LLC_pooling.m

% ========================================================================
% Pooling the llc codes to form the image feature
% USAGE: [beta] = LLC_pooling(feaSet, B, pyramid, knn)
% Inputs
%       feaSet      -the coordinated local descriptors
%       B           -the codebook for llc coding
%       pyramid     -the spatial pyramid structure
%       knn         -the number of neighbors for llc coding
% Outputs
%       beta        -the output image feature
%
% Written by Jianchao Yang @ IFP UIUC
% May, 2010
% ========================================================================

function [beta] = LLC_pooling(feaSet, B, pyramid, knn)

dSize = size(B, 2);
nSmp = size(feaSet.feaArr, 2);

img_width = feaSet.width;
img_height = feaSet.height;
idxBin = zeros(nSmp, 1);

% llc coding
llc_codes = LLC_coding_appr(B', feaSet.feaArr', knn);
llc_codes = llc_codes';

% spatial levels
pLevels = length(pyramid);
% spatial bins on each level
pBins = pyramid.^2;
% total spatial bins
tBins = sum(pBins);

beta = zeros(dSize, tBins);
bId = 0;

for iter1 = 1:pLevels,
    
    nBins = pBins(iter1);
    
    wUnit = img_width / pyramid(iter1);
    hUnit = img_height / pyramid(iter1);
    
    % find to which spatial bin each local descriptor belongs
    xBin = ceil(feaSet.x / wUnit);
    yBin = ceil(feaSet.y / hUnit);
    idxBin = (yBin - 1)*pyramid(iter1) + xBin;
    
    for iter2 = 1:nBins,     
        bId = bId + 1;
        sidxBin = find(idxBin == iter2);
        if isempty(sidxBin),
            continue;
        end      
        beta(:, bId) = max(llc_codes(:, sidxBin), [], 2);
    end
end

if bId ~= tBins,
    error('Index number error!');
end

beta = beta(:);
beta = beta./sqrt(sum(beta.^2));