## Developer Reference

• 2021
• 06/28/2021
• Public Content
Contents

# Real - Complex Packed (RCPack2D) Format

The forward Fourier transform of a real two-dimensional image data yields a matrix of complex results which has conjugate-symmetric properties. Intel IPP functions use packed format RCPack2D for storing and retrieving data of this type. Accordingly, real flavors of the inverse Fourier transform functions convert packed complex conjugate-symmetric data back to its real origin. The RCPack2D format exploits the complex conjugate symmetry of the transformed data to store only a half of the resulting Fourier coefficients. For the
N
by
M
transform, the respective FFT and DFT functions actually store real and imaginary parts of the complex Fourier coefficients
A(i,j)
for
i = 0,...,M-1; j = 0,... N/2
in a single real array of dimensions (
N
,
M)
. The RCPack2D storage format is slightly different for odd and even
M
and is arranged in accordance with the following tables:
RCPack2D Storage for Odd Number of Rows
Re A(0,0)
Re A(0,1)
Im A(0,1)
...
Re A(0,(N-1)/2)
Im A(0,(N-1)/2)
Re A(0,N/2)
Re A(1,0)
Re A(1,1)
Im A(1,1)
...
Re A(1,(N-1)/2)
Im A(1,(N-1)/2)
Re A(1,N/2)
Im A(1,0)
Re A(2,1)
Im A(2,1)
...
Re A(2,(N-1)/2)
Im A(2,(N-1)/2)
Im A(1,N/2)
...
...
...
...
...
...
...
Re A(M/2,0)
Re A(M-2,1)
Im A(M-2,1)
...
Re A(M-2,(N-1)/2)
Im A(M-2,(N-1)/2)
Re A(M/2,N/2)
Im A(M/2,0)
Re A(M-1,1)
Im A(M-1,1)
...
Re A(M-1,(N-1)/2)
Im A(M-1,(N-1)/2)
Im A(M/2,N/2)
RCPack2D Storage for Even Number of Rows
Re A(0,0)
Re A(0,1)
Im A(0,1)
...
Re A(0,(N-1)/2)
Im A(0,(N-1)/2)
Re A(0,N/2)
Re A(1,0)
Re A(1,1)
Im A(1,1)
...
Re A(1,(N-1)/2)
Im A(1,(N-1)/2)
Re A(1,N/2)
Im A(1,0)
Re A(2,1)
Im A(2,1)
...
Re A(2,(N-1)/2)
Im A(2,(N-1)/2)
Im A(1,N/2)
...
...
...
...
...
...
...
Re A(M/2-1,0)
Re A(M-3,1)
Im A(M-3,1)
...
Re A(M-3,(N-1)/2)
Im A(M-3,(N-1)/2)
Re A(M/2-1,N/2)
Im A(M/2-1,0)
Re A(M-2,1)
Im A(M-2,1)
...
Re A(M-2,(N-1)/2)
Im A(M-2,(N-1)/2)
Im A(M/2-1,N/2)
Re A(M/2,0)
Re A(M-1,1)
Im A(M-1,1)
...
Re A(M-1,(N-1)/2)
Im A(M-1,(N-1)/2)
Re A(M/2,N/2)
The shaded columns to the right side of the tables indicate values for even
N
only.
Note the above tables show the arrangement of coefficients for one channel. For multichannel images the channel coefficients are clustered and stored consecutively, for example, for 3-channel image they are stored in the following way:
C1-Re
A(0,0)
;
C2-R
e
A(0,0)
;
C3-Re
A(0,0)
;
C1-Re
A(0,1)
;
C2-Re
A(0,1)
;
C3-Re
A(0,1)
;
C1-Im
A(0,1
);
C2-Im
A(0,1
); ...
The remaining Fourier coefficients are obtained using the following relationships based on conjugate-symmetric properties:
A(i,j) = conj(A(M-i,N-j)), i = 1,..., M-1; j = 1,..., N-1
A(0,j) = conj(A(0,N-j)), j = 1,..., N-1
A(i,0) = conj(A(M-i,0)), i = 1,..., M-1

#### Product and Performance Information

1

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