无法识别的 function 或变量“f”错误

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