dct

Discrete cosine transform

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Syntax

y = dct(x)

y = dct(x,n)

y = dct(x,n,dim)

y = dct(___,'Type',dcttype)

Description

example

y = dct(x) returns the unitary discrete cosine transform of input array x. The output y has the same size as x. If x has more than one dimension, then dct operates along the first array dimension with size greater than 1.

y = dct(x,n) zero-pads or truncates the relevant dimension of x to length n before transforming.

example

y = dct(x,n,dim) computes the transform along dimension dim. To input a dimension and use the default value of n, specify the second argument as empty, [].

example

y = dct(___,'Type',dcttype) specifies the type of discrete cosine transform to compute. See Discrete Cosine Transform for details. This option can be combined with any of the previous syntaxes.

Examples

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Energy Stored in DCT Coefficients

Open Live Script

Find how many DCT coefficients represent 99% of the energy in a sequence.

x = (1:100) + 50*cos((1:100)*2*pi/40);
X = dct(x);
[XX,ind] = sort(abs(X),'descend');
i = 1;
while norm(X(ind(1:i)))/norm(X) < 0.99
   i = i + 1;
end
needed = i;

Reconstruct the signal and compare it to the original signal.

X(ind(needed+1:end)) = 0;
xx = idct(X);

plot([x;xx]')
legend('Original',['Reconstructed, N = ' int2str(needed)], ...
       'Location','SouthEast')

Image Data Compression

Open Live Script

Load a file that contains depth measurements of a mold used to mint a United States penny. The data, taken at the National Institute of Standards and Technology, are sampled on a 128-by-128 grid. Display the data.

load penny

surf(P)
view(2)
colormap copper
shading interp
axis ij square off

Compute the discrete cosine transform of the image data. Operate first along the rows and then along the columns.

Q = dct(P,[],1);
R = dct(Q,[],2);

Find what percentage of DCT coefficients contain 99.98% of the energy in the image.

X = R(:);

[~,ind] = sort(abs(R(:)),'descend');
coeffs = 1;
while norm(X(ind(1:coeffs)))/norm(X) < 0.9998
   coeffs = coeffs + 1;
end
fprintf('%3.1f%% of the coefficients are sufficient\n',coeffs/numel(R)*100)

21.8% of the coefficients are sufficient

Reconstruct the image using only the necessary coefficients.

R(abs(R) < abs(X(ind(coeffs)))) = 0;

S = idct(R,[],2);
T = idct(S,[],1);

Display the reconstructed image.

surf(T)
view(2)
shading interp
axis ij square off

Image Resizing

Open Live Script

load penny

surf(P)
view(2)
colormap copper
shading interp
axis ij square off

Compute the discrete cosine transform of the image data using the DCT-1 variant. Operate first along the rows and then along the columns.

Q = dct(P,[],1,'Type',1);
R = dct(Q,[],2,'Type',1);

Invert the transform. Truncate the inverse so that each dimension of the reconstructed image is one-half the length of the original.

S = idct(R,size(P,2)/2,2,'Type',1);
T = idct(S,size(P,1)/2,1,'Type',1);

Invert the transform again. Zero-pad the inverse so that each dimension of the reconstructed image is twice the length of the original.

U = idct(R,size(P,2)*2,2,'Type',1);
V = idct(U,size(P,1)*2,1,'Type',1);

Display the original and reconstructed images.

surf(V)
view(2)
shading interp
hold on

surf(P)
view(2)
shading interp

surf(T)
view(2)
shading interp
hold off
axis ij equal off

Input Arguments

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`x` — Input array
vector | matrix | N-D array | `gpuArray` object

Input array, specified as a real-valued or complex-valued vector, matrix, N-D array, or gpuArray object.

See Run MATLAB Functions on a GPU (Parallel Computing Toolbox) and GPU Support by Release (Parallel Computing Toolbox) for details on gpuArray (Parallel Computing Toolbox) objects.

Example: sin(2*pi*(0:255)/4) specifies a sinusoid as a row vector.

Example: sin(2*pi*[0.1;0.3]*(0:39))' specifies a two-channel sinusoid.

Data Types: single | double
Complex Number Support: Yes

`n` — Transform length
positive integer scalar

Transform length, specified as a positive integer scalar.

Data Types: single | double

`dim` — Dimension to operate along
positive integer scalar

Dimension to operate along, specified as a positive integer scalar.

Data Types: single | double

`dcttype` — Discrete cosine transform type
`2` (default) | `1` | `3` | `4`

Discrete cosine transform type, specified as a positive integer scalar from 1 to 4. See Discrete Cosine Transform for the definitions of the different types of DCT.

