smooth_peak {FunChIP} | R Documentation

## Spline smoothing of the peak

### Description

It approximates the read counts associated to every peak with a suitable B-spline function, so that a smoothing representation of the peaks is obtained. The first derivative of the spline is also computed. To obtain a smooth representation, the peak is extended and new initial and final points are identified. See the Vignette of the FunChIP package for a graphical representation of the spline approximation.

### Usage

    ## S4 method for signature 'GRanges'
    smooth_peak(object, n.breaks = 100, subsample = TRUE,
        subsample.data = 100, order = 4,
        lambda = (10^(seq(-5,5, by = 0.5))),
        GCV.derivatives = TRUE , plot.GCV = FALSE, rescale = FALSE)

### Arguments

`object` | GRanges object. It must contain the metadata column `counts`.
`n.breaks` | integer. Number of breaks, or knots, for the B-spline basis domain definition. Default is `100`.
`subsample` | logical. If `TRUE`, only a random subset (of size fixed by the parameter `subsample.data`) is used to identify the optimal value of lambda for the penalization via cross-validation. If `subsample=FALSE`, all the peaks of the GRanges data will be used. To contain running times, it is suggested to maintain the default value `subsample = TRUE`.
`subsample.data` | integer. Number of data used for the cross-validation (if `subsample.data` is `TRUE`). Default value is 100. If `subsample = FALSE`, all data points will be used and `subsample.data` is ignored.
`order` | integer. Order of the B-spline basis used for the smoothing. The order is one higher than the degree of the spline. Default is 4 (cubic splines).
`lambda` | vector (or single value). Contains all the possible values of the smoothing parameter to be considered for the final choice. If a single value is provided, this will be automatically chosen for the smoothing. Default value is 10^{\textrm{\code{seq(-5,5,by=0.5)}}} to analyze a sufficiently wide set of values. See details below.
`GCV.derivatives` | logical. If `TRUE` the Generalized Cross Validation index (GCV) on the derivatives is considered as criteria to identify λ, otherwise the GCV is computed on the data. Default is `TRUE`.
`plot.GCV` | logical. If `TRUE`, the plot of the GCV of the data and derivatives is shown as a function of λ. Default value is `FALSE`.
`rescale` | logical. If `TRUE` scaled peaks are also provided. From the spline approximation of the peak a new curve is defined. It is obtained scaling both the abscissa grid and the values of the coverages of the splines. All the scaled peaks have a common grid of width equal to the minimum width of the origninal splines and area equal to 1. Default is `FALSE`.

### Details

It creates a piece-wise polynomial of fixed `order` s approximating the data (B-spline expansion, Ramsay and Silverman, 2005). Given the point wise defined function f: (x,f(x)), the `smooth_peak` method returns the evaluation of s on the x grid (s(x)) minimizing, for a fixed λ,

ERR(λ) = || f - s ||^2_{L^2} + λ ||s''||^2_{L_2}

, with s'' being the second derivative of the function s and ||s||_{L^2} the L^2 norm of the function, i.e. the integral on the domain of s of s^2.

The choice of λ is crucial for the definition of the spline, and it can be selected by minimizing the Generalized Cross-Validation index

GCV(λ) = (n SSE)/(n-df(λ))^2

, with SSE the error computed as

SSE = || f - s ||^2_{L^2}

, if `GCV.derivatives = FALSE`, or

SSE = || grad(f) - s' ||^2_{L^2}

, if `GCV.derivatives = TRUE`, and df(λ) is the number of the degrees of freedom of the basis expansion automatically computed from s. For further details on the cross-validation procedure and on the computation of the number of degrees of freedom see Ramsay and Silverman, 2005.

If `plot.GCV` is `TRUE`, the plot of the GCV index as a function of λ is presented, which can be used to identify the optimal value of the parameter. If the plot is decreasing in λ, one could consider to increase the allowed values of λ to find the minimum of the curve.

### Value

the GRanges `object` with new metadata columns:

- `width_spline` integer. Value containing the width of the smoothed peak, i.e. the number of non-zero values of the spline approximation. This value is not necessarily equal to the original width of the peak, as the approximation can stretch outside the original width of the peak: to ensure smoothness some 0 values can be introduced at the edges of the region.

- `spline` vector. Evaluation of the spline on the grid of size `width_spline`.

- `spline_der` vector. Evaluation of the derivatives of the spline on the grid of size `width_spline`.

- `start_spline` integer. Genomic coordinate of the initial point of the spline approximation.

- `end_spline` integer. Genomic coordinate of the final point of the spline approximation.

If `rescale` is `TRUE` two more metadata columns are added:

- `spline_rescaled` vector. Evaluation of the scaled peaks functions on a grid of width equal to the minimum of `width_spline`.

- `spline_der_rescaled` vector. Evaluation of the derivatives of the scaled peaks on a grid of width equal to the minimum of `width_spline`.

### Author(s)

Alice Parodi, Marco J. Morelli, Laura M. Sangalli, Piercesare Secchi, Simone Vantini

### References

Ramsay, J.O., Silverman, B.W., 2005. Functional Data Analysis, 2nd ed. Springer, New York, NY.

### Examples

    # load the data
    data(peaks)

    # it computes the spline approximation
    # of the pealks given the
    # GRange with the metadata counts.
    # It is obtained by the pileup_peak method

    # Default paramters are used: GCV is
    # computed on the derivatives.

    peaks.spline <- smooth_peak(peaks.data, lambda = 10^(-4:6),
                                subsample.data = 50, GCV.derivatives = TRUE )

    peaks.spline.scaled <- smooth_peak(peaks.data, lambda = 10^(-4:6),
                                subsample.data = 50, GCV.derivatives = TRUE, rescale = TRUE )

[Package FunChIP version 1.6.0 Index]