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# dct

Discrete cosine transform.

# idct

Inverse discrete cosine transform.

### Syntax

X=dct(A [,sign] [,option]) X=dct(A,sign,selection [,option]) X=dct(A,sign,dims,incr [,option]) X=idct(A [,option]) X=idct(A,selection [,option]) X=idct(A,dims,incr [,option])

### Arguments

- A
a real or complex vector or real or complex array (vector, matrix or N-D array.

- X
- a real or complex array with same shape as
`A`

. - sign
- an integer. with possible values
`1`

or`-1`

. Select direct or inverse transform. The default value is`-1`

(direct transform). - selection
- a vector containing index on
`A`

array dimensions. See the Description part for details. - dims
- a vector of positive numbers with integer values, or a
vector of positive integers. See the Description part for details.
Each element must be a divisor of the total number of elements of

`A`

.The product of the elements must be less than the total number of elements of

`A`

. - incr
- a vector of positive numbers with integer values, or a
vector of positive integers. See the Description part for
details.
`incr`

must have the same number of elements than`dims`

.Each element must be a divisor of the total number of elements of

`A`

.The

`incr`

elements must be in strictly increasing order. - option
- a character string. with possible values
`"dct1"`

,`"dct2"`

,`"dct4"`

or`"dct"`

for direct transform and`"dct1"`

,`"dct3"`

,`"dct4"`

or`"idct"`

for inverse transform. The default value is`"dct"`

for direct transform and`"idct"`

for inverse transform. See the Description part for details.

### Description

### Transform description

This function realizes direct or
inverse 1-D or N-D Discrete Cosine Transforms with shift depending on the `option`

parameter value. For a 1-D array of length :

For

`"dct1"`

the function computes the unnormalized DCT-I transform:For

`"dct2"`

the function computes the unnormalized DCT-II transform:For

`"dct3"`

the function computes the unnormalized DCT-III transform:For

`"dct4"`

the function computes the unnormalized DCT-IV transform:For

`"dct"`

the function computes the normalized DCT-II transform:For

`"idct"`

the function computes the normalized DCT-III transform:

The multi-dimensional DCT transforms , in general, are the separable product of the given 1d transform along each dimension of the array. For unnormalized transforms , computing the forward followed by the backward/inverse multi-dimensional transform will result in the original array scaled by the product of the dimension sizes.

The normalized multi-dimensional DCT transform of an array
`A`

with dimensions
is given by

The normalized multi-dimensional DCT inverse transform of an
array `A`

with dimensions is given by

### Syntax description

- Short syntax
- direct
`X=dct(A,-1 [,option])`

or`X=dct(A [,option])`

gives a direct transform according to the option value. The default is normalized DCT-II direct transform.If

`A`

is a vector (only one dimension greater than 1) a 1-d transform is performed and in the other cases a n-dimensional transform is done.(the

`-1`

argument refers to the sign of the exponent..., NOT to "inverse"),- inverse
`X=dct(A,1 [,option])`

or`X=idct(A [,option])`

performs the inverse transform.If

`A`

is a vector (only one dimension greater than 1) a 1-d transform is performed and in the other cases a n-dimensional transform is done.

- Long syntax for DCT along specified dimensions
`X=dct(A,sign,selection [,option])`

allows to perform efficiently all direct or inverse dct of the "slices" of`A`

along selected dimensions.For example, if

`A`

is a 3-D array`X=dct(A,-1,2)`

is equivalent to:and

`X=dct(A,-1,[1 3])`

is equivalent to:`X=dct(A,sign,dims,incr)`

is an old syntax that also allows to perform all direct or inverse dct of the slices of`A`

along selected dimensions.For example, if

`A`

is an array with`n1*n2*n3`

elements`X=dct(A,-1,n1,1)`

is equivalent to`X=dct(matrix(A,[n1,n2,n3]),-1,1)`

. and`X=dct(A,-1,[n1 n3],[1 n1*n2])`

is equivalent to`X=dct(matrix(A,[n1,n2,n3]),-1,[1,3])`

.

### Optimizing dct

Remark: function automatically stores his last parameters in memory to re-use it in a second time. This improves greatly the time computation when consecutives calls (with same parameters) are performed.

It is possible to go further in dct optimization using get_fftw_wisdom, set_fftw_wisdom functions.

### Algorithms

This function uses the fftw3 library.

### Examples

1-D dct

//Frequency components of a signal //---------------------------------- // build a sampled at 1000hz containing pure frequencies // at 50 and 70 Hz sample_rate=1000; t = 0:1/sample_rate:0.6; N=size(t,'*'); //number of samples s=sin(2*%pi*50*t)+sin(2*%pi*70*t+%pi/4)+grand(1,N,'nor',0,1); d=dct(s); // zero low energy components d(abs(d)<1)=0; size(find(y1<>0),'*') //only 30 non zero coefficients out of 600 clf;plot(s,'b'),plot(dct(d,1),'r')

2-D dct

function z=__milk_drop(x, y) sq = x.^2+y.^2; z = exp( exp(-sq).*(exp(cos(sq).^20)+8*sin(sq).^20+2*sin(2*(sq)).^8) ); endfunction x = -2:0.1:2; [X,Y] = ndgrid(x,x); A = __milk_drop(X,Y); d = dct(A); d(abs(d)<1)=0; size(find(d<>0),'*') A1 = dct(d,1); clf gcf().color_map = graycolormap(128); subplot(121), grayplot(x,x,A) subplot(122), grayplot(x,x,A1)

### See also

- fft — Transformée de Fourier discrète rapide.
- dst — Discrete sine transform.
- fftw_flags — choix de la méthode pour la sélection de l'algorithme de planification pour la fft
- get_fftw_wisdom — retourne le wisdom fftw
- set_fftw_wisdom — charge un wisdom fftw
- fftw_forget_wisdom — Réinitialise le wisdom fftw

### Bibliography

Matteo Frigo and Steven G. Johnson, "FFTW Documentation" http://www.fftw.org/#documentation

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