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

I am getting the following error message: Index in position 1 is invalid. Array indices must be positive integers or logical values.

My code is:

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?

[Walter Roberson]

How do you know that rotCoords cannot be 0 ?

[hema]

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

A([i,j],:)=A([j,i],:);

my code is:

for i=0:20

for j=0:20

if i==j

A=zeros(i,j);

else

A([i,j],:)=A([j,i],:);

end

please help me

[Walter Roberson]

0 is not a valid index

Note that A=zeros(i,j); overwrites all of the array A

Answer 1

[Walter Roberson]

You have

Fmid = ceil(Fl/2);

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

rotCoords = Fmid+round(x*sin(thetas(i)) + y*cos(thetas(i)));

when sin(thetas) is 1 then cos(thetas) is 0. so rotCoords would be Fmid + round(x*1 + y*0) which could be as low as ceil(Fl/2) + ceil(-Fl/2) which would be 0. Therefore rotCoords can come out 0, which is not a valid index.

In practice your rotCoords are in the range 0 to 259, where 259 is Fl-1

Answer 2

[Kazi Shakib]

V=imresize(im,[256,256]);
[m1,n1] = size(V);
blocksize=2;
nob=m1*n1/(blocksize*blocksize);
kk1=0;
for i1=1:(m1/blocksize)
  for j1=1:(n1/blocksize)
  J=V((blocksize*(1i-1)+1:blocksize*(1i-1)+blocksize),(blocksize*(1j-1)+1:blocksize*(1j-1)+blocksize));

    kk1=kk1+(m1/blocksize);
    end
  end

I have the same probz here..what to do??

[Walter Roberson]

1i is an imaginary number, so you are trying to index at an imaginary number.

Answer 3

[William Harris]

Hi, I'm getting the same error when I use roi_5, the contents of which are shown below, to index into an image. I'm assuming that the issue is a result of the '10e-09*' at the start. Does anyone know how to deal with this? Or is something else likely to be causing the error?

roi_5 is defined as;

roi_5 = R1rho(22:30, 28:36);

The contents of roi_5:

roi_5 =

   1.0e-09 *

    0.0313    0.0342    0.0347    0.0307    0.0337    0.0318    0.0355    0.0381    0.0394
    0.0329    0.0345    0.0411    0.0412    0.0458    0.0432    0.0456    0.0458    0.0405
    0.0383    0.0473    0.0453    0.0393    0.0486    0.0445    0.0455    0.0570    0.0496
    0.0415    0.0499    0.0559    0.0541    0.0656    0.0597    0.0627    0.0692    0.0549
    0.0457    0.0611    0.0572    0.0508    0.0676    0.0587    0.0636    0.0794    0.0665
    0.0491    0.0581    0.0687    0.0650    0.0815    0.0725    0.0811    0.0872    0.0685
    0.0638    0.0678    0.0741    0.0655    0.0787    0.0749    0.0777    0.0886    0.0858