# scipy.fftpack.dct¶

scipy.fftpack.dct(x, type=2, n=None, axis=-1, norm=None, overwrite_x=0)[source]

Return the Discrete Cosine Transform of arbitrary type sequence x.

Parameters : x : array_like The input array. type : {1, 2, 3}, optional Type of the DCT (see Notes). Default type is 2. n : int, optional Length of the transform. axis : int, optional Axis over which to compute the transform. norm : {None, ‘ortho’}, optional Normalization mode (see Notes). Default is None. overwrite_x : bool, optional If True the contents of x can be destroyed. (default=False) y : ndarray of real The transformed input array.

See also

idct

Notes

For a single dimension array x, dct(x, norm='ortho') is equal to MATLAB dct(x).

There are theoretically 8 types of the DCT, only the first 3 types are implemented in scipy. ‘The’ DCT generally refers to DCT type 2, and ‘the’ Inverse DCT generally refers to DCT type 3.

type I

There are several definitions of the DCT-I; we use the following (for norm=None):

```                                   N-2
y[k] = x[0] + (-1)**k x[N-1] + 2 * sum x[n]*cos(pi*k*n/(N-1))
n=1```

Only None is supported as normalization mode for DCT-I. Note also that the DCT-I is only supported for input size > 1

type II

There are several definitions of the DCT-II; we use the following (for norm=None):

```          N-1
y[k] = 2* sum x[n]*cos(pi*k*(2n+1)/(2*N)), 0 <= k < N.
n=0```

If norm='ortho', y[k] is multiplied by a scaling factor f:

```f = sqrt(1/(4*N)) if k = 0,
f = sqrt(1/(2*N)) otherwise.```

Which makes the corresponding matrix of coefficients orthonormal (OO' = Id).

type III

There are several definitions, we use the following (for norm=None):

```                  N-1
y[k] = x[0] + 2 * sum x[n]*cos(pi*(k+0.5)*n/N), 0 <= k < N.
n=1```

or, for norm='ortho' and 0 <= k < N:

```                                    N-1
y[k] = x[0] / sqrt(N) + sqrt(1/N) * sum x[n]*cos(pi*(k+0.5)*n/N)
n=1```

The (unnormalized) DCT-III is the inverse of the (unnormalized) DCT-II, up to a factor 2N. The orthonormalized DCT-III is exactly the inverse of the orthonormalized DCT-II.

References

http://en.wikipedia.org/wiki/Discrete_cosine_transform

‘A Fast Cosine Transform in One and Two Dimensions’, by J. Makhoul, IEEE Transactions on acoustics, speech and signal processing vol. 28(1), pp. 27-34, http://dx.doi.org/10.1109/TASSP.1980.1163351 (1980).

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