, cvflann::flann_distance_t distType=cvflann::FLANN_DIST_L2);

其参数有一些特殊要求：

1.对于params的取值为 AutotunedIndexParams、LinearIndexParams、KDTreeIndexParams时需要使用float型的features

2.当param为LshIndexParams时，features是uchar的Mat

原因是Index的构造函数其实调用了其build方法：

void Index::build(InputArray _data, const IndexParams& params, flann_distance_t _distType)
{
    release();
    algo = getParam<flann_algorithm_t>(params, "algorithm", FLANN_INDEX_LINEAR);
    if( algo == FLANN_INDEX_SAVED )
    {
        load(_data, getParam<String>(params, "filename", String()));
        return;
    }


    Mat data = _data.getMat();
    index = 0;
    featureType = data.type();
    distType = _distType;


    if ( algo == FLANN_INDEX_LSH)
    {
        distType = FLANN_DIST_HAMMING;
    }


    switch( distType )
    {
    case FLANN_DIST_HAMMING:
        buildIndex< HammingDistance >(index, data, params);
        break;
    case FLANN_DIST_L2:
        buildIndex< ::cvflann::L2<float> >(index, data, params);
        break;
    case FLANN_DIST_L1:
        buildIndex< ::cvflann::L1<float> >(index, data, params);
        break;
#if MINIFLANN_SUPPORT_EXOTIC_DISTANCE_TYPES
    case FLANN_DIST_MAX:
        buildIndex< ::cvflann::MaxDistance<float> >(index, data, params);
        break;
    case FLANN_DIST_HIST_INTERSECT:
        buildIndex< ::cvflann::HistIntersectionDistance<float> >(index, data, params);
        break;
    case FLANN_DIST_HELLINGER:
        buildIndex< ::cvflann::HellingerDistance<float> >(index, data, params);
        break;
    case FLANN_DIST_CHI_SQUARE:
        buildIndex< ::cvflann::ChiSquareDistance<float> >(index, data, params);
        break;
    case FLANN_DIST_KL:
        buildIndex< ::cvflann::KL_Divergence<float> >(index, data, params);
        break;
#endif
    default:
        CV_Error(Error::StsBadArg, "Unknown/unsupported distance type");
    }
}

从中可以看出，除FLANN_DIST_HAMMING外，其余数据类型都为float型

对于FLANN_DIST_HAMMING其中：

  if ( algo == FLANN_INDEX_LSH)
    {
        distType = FLANN_DIST_HAMMING;

    }

 switch( distType )
    {
    case FLANN_DIST_HAMMING:
        buildIndex< HammingDistance >(index, data, params);
        break;

HammingDistance的定义为：typedef ::cvflann::Hamming<uchar> HammingDistance;

因此，使用LshIndexParams时，features的类型要为uchar。