# lwt2

## Description

`[`

performs the 2-D lifting wavelet transform (LWT) of the real- or complex-valued
matrix `ll`

,`lh`

,`hl`

,`hh`

] = lwt2(`x`

)`x`

using the `db1`

wavelet. The function
performs the decomposition first along the rows in `x`

and then
along the columns. The default decomposition level depends on the size of
`x`

. For more information, see Level. The function returns the approximation coefficients at the
coarsest scale and the horizontal, vertical, and diagonal detail coefficients by
level.

If `x`

is a single-precision input, the numeric type of the
coefficients is single precision. Otherwise, the numeric type is double
precision.

`[___] = lwt2(`

specifies options using one or more name-value arguments. For example,
`x`

,`Name=Value`

)`lwt2(x,Wavelet="db2",Level=3)`

performs 2-D LWT using the
`db2`

wavelet and a level 3 decomposition.

## Examples

## Input Arguments

## Output Arguments

## Algorithms

At each stage of a 2-D wavelet decomposition, the approximation coefficients at level
*j* are decomposed into four components: the approximation at level
*j*+1 and the details in three orientations (horizontal, vertical,
and diagonal). Each component is the result of convolving the rows and columns of the
level *j* approximation with the appropriate combination of lowpass and
highpass filters, *LoD* and *HiD*, respectively,
followed by downsampling:

Approximation — Convolve the rows and columns with a lowpass filter (

`ll`

)Horizontal — Convolve the rows with a lowpass filter, and convolve the columns with a highpass filter (

`lh`

)Vertical — Convolve the rows with a highpass filter, and convolve the columns with a lowpass filter (

`hl`

)Diagonal — Convolve the rows and columns with a highpass filter (

`hh`

)

The following chart describes the basic decomposition steps.

where

— Downsample columns: keep the even-indexed columns

— Downsample rows: keep the even-indexed rows

— Convolve the rows of the entry with filter

*X*— Convolve the columns of the entry with filter

*X*

The decomposition is initialized by setting the approximation
coefficients equal to the image *s*: *cA*_{0} =
*s*.

## References

[1] Daubechies, Ingrid.
*Ten Lectures on Wavelets*. CBMS-NSF Regional Conference Series
in Applied Mathematics 61. Philadelphia, Pa: Society for Industrial and Applied
Mathematics, 1992.

[2] Mallat, S.G. “A Theory for
Multiresolution Signal Decomposition: The Wavelet Representation.” *IEEE
Transactions on Pattern Analysis and Machine Intelligence* 11, no. 7
(July 1989): 674–93. https://doi.org/10.1109/34.192463.

[3] Strang, Gilbert, and Truong
Nguyen. *Wavelets and Filter Banks*. Rev. ed. Wellesley, Mass:
Wellesley-Cambridge Press, 1997.

[4] Sweldens, Wim. “The Lifting
Scheme: A Construction of Second Generation Wavelets.” *SIAM Journal on
Mathematical Analysis* 29, no. 2 (March 1998): 511–46. https://doi.org/10.1137/S0036141095289051.

## Extended Capabilities

## Version History

**Introduced in R2021b**

## See Also

`ilwt2`

| `lwtcoef2`

| `haart2`

| `ihaart2`

| `liftingScheme`