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'); }

[Image Analyst]

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Answer 1

[Matt J]

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.

[P]

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

[Matt J]

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)

[P]

K>> i,j,size(f300)

i =

   931

j =

     1

ans =

   931   711

[P]

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

[Matt J]

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.

[P]

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

[Matt J]

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?

[P]

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