Bilinear interpolation error - index exceeds matrix dimensions

I am supposed to do the bilinear interpolation without using imresize or interp. I am getting the error 'Index exceeds matrix dimensions' The padding was to avoid that.

{ f300=imread('Chrono');

fbilin(1:3:2769,1:3:2109)=f300(1:923,1:703);

f300=padarray(f300,[4 4],'replicate','both');

for j=1:3:2109

   for i=1:3:2769
    f11=f300(i,j+2);
    f12=f300(i,j+2);
    f21=f300(i+2,j);
    f22=f300(i+2,j+2);
    fbilin(i+1,j+1)=(f11*((i+2)-(i+1))*((j+2)-(j+1))+...
        f21*((i+1)-(i))*((j+2)-(j+1))+...
        f12*((i+2)-(i+1))*((j+1)-(j))+...
        f22*((i+1)-(i))*((j+1)-(j)))./((i+2)-(i))*((j+2)-(j));
  end
end

subplot(2,3,5), imagesc(fbilin), title('Chronometer Bilinear'); }

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Answer 1

Use DBSTOP to halt execution at the point where the error occurs. Check the values of the indices at that point. Also check the size of the (supposedly padded) arrays.

error in line f21=f300(i+2,j); my padded matrix is of size 932x711.

If you have used "dbstop if error" you should have reached a K>> prompt. Enter the following at the K>> prompt and paste for us the exact output

K>> i,j, size(f300)

K>> i,j,size(f300)

i =

   931

j =

     1

ans =

   931   711

if i swap my i and j in the sequence of the loop my dbstop result is K>> i,j,size(f300)

i =

     1

j =

   712

ans =

   931   711

Well, it's now clear that your indices have run over too large a range, right? If size(f300,1)=931, then there's no way that i+2=933 can be used to index it. Similarly in your second case, j=712 is not a valid index into an array whose dimension is only 711.

i agree. I just cannot figure out how to run this loop to get the image.

Well, one thing should be clear. If the size of the array is 931, you should probably not be running i up to 2769, and similarly for j. Where did these numbers come from anyway?

you are right. instead of f300 it should be fbilin. Thank you.