Unrecognized function or variable 'f' error - matlab

Question

I am new to Matlab and I am trying to implement Hough transform without using Matlab builtin function; hence I am learning possible ways to do so. I found a function written in a book that seems to be correct. I got an error and cannot figure it out.
I want to implement hough transform function and later apply it on to a grayscale image
% This functions makes use of sparse matrices
function [h, theta, rho] = HT(f, dtheta, drho)
% [H, THETA, RHO] = HOUGH(F, DTHETA, DRHO) computes the hough
% transform of the image F. DTHETA specifies the spacing (in degrees) of
% the the hough transform bins along the theta axis.
% DRHO specifies the spacing of the hough transform bins along the rho
% axis. H is the Hough transform matrix. It is NRHO-by-NTHETA, where
% NRHO = 2*ceil(norm(size(F))/DRHO) - 1, and NTHETA = 2*ceil(90/DTHETA).
if nargin < 3
drho = 1;
end
if nargin < 2
dtheta = 1;
end
f = double(f);
[M,N] = size(f);
theta = linspace(-90, 0, ceil(90/dtheta) + 1);
theta = [theta -fliplr(theta(2:end - 1))];
ntheta = length(theta);
D = sqrt((M - 1)^2 + (N - 1)^2);
q = ceil(D/drho);
nrho = 2*q - 1;
rho = linspace(-q*drho, q*drho, nrho);
[x, y, val] = find(f);
x = x - 1; y = y-1;
%Initialize output.
h = zeros(nrho, length(theta));
% To avoid excessive memory usage, process 1000 nonzero pixel
% values at a time.
for k = 1:ceil(length(val)/1000)
first = (k-1)*1000 + 1;
last = min(first+999, length(x));
x_matrix = repmat(x(first:last), 1, ntheta);
y_matrix = repmat(y(first:last), 1, ntheta);
val_matrix = repmat(val(first:last), 1, ntheta);
theta_matrix = repmat(theta, size(x_matrix, 1), 1)*pi/180;
rho_matrix = x_matrix.*cos(theta_matrix) + ...
y_matrix.*sin(theta_matrix);
slope = (nrho - 1)/(rho(end) - rho(1));
rho_bin_index = round(slope*(rho_matrix - rho(1)) + 1);
theta_bin_index = repmat(1:ntheta, size(x_matrix, 1), 1);
h = h + full(sparse(rho_bin_index(:), theta_bin_index(:), ...
val_matrix(:), nrho, ntheta));
end
%Illustration of Hough transform on a simple binary image
f = zeros(101, 101);
f(1, 1) = 1; f(101, 1) = 1; f(1, 101) = 1;
f(101, 101) = 1; f(51, 51) = 1;
% Compute & display the hough
H = HT(f);
imshow(H, [])
%Label the axis
[H, theta, rho] = HT(f);
imshow(theta, rho, H, [ ], 'notruesize')
axis on, axis normal
xlabel('\theta', ylabel('\rho'))
I expect the hough transform to be displayed using imshow but I get this error
HT(f, dtheta, drho)
Unrecognized function or variable 'f'.

Answer 1

What you have here is a combination of script and functions in a single .m file. This is allowed starting from Matlab R2016b, but the rule is to have the function definitions after the script is finished.
So your code should look like this:
%Illustration of Hough transform on a simple binary image
f = zeros(101, 101);
f(1, 1) = 1; f(101, 1) = 1; f(1, 101) = 1;
f(101, 101) = 1; f(51, 51) = 1;
% Compute & display the hough
H = HT(f);
imshow(H, [])
%Label the axis
[H, theta, rho] = HT(f);
imshow(theta, rho, H, [ ], 'notruesize')
axis on, axis normal
xlabel('\theta', ylabel('\rho'))
function [h, theta, rho] = HT(f, dtheta, drho)
if nargin < 3
drho = 1;
end
if nargin < 2
dtheta = 1;
end
%.... the rest of the function
end