% [image, descriptors, locs] = sift(imageFile)
%
% This function reads an image and returns its SIFT keypoints.
% Input parameters:
% imageFile: the file name for the image.
%
% Returned:
% image: the image array in double format
% descriptors: a K-by-128 matrix, where each row gives an invariant
% descriptor for one of the K keypoints. The descriptor is a vector
% of 128 values normalized to unit length.
% locs: K-by-4 matrix, in which each row has the 4 values for a
% keypoint location (row, column, scale, orientation). The
% orientation is in the range [-PI, PI] radians.
%
% Credits: Thanks for initial version of this program to D. Alvaro and
% J.J. Guerrero, Universidad de Zaragoza (modified by D. Lowe)
function [image, descriptors, locs] = sift(imageFile)
% Load image
image = imread(imageFile);
% If you have the Image Processing Toolbox, you can uncomment the following
% lines to allow input of color images, which will be converted to grayscale.
if isrgb(image)
image = rgb2gray(image);
end
[rows, cols] = size(image);
% Convert into PGM imagefile, readable by "keypoints" executable
f = fopen('tmp.pgm', 'w');
if f == -1
error('Could not create file tmp.pgm.');
end
fprintf(f, 'P5\n%d\n%d\n255\n', cols, rows);
fwrite(f, image', 'uint8');
fclose(f);
% Call keypoints executable
if isunix
command = '!./sift ';
else
command = '!siftWin32 ';
end
command = [command ' <tmp.pgm >tmp.key'];
eval(command);
% Open tmp.key and check its header
g = fopen('tmp.key', 'r');
if g == -1
error('Could not open file tmp.key.');
end
[header, count] = fscanf(g, '%d %d', [1 2]);
if count ~= 2
error('Invalid keypoint file beginning.');
end
num = header(1);
len = header(2);
if len ~= 128
error('Keypoint descriptor length invalid (should be 128).');
end
% Creates the two output matrices (use known size for efficiency)
locs = double(zeros(num, 4));
descriptors = double(zeros(num, 128));
% Parse tmp.key
for i = 1:num
[vector, count] = fscanf(g, '%f %f %f %f', [1 4]); %row col scale ori
if count ~= 4
error('Invalid keypoint file format');
end
locs(i, :) = vector(1, :);
[descrip, count] = fscanf(g, '%d', [1 len]);
if (count ~= 128)
error('Invalid keypoint file value.');
end
% Normalize each input vector to unit length
descrip = descrip / sqrt(sum(descrip.^2));
descriptors(i, :) = descrip(1, :);
end
fclose(g);