Data Types: single | double

Output Arguments

collapse all

`y` — Discrete cosine transform
vector | matrix | N-D array | `gpuArray` object

Discrete cosine transform, returned as a real-valued or complex-valued vector, matrix, N-D array, or gpuArray object.

More About

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Discrete Cosine Transform

The discrete cosine transform (DCT) is closely related to the discrete Fourier transform. You can often reconstruct a sequence very accurately from only a few DCT coefficients. This property is useful for applications requiring data reduction.

The DCT has four standard variants. For a signal x of length N, and with δ_kℓ the Kronecker delta, the transforms are defined by:

DCT-1:

$y (k) = \sqrt{\frac{2}{N - 1}} \sum_{n = 1}^{N} x (n) \frac{1}{\sqrt{1 + δ_{n 1} + δ_{n N}}} \frac{1}{\sqrt{1 + δ_{k 1} + δ_{k N}}} \cos (\frac{π}{N - 1} (n - 1) (k - 1))$
DCT-2:

$y (k) = \sqrt{\frac{2}{N}} \sum_{n = 1}^{N} x (n) \frac{1}{\sqrt{1 + δ_{k 1}}} \cos (\frac{π}{2 N} (2 n - 1) (k - 1))$
DCT-3:

$y (k) = \sqrt{\frac{2}{N}} \sum_{n = 1}^{N} x (n) \frac{1}{\sqrt{1 + δ_{n 1}}} \cos (\frac{π}{2 N} (n - 1) (2 k - 1))$
DCT-4:

$y (k) = \sqrt{\frac{2}{N}} \sum_{n = 1}^{N} x (n) \cos (\frac{π}{4 N} (2 n - 1) (2 k - 1))$

The series are indexed from n = 1 and k = 1 instead of the usual n = 0 and k = 0, because MATLAB^® vectors run from 1 to N instead of from 0 to N – 1.

All variants of the DCT are unitary (or, equivalently, orthogonal): To find their inverses, switch k and n in each definition. DCT-1 and DCT-4 are their own inverses. DCT-2 and DCT-3 are inverses of each other.

References

[1] Jain, A. K. Fundamentals of Digital Image Processing. Englewood Cliffs, NJ: Prentice-Hall, 1989.

[2] Oppenheim, Alan V., Ronald W. Schafer, and John R. Buck. Discrete-Time Signal Processing. 2nd Ed. Upper Saddle River, NJ: Prentice Hall, 1999.

[3] Pennebaker, W. B., and J. L. Mitchell. JPEG Still Image Data Compression Standard. New York: Van Nostrand Reinhold, 1993.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

C and C++ code generation for dct requires DSP System Toolbox™ software.
The length of the transform dimension must be a power of two. If specified, the pad or truncation value must be constant. Expressions or variables are allowed if their values do not change.
Inputs must be double precision.
Only DCT-2 is allowed.

GPU Arrays
Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.

Usage notes and limitations:

N-D input arrays are not supported.
The dim and dcttype input arguments are not supported.

For more information, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox).

Documentation

dct

Syntax

Description

Examples

Energy Stored in DCT Coefficients

Image Data Compression

Image Resizing

Input Arguments

`x` — Input array
vector | matrix | N-D array | `gpuArray` object

`n` — Transform length
positive integer scalar

`dim` — Dimension to operate along
positive integer scalar

`dcttype` — Discrete cosine transform type
`2` (default) | `1` | `3` | `4`

Output Arguments

`y` — Discrete cosine transform
vector | matrix | N-D array | `gpuArray` object

More About

Discrete Cosine Transform

References

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

GPU Arrays
Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.

See Also

Topics

Signal Processing Toolbox Documentation

Support

Documentation

dct

Syntax

Description

Examples

Energy Stored in DCT Coefficients

Image Data Compression

Image Resizing

Input Arguments

x — Input array vector | matrix | N-D array | gpuArray object

n — Transform length positive integer scalar

dim — Dimension to operate along positive integer scalar

dcttype — Discrete cosine transform type 2 (default) | 1 | 3 | 4

Output Arguments

y — Discrete cosine transform vector | matrix | N-D array | gpuArray object

More About

Discrete Cosine Transform

References

Extended Capabilities

C/C++ Code Generation Generate C and C++ code using MATLAB® Coder™.

GPU Arrays Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.

See Also

Topics

Signal Processing Toolbox Documentation

Support

`x` — Input array
vector | matrix | N-D array | `gpuArray` object

`n` — Transform length
positive integer scalar

`dim` — Dimension to operate along
positive integer scalar

`dcttype` — Discrete cosine transform type
`2` (default) | `1` | `3` | `4`

`y` — Discrete cosine transform
vector | matrix | N-D array | `gpuArray` object

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

GPU Arrays
Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.