MaNoVe.wst: Make Node Vector (using Coifman-Wickerhauser best-basis type...

In wavethresh: Wavelets Statistics and Transforms

Make Node Vector (using Coifman-Wickerhauser best-basis type algorithm) on nondecimated wavelet transform object

Description

This method chooses a "best-basis" using the Coifman-Wickerhauser (1992) algorithm applied to nondecimated wavelet transform, wst.object, objects.

Usage

## S3 method for class 'wst'
MaNoVe(wst, entropy=Shannon.entropy, verbose=FALSE,
	stopper=FALSE, alg="C", ...)

Arguments

wst	The wst object for which you wish to find the best basis for.
entropy	The function used for computing the entropy of a vector
verbose	Whether or not to print out informative messages
stopper	Whether the computations are temporarily stopped after each packet. This can be useful in conjunction with the verbose argument so as to see computations proceed one packet at a time.
alg	If "C" then fast compiled C code is used (in which case the entropy function is ignored and the C code uses an internal Shannon entropy. Otherwise, slower R code is used but an arbitrary entropy argument can be used
...	Other arguments

Details

Description says all

Value

A wavelet node vector object, of class nv, a basis description. This can be fed into a basis inversion using, say, the function InvBasis.

Author(s)

G P Nason

See Also

InvBasis, MaNoVe, MaNoVe.wp, Shannon.entropy, wst.object, wst

Examples

#
# What follows is a simulated denoising example. We first create our
# "true" underlying signal, v. Then we add some noise to it with a signal
# to noise ratio of 6. Then we take the packet-ordered non-decimated wavelet
# transform and then threshold that.
#
# Then, to illustrate this function, we compute a "best-basis" node vector
# and use that to invert the packet-ordered NDWT using this basis. As a
# comparison we also use the Average Basis method
# (cf Coifman and Donoho, 1995). 
#
# NOTE: It is IMPORTANT to note that this example DOES not necessarily
# use an appropriate or good threshold or necessarily the right underlying
# wavelet. I am trying to show the general idea and please do not "quote" this
# example in literature saying that this is the way that WaveThresh (or
# any of the associated authors whose methods it attempts to implement)
# does it. Proper denoising requires a lot of care and thought.
#
#
# Here we go....
#
# Create an example vector (the Donoho and Johnstone heavisine function)
#
v <- DJ.EX()$heavi
#
# Add some noise with a SNR of 6
#
vnoise <- v + rnorm(length(v), 0, sd=sqrt(var(v))/6)
#
# Take packet-ordered non-decimated wavelet transform (note default wavelet
# used which might not be the best option for denoising performance).
#
vnwst <- wst(vnoise)
#
# Let's take a look at the wavelet coefficients of vnoise
#
## Not run: plot(vnwst)

#
# Wow! A huge number of coefficients, but mostly all noise.
#
#
# Threshold the resultant NDWT object.
# (Once again default arguments are used which are certainly not optimal).
#
vnwstT <- threshold(vnwst)
#
# Let's have a look at the thresholded wavelet coefficients
#
## Not run: plot(vnwstT)

#
# Ok, a lot of the coefficients have been removed as one would expect with
# universal thresholding
#
#
# Now select packets for a basis using a Coifman-Wickerhauser algorithm
#
vnnv <- MaNoVe(vnwstT)
#
# Let's have a look at which packets got selected
#
vnnv
# Level :  9  Action is  R (getpacket Index:  1 )
# Level :  8  Action is  L (getpacket Index:  2 )
# Level :  7  Action is  L (getpacket Index:  4 )
# Level :  6  Action is  L (getpacket Index:  8 )
# Level :  5  Action is  R (getpacket Index:  17 )
# Level :  4  Action is  L (getpacket Index:  34 )
# Level :  3  Action is  L (getpacket Index:  68 )
# Level :  2  Action is  R (getpacket Index:  137 )
# Level :  1  Action is  R (getpacket Index:  275 )
# There are  10  reconstruction steps
#
# So, its not the regular decimated wavelet transform!
#
# Let's invert the representation with respect to this basis defined by
# vnnv
#
vnwrIB <- InvBasis(vnwstT, vnnv)
#
# And also, for completeness let's do an Average Basis reconstruction.
#
vnwrAB <- AvBasis(vnwstT)
#
# Let's look at the Integrated Squared Error in each case.
#
sum( (v - vnwrIB)^2)
# [1] 386.2501
#
sum( (v - vnwrAB)^2)
# [1] 328.4520
#
# So, for this limited example the average basis method does better. Of course,
# for *your* simulation it could be the other way round. "Occasionally", the
# inverse basis method does better. When does this happen? A good question.
#
# Let's plot the reconstructions and also the original
#
## Not run: plot(vnwrIB, type="l")
## Not run: lines(vnwrAB, lty=2)
## Not run: lines(v, lty=3)

#
# The dotted line is the original. Neither reconstruction picks up the
# spikes in heavisine very well. The average basis method does track the
# original signal more closely though.
#

wavethresh documentation built on Sept. 11, 2024, 9:33 p.m.