Problem with matrix indexing in for loop

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

Answer 1

[per isakson]

Here are some links on debugging in Matlab

• Debugging in MATLAB
• Debugging session
• Debugging Approaches

BTW:

    >> x_f(1:5)
    ans =
        -4    -3    -2    -1     0

Try

    >> dbstop if error

Answer 2

[Image Analyst]

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???

[Parag]

thanks for your reply. I am trying to rotate this simulated image using rotation matrix.

[Image Analyst]

Use imrotate() - that's by far the easiest. If you want to do it manually, use the rotation matrix [cos, -sin, sin, cos] like you learned in math class. Don't do it like you tried. Don't.

[Parag]

Thanks ..I did it with imrotate. for making forward projection I am trying this. I will try now with rotation matrix.

Thanks

[Parag]

I did it using same logic in labview but here I am getting problems.Thanks for suggestion

[Image Analyst]

I would be surprised if that produced a rotated image in labview.

Answer 3

[Parag]

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.

[per isakson]

Check the links I supplied!