signal processing - zero padding fft -

I have a picture of 786 * 786 size and I would like to make the pad to make it 1024 * 1024 so that I I can use my Radix2 algorithm. In the input side, I basically add zero to the end so that length will be 1024 * 1024 and my FFT will produce 1024 * 1024 output, my question is how do I get the correct 786 * 786 output?

Thank you for answering.

The output of FFT is still correct , even if you tick the input Gave. The difference is that the output is interpolated in the frequency domain, so you have a more clear resolution (obviously more information is not magically received - you are looking at extra points due to interpolation Are). If you do inverse FFT, then you will return to your original zero pad input.