find_nearest_neighbours.m

％ [nearest_neighbours] = find_nearest_neighbours( database, desc, max_dist ) ％ Find the indices of

function [nearest_neighbours ratio] = find_nearest_neighbours( database, desc, dist_ratio )
% [nearest_neighbours] = find_nearest_neighbours( database, desc, max_dist )
% Find the indices of the nearest neighbours of the given desriptors in the
% specified database.  Uses euclidean distance.
%
% Input:
% database - descriptor database created by add_descriptors_to_database.
% desc - descriptors from the SIFT function.
% dist_ratio - maximum ratio between distances of nearest and second closest
%   neighbour for a match to be allowed.
%
% Output:
% nearest_neighbours - indices of the nearest neighbours for the descriptors
%   (descriptors with no neighbour closer than max_dist will have index 0).
%
% Thomas F. El-Maraghi
% May 2004

if ~exist( 'dist_ratio' )
   dist_ratio = 0.8;
end

nearest_neighbours = zeros(size(desc,1),1);
ratio = zeros(size(desc,1),1);
for k = 1:size(desc,1)
   dist = sqrt(sum((database.desc - repmat(desc(k,:),size(database.desc,1),1)).^2,2));
   [nn1_dist idx] = min(dist);
   dist(idx) = max(dist);
   nn2_dist = min(dist);
   if nn1_dist/nn2_dist >= dist_ratio
      idx = 0;
   end
   nearest_neighbours(k) = idx;
   ratio(k) = nn1_dist;
end