Can't transform properly using kiss fft

Hi,
I'm trying to convert an image to frequency domain and back using kiss FFT.
The problem is that when I ONLY apply the forward transform than (just after 
that) the inverse transform I obtain a brightened image.
I'm using complex numbers this way:

const int dims[2] = {h,w}; // dimensions of fft ( height, width)
const int ndims = 2;  // number of dimensions. here 2 (2D)
kiss_fftnd_cfg  stf = kiss_fftnd_alloc(dims, ndims, 0, 0, 0), // forward 2d 
fft configure
  sti = kiss_fftnd_alloc(dims, ndims, 1, 0, 0); // inverse 2d fft configure
kiss_fft_cpx   *buf = new kiss_fft_cpx[w * h],
        *out = new kiss_fft_cpx[w * h];

 get Luminance from image and put it inside the array buf like follows:

 for(every pixel at location x)
 {
  // for simplicity here I don't center the spectrum
  buf[x].r = Luminance[x];
  buf[x].i = 0.0;
 }


 // forward fft
 kiss_fftnd(stf, buf, out);
 // inverse fft
 kiss_fftnd(sti, out, buf);

 for(every pixel at location x)
  // I use just the real part
  Luminance[x] = buf[x].r ( clipped in the range [0,255] );

 set luminance back inside the image

 free allocated data;

This is a very simple code but it doesn't work like it's supposed to.
I hope someone can help me.

Thanks in advance.

I solved the problem myself, but to think that I had to apply a convolution 
just to normalize the values...

"4N" <xxxx@yyyy.zzz> ha scritto nel messaggio 
news:47455386$0$36447$4fafbaef@reader5.news.tin.it...
> Hi,
> I'm trying to convert an image to frequency domain and back using kiss 
> FFT.
> The problem is that when I ONLY apply the forward transform than (just 
> after that) the inverse transform I obtain a brightened image.
> I'm using complex numbers this way:
>
> const int dims[2] = {h,w}; // dimensions of fft ( height, width)
> const int ndims = 2;  // number of dimensions. here 2 (2D)
> kiss_fftnd_cfg  stf = kiss_fftnd_alloc(dims, ndims, 0, 0, 0), // forward 
> 2d fft configure
>  sti = kiss_fftnd_alloc(dims, ndims, 1, 0, 0); // inverse 2d fft configure
> kiss_fft_cpx   *buf = new kiss_fft_cpx[w * h],
>        *out = new kiss_fft_cpx[w * h];
>
> get Luminance from image and put it inside the array buf like follows:
>
> for(every pixel at location x)
> {
>  // for simplicity here I don't center the spectrum
>  buf[x].r = Luminance[x];
>  buf[x].i = 0.0;
> }
>
>
> // forward fft
> kiss_fftnd(stf, buf, out);
> // inverse fft
> kiss_fftnd(sti, out, buf);
>
> for(every pixel at location x)
>  // I use just the real part
>  Luminance[x] = buf[x].r ( clipped in the range [0,255] );
>
> set luminance back inside the image
>
> free allocated data;
>
> This is a very simple code but it doesn't work like it's supposed to.
> I hope someone can help me.
>
> Thanks in advance.
>
> 

4N wrote:
> I solved the problem myself, but to think that I had to apply a convolution 
> just to normalize the values...


I'm not sure why you would need "convolution just to normalize the 
values". From what I can tell, your problem was simply scaling.  Many 
FFT libraries have a non-unity gain when doing ifft( fft(x) ).

Did you read the README file?  I know some people are morally opposed to 
reading documentation, but it is barely 100 lines long. It has a 
whopping two FAQs.  The second one would've solved your problem:


"   Q: Why don't I get the output I expect?
         A: The two most common causes of this are
                 1) scaling : is there a constant multiplier between 
what you got and what you want?
                 2) mixed build environment -- all code must be compiled 
with same preprocessor definitions for FIXED_POINT and kiss_fft_scalar





However ... with that said, one of these days I will probably make 
kiss_fft do round-trip scaling.  I get the scaling question often enough 
to make me realize that the library is not as fool-proof as it could be.


-- Mark