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### 2D FFT of images

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```Hi there,

I have a few questions relating to the use of the fft2 function in MATLAB. Basically when I use the fft2 function on an image and then multiply by the complex conjugate I still get some imaginary numbers appearing in the array. Is there a reason for this?

In addition to this when I look at the resulting image it turns out that I have negative results as well in the power spectrum.

Here is a rough example of the commands I used:

fft = fft2(image)
fftshifted = fftshift(fft)
fftpower = fftshifted*conj(fftshifted)

Would really appreciate if anyone has any suggestions as to what might be causing these problems.
```
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```Tom wrote:

> I have a few questions relating to the use of the fft2 function in
> MATLAB. Basically when I use the fft2 function on an image and then
> multiply by the complex conjugate I still get some imaginary numbers
> appearing in the array. Is there a reason for this?

Round-off error? How big are the imaginary parts compared to the
conjugate? e.g., are they on the order of 10^(-14) times the maximum
value of the conjugate (include the DC offset in that maximum...)
```
 0

```"Tom" <tgreig@gmail.com> wrote in message <hop49h\$hfk\$1@fred.mathworks.com>...
> Hi there,
>
> I have a few questions relating to the use of the fft2 function in MATLAB. Basically when I use the fft2 function on an image and then multiply by the complex conjugate I still get some imaginary numbers appearing in the array. Is there a reason for this?
============

Yes. Finite precision machine arithemtic.

> In addition to this when I look at the resulting image it turns out that I have negative results as well in the power spectrum.
=============

but they're presumably negligably small
```
 0

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