When I run this code an error occurs that says array indices must be positive or logical values.

clc;
close all;
clear;

PD=phantom('Modified Shepp-Logan');
padimage = [2,2];
PD= padarray(PD,padimage);
subplot(2,3,1)
imagesc(PD);
colormap('gray');
title('Modified Shepp-Logan')


freq = 2;
thetas = 0:freq:180;
gtheta  = length(thetas);
gl = size(PD,1);

sinogram = zeros(gl,gtheta);
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');

thetas=0;
Fl = size(sinogram,1);
Ftheta  = length(thetas); 
thetas = (pi/180)*thetas;

g0 = zeros(Fl,Fl);
Fmid = ceil(Fl/2);
[x,y] = meshgrid(ceil((-Fl)/2):ceil(Fl/2-1));

for i = 1:Ftheta
    rotCoords = Fmid+round(x*sin(thetas(i)) + y*cos(thetas(i)));
    rotCoords=floor(abs(rotCoords));
   g0 = g0 + sinogram(rotCoords,i)./Ftheta;
end

subplot(2,3,3);
imagesc(g0);
title('Simple backprojection')


% Here is the error

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

Error in P4P2 (line 41)
   g0 = g0 + sinogram(rotCoords,i)./Ftheta;

Answer 1

[David Hill]

Not sure what you are trying to do, but rotCoords is 260x260 and sinogram is a 260x91. You cannot index inside sinogram with rotCoords, you will get that error message.

[Levi Stevens]

If the frequency changes to freq = .6941538461153; then the matrix size becomes 260x260. However, the same error still appears.

[David Hill]

g0 = g0 + sinogram(rotCoords+1)./Ftheta;%rotCoords has values of 0 to 259. You cannot linear index with zero. Adding 1 will allow linear indexing into sinogram.

[Levi Stevens]

I'm trying to get the sample back projection to be similar to Modified Shepp-Logan as I increase the number of angles used. If I comment out the thetas=0 part of the code, I get the resulting image shown here. This is the closest I can seem to get to the modified Shepp-Logan.

Answer 2

[Image Analyst]

This is one of our most asked FAQ's. So see the FAQ on this error for a thorough discussion:

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22