Josh Allen asked . 2023-04-01

Index in position 1 is invalid. Array indices must be positive integers or logical values?

clc;
close all;
clear;
% The radon transform of Circle Phantom
PD=phantom('Modified Shepp-Logan');
% pad the image with zeros
padimage = [2,2];
PD= padarray(PD,padimage);
% Original image
subplot(2,3,1)
imagesc(PD);
colormap('gray');
title('Circle Phantom')
xlabel('X')
ylabel('Y')
% Theta is 0:180 with the increment of 0.5 degrees
freq = 2;
thetas = 0:freq:180;
% compute sinogram / radon transformation
gtheta  = length(thetas);
gl = size(PD,1);
sinogram = zeros(gl,gtheta);
% loop for the number of angles
for i = 1:length(thetas)
   tmpImage      = imrotate(PD,-thetas(i),'bilinear','crop');
   sinogram(:,i) = sum(tmpImage);
end
subplot(2,3,2)
imagesc(sinogram);
title('Circle Sinogram');
xlabel('l');
ylabel('\theta');
% The inverse radon transform with only 1 back projection that is theta = 0
thetas=0;
Fl = size(sinogram,1);
Ftheta  = length(thetas); 
% convert thetas to radians
thetas = (pi/180)*thetas;
% set up the backprojected image
g0 = zeros(Fl,Fl);
% find the middle index of the projections
Fmid = ceil(Fl/2);
% set up the coords of the image
[x,y] = meshgrid(ceil(-Fl/2):ceil(Fl/2-1));
% loop over each projection
for i = 1:Ftheta
    % Using the back projection formula
    rotCoords = Fmid+round(x*sin(thetas(i)) + y*cos(thetas(i)));
%     % check which coords are in bounds
%     indices   = find((rotCoords > 0) & (rotCoords <= Fl));
%     newCoords = rotCoords(indices);
    % summation
    rotCoords=floor(abs(rotCoords));
   g0 = g0 + sinogram(rotCoords,i)./Ftheta;
end
subplot(2,3,3);
imagesc(g0);
title('Simple backprojection')
xlabel('X')
ylabel('Y')

The error message is occurring for line 54:

Index in position 1 is invalid. Array indices must be positive integers or logical values.

Error in CT_HW3_BP (line 54)
   g0 = g0 + sinogram(rotCoords,i)./Ftheta;

Can anyone tell me what the issue is here?

matlab , error , MATLAB , Language Fundamentals , Matrices and Arrays