Fourier Transform - Help Request

[countzero]

I want to write a c function which implements fourier transform in 2d in a quick manner. I wrote my own version by looking at the formula and it seems to work ok. However I was not very pleased with the speed so I wrote a fast fourier function from the book numerical recipes in c. However, when I check the results don't match. There is a -1 coefficient difference in the imaginary set with my inverse fourier and numerical recipes in c's fast fourier version.  I was wondering how can I go about and check which one is correct ? does anyone have an idea ?

numerical recipes in c
wikipedia fourier transform(where I implemented the formula from)
another page with fft c code(probably buggy though)

my humble code for those willing to debug : (which assumes all data is real)

Code: [Select]



double m2pi = -2.0*pi;
for (u=0; u<M; u++)
for (v=0; v<N; v++) {
temp = 0, itemp = 0;
for (x=0; x<M; x++)
for (y=0; y<N; y++) {
temp += A[x][y].re*cos(m2pi*u*x/M+m2pi*v*y/N);
itemp += A[x][y].re*sin(m2pi*u*x/M+m2pi*v*y/N);
}
B[u][v].re = temp;
B[u][v].im = itemp;
}


and the inverse

Code: [Select]


double m2pi = 2.0*pi;
for (u=0; u for (v=0; v temp = 0, itemp = 0;
for (x=0; x for (y=0; y constant = m2pi*u*x/M+m2pi*v*y/N;
cos_coef = cos(constant);
sin_coef = sin(constant);
temp += B[x][y].re*cos_coef - B[x][y].im*sin_coef;
itemp += B[x][y].im*cos_coef + B[x][y].re*sin_coef;
}
C[u][v].re = temp/(M*N);
C[u][v].im = itemp/(M*N);
}



[countzero]

yes, it was a nuisiance but I believe I fixed it.

now, when I use the numerical recipes code, it just works both inverse and forward. I believe I'm doing something wrong in my code, cause I get the negative of the imaginary part. However I can still get the original sequence with the inverse transform (my code). my code vs num. rep. just doesn't work because of the difference of the imaginary parts.

no, it's not for image processing. It was for somekind of contest, which I already failed to submit in time.

[countzero]

ok, I just setup matlab to test results and it turns out that my code was correct after all and the num reps was wrong. probably I made a mistake when I was fiddling with the offsets. Now I'm trying to understand what actually num. rep. code does and debug it. I wonder if it would be easier if I set about to writing my own fft code.