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

Question

Levi Stevens 2022-3-22

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https://ww2.mathworks.cn/matlabcentral/answers/1678044-when-i-run-this-code-an-error-occurs-that-says-array-indices-must-be-positive-or-logical-values

回答： Image Analyst 2022-3-23

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;

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Answer 1

David Hill 2022-3-22

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https://ww2.mathworks.cn/matlabcentral/answers/1678044-when-i-run-this-code-an-error-occurs-that-says-array-indices-must-be-positive-or-logical-values#answer_924424

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.

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David Hill 2022-3-22

在 MATLAB Online 中打开

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 2022-3-23

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.

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