2D Forward and Inverse Discrete Wavelet Transform

Single level dwt2

pywt.dwt2(data, wavelet[, mode='sym'])

The dwt2() function performs single level 2D Discrete Wavelet Transform.

Parameters:

• data – 2D input data.
• wavelet – Wavelet to use in the transform. This can be a name of the wavelet from the wavelist() list or a Wavelet object instance.
• mode – Signal extension mode to deal with the border distortion problem. See MODES for details. This is only important when DWT was performed in periodization mode.

Returns one average and three details 2D coefficients arrays. The coefficients arrays are organized in tuples in the following form:

(cA, (cH, cV, cD))

where cA, cH, cV, cD denote approximation, horizontal detail, vertical detail and diagonal detail coefficients respectively.

The relation to the other common data layout where all the approximation and details coefficients are stored in one big 2D array is as follows:

                            -------------------
                            |        |        |
                            | cA(LL) | cH(LH) |
                            |        |        |
(cA, (cH, cV, cD))  <--->   -------------------
                            |        |        |
                            | cV(HL) | cD(HH) |
                            |        |        |
                            -------------------

PyWavelets does not follow this pattern because of pure practical reasons of simple access to particular type of the output coefficients.

Example:

>>> import pywt, numpy
>>> data = numpy.ones((4,4), dtype=numpy.float64)
>>> coeffs = pywt.dwt2(data, 'haar')
>>> cA, (cH, cV, cD) = coeffs
>>> print cA
[[ 2.  2.]
 [ 2.  2.]]
>>> print cV
[[ 0.  0.]
 [ 0.  0.]]

Single level idwt2

pywt.idwt2(coeffs, wavelet[, mode='sym'])

The idwt2() function reconstructs data from the given coefficients set by performing single level 2D Inverse Discrete Wavelet Transform.

Parameters:

• coeffs –

  A tuple with approximation coefficients and three details coefficients 2D arrays like from dwt2():

  (cA, (cH, cV, cD))
  

• wavelet – Wavelet to use in the transform. This can be a name of the wavelet from the wavelist() list or a Wavelet object instance.
• mode – Signal extension mode to deal with the border distortion problem. See MODES for details. This is only important when the dwt() was performed in the periodization mode.

Example:

>>> import pywt, numpy
>>> data = numpy.array([[1,2], [3,4]], dtype=numpy.float64)
>>> coeffs = pywt.dwt2(data, 'haar')
>>> print pywt.idwt2(coeffs, 'haar')
[[ 1.  2.]
 [ 3.  4.]]

2D multilevel decomposition using wavedec2

pywt.wavedec2(data, wavelet[, mode='sym'[, level=None]])

Performs multilevel 2D Discrete Wavelet Transform decomposition and returns coefficients list:

[cAn, (cHn, cVn, cDn), ..., (cH1, cV1, cD1)]

where n denotes the level of decomposition and cA, cH, cV and cD are approximation, horizontal detail, vertical detail and diagonal detail coefficients arrays respectively.

Parameters:

• data – Input signal can be NumPy array, Python list or other iterable object. Both single and double precision floating-point data types are supported and the output type depends on the input type. If the input data is not in one of these types it will be converted to the default double precision data format before performing computations.
• wavelet – Wavelet to use in the transform. This can be a name of the wavelet from the wavelist() list or a Wavelet object instance.
• mode – Signal extension mode to deal with the border distortion problem. See MODES for details.
• level – Decomposition level. This should not be greater than the reasonable maximum value computed with the dwt_max_level() function for the smaller dimension of the input data.

Example:

>>> import pywt, numpy
>>> coeffs = pywt.wavedec2(numpy.ones((8,8)), 'db1', level=2)
>>> cA2, (cH2, cV2, cD2), (cH1, cV1, cD1) = coeffs
>>> print cA2
[[ 4.  4.]
 [ 4.  4.]]

2D multilevel reconstruction using waverec2

pywt.waverec2(coeffs, wavelet[, mode='sym'])

Performs multilevel reconstruction from the given coefficients set.

Parameters:

• coeffs –

  Coefficients set must be in the form like that from wavedec2() decomposition:

  [cAn, (cHn, cVn, cDn), ..., (cH1, cV1, cD1)]
  

• wavelet – Wavelet to use in the transform. This can be a name of the wavelet from the wavelist() list or a Wavelet object instance.
• mode – Signal extension mode to deal with the border distortion problem. See MODES for details.

Example:

>>> import pywt, numpy
>>> coeffs = pywt.wavedec2(numpy.ones((4,4)), 'db1')
>>> print "levels:", len(coeffs)-1
levels: 2
>>> print pywt.waverec2(coeffs, 'db1')
[[ 1.  1.  1.  1.]
 [ 1.  1.  1.  1.]
 [ 1.  1.  1.  1.]
 [ 1.  1.  1.  1.]]