Problem with matrix indexing in for loop

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Parag
Parag 2014-6-29
评论： Image Analyst 2014-6-30
HI guys
In this code, I am getting problem when I am trying to find index of object . In for loop if I comment out img=img+obj(x_f,y_f) then Index x_f and y_f shows from 1 to 512 but when I remove comment then indexes changed. I don't know how it is happening in for loop. Can anyone please help to sort out problem. I will appreciate your help
obj=zeros(512);
obj(230:250,230:250)=5;
%theta = pi/2*180;
figure;
subplot(2,2,1);
imshow(obj);
drawnow;
N=512;
x=[-N/2:N/2-1];
y=[-N/2:N/2-1];
theta=pi/180;
img=zeros(512);
for i=1:180
x_f=round( x .* cos(theta*i) + y.* sin(theta * i)+256);
y_f=round( y .* cos(theta*i)- x .* sin(theta * i)+256);
img=img+obj(x_f,y_f);
% subplot(2,2,2);
% imshow(img);
% drawnow;
end

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回答（3 个）

per isakson
per isakson 2014-6-29
编辑：per isakson 2014-6-29
Here are some links on debugging in Matlab
BTW:
>> x_f(1:5)
ans =
-4 -3 -2 -1 0
Try
>> dbstop if error
Image Analyst
Image Analyst 2014-6-29
x_f and y_f will not change depending on whether the img line is in there or commented out. However the values do go negative and you can't have negative indexes into img. What are you trying to do anyway? Draw sine waves into an image???
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Parag
Parag 2014-6-29
I did it using same logic in labview but here I am getting problems.Thanks for suggestion
Image Analyst
Image Analyst 2014-6-30
I would be surprised if that produced a rotated image in labview.

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Parag
Parag 2014-6-29
Thank you for your reply.
I could not able to figure out the behavior of matlab in for loop. When I do that similar indexing without using for loop it work for me.

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