# wrap examples

## Raymaekers, J. and Rousseeuw, P.J.

### 2026-02-25

# Introduction

This file contains some examples of the functions related to the Wrap routine. More specifically the functions `wrap` and `estLocScale`.

    library("cellWise")

Simple example: Stars data

    # load the data

    X <- robustbase::starsCYG; plot(X)

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    cutoff <- sqrt(qchisq(0.975, ncol(X)))
    X.mean <- colMeans(X)
    X.cov  <- cov(X)
    cor(X)

    ##               log.Te  log.light
    ## log.Te     1.0000000 -0.2104133
    ## log.light -0.2104133  1.0000000

    locScaleX <- estLocScale(X)
    Xw <- wrap(X, locScaleX$loc, locScaleX$scale)$Xw
    Xw.mean <- colMeans(Xw)
    Xw.cov <- cov(Xw)
    cor(Xw)

    ##           [,1]      [,2]
    ## [1,] 1.0000000 0.5732006
    ## [2,] 0.5732006 1.0000000

    # classical Mahalanobis distances
    #

    md <- sqrt(mahalanobis(X, center = X.mean, cov = X.cov))
    plot(1:nrow(X), md, pch = 16, cex = 0.9,
         ylab = "Mahalanobis distance",
         xlab = "Index",
         main = "Mahalanobis distances",
         xlim = c(0, nrow(X)), ylim = c(0, max(md, cutoff) + 0.2), col = "black")
    abline(h = cutoff, lwd = 1.5, col = "red")

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    # wrapping-based robust distances
    #

    rd <- sqrt(mahalanobis(X, center = Xw.mean, cov = Xw.cov))
    plot(1:nrow(X), rd, pch = 16, cex = 0.9,
         ylab = "robust distance",
         xlab = "Index",
         main = "Wrapping-based robust distances",
         xlim = c(0, nrow(X)), ylim = c(0, max(rd, cutoff) + 0.2), col = "black")
    abline(h = cutoff, lwd = 1.5, col = "red")

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    # classical and wrapped tolerance ellipse
    #

    ell1 <- ellipse::ellipse(X.cov, centre = X.mean, t = cutoff, npoints = 120)
    ell2 <- ellipse::ellipse(Xw.cov, centre = Xw.mean, t = cutoff, npoints = 120)
    xmin <- min(min(X[, 1]), min(ell1[, 1]), min(ell2[, 1]))
    xmax <- max(max(X[, 1]), max(ell1[, 1]), max(ell2[, 1]))
    ymin <- min(min(X[, 2]), min(ell1[, 2]), min(ell2[, 2]))
    ymax <- max(max(X[, 2]), max(ell1[, 2]), max(ell2[, 2]))
    myxlim <- c(xmin, xmax); myylim <- c(ymin, ymax)
    plot(X, xlab = "X1", ylab = "X2", main = "Stars: classical and wrapping ellipse",
         pch = 16, col = "black", xlim = myxlim, ylim = myylim)
    points(ell1, type = "l", lwd = 1.5, col = "red")
    points(ell2, type = "l", lwd = 1.5, col = "blue")

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Bushfire dataset: a 5-dimensional example

    X <- as.matrix(robustbase::bushfire); pairs(X)

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    cutoff <- sqrt(qchisq(0.975, ncol(X))); cutoff

    ## [1] 3.582248

    X.mean <- colMeans(X); X.cov <- cov(X)
    round(cor(X), 2)

    ##       V1    V2    V3    V4    V5
    ## V1  1.00  0.80 -0.59 -0.49 -0.49
    ## V2  0.80  1.00 -0.53 -0.53 -0.52
    ## V3 -0.59 -0.53  1.00  0.97  0.98
    ## V4 -0.49 -0.53  0.97  1.00  1.00
    ## V5 -0.49 -0.52  0.98  1.00  1.00

    estX <- cellWise::estLocScale(X)
    Xw <- cellWise::wrap(X, estX$loc, estX$scale)$Xw
    Xw.mean <- colMeans(Xw); Xw.cov <- cov(Xw)
    round(cor(Xw), 2)

    ##       [,1]  [,2]  [,3]  [,4]  [,5]
    ## [1,]  1.00  0.78 -0.62 -0.52 -0.53
    ## [2,]  0.78  1.00 -0.25 -0.14 -0.15
    ## [3,] -0.62 -0.25  1.00  0.98  0.99
    ## [4,] -0.52 -0.14  0.98  1.00  1.00
    ## [5,] -0.53 -0.15  0.99  1.00  1.00

    wd <- sqrt(mahalanobis(X, center = Xw.mean, cov = Xw.cov))
    weights <- (wd < cutoff) + 0
    # Reweighting functions:
    wmean <- function(X, w) { # weighted mean
      wmean1 <- function(y, w) { sum(y * w) / sum(w) }
      apply(X, 2, FUN = wmean1, w = w)
    }
    wcov <- function(X, w) { # weighted cov
      Xc <- sweep(X, 2, wmean(X, w), "-")
      Xc <- sweep(Xc, 1, sqrt(w), "*")
      t(Xc) %*% Xc / sum(w)
    }
    X.wmean <- wmean(X, weights)
    X.wcov <- wcov(X, weights)
    round(cov2cor(X.wcov), 2)

    ##       V1    V2    V3    V4    V5
    ## V1  1.00  0.86 -0.74 -0.62 -0.63
    ## V2  0.86  1.00 -0.34 -0.19 -0.20
    ## V3 -0.74 -0.34  1.00  0.97  0.98
    ## V4 -0.62 -0.19  0.97  1.00  1.00
    ## V5 -0.63 -0.20  0.98  1.00  1.00

    # Classical Mahalanobis distances
    #
    md <- sqrt(mahalanobis(X, center = X.mean, cov = X.cov))
    plot(1:nrow(X), md, pch = 16, cex = 0.9,
         ylab = "Mahalanobis distance", xlab = "Index",
         main = "Mahalanobis distances", xlim = c(0, nrow(X)),
         ylim = c(0, max(md, cutoff) + 0.2), col = "black")
    abline(h = cutoff, lwd = 1.5, col = "red")

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    # No outliers stand out here.

    # Wrapping-based robust distances
    #
    rwd <- sqrt(mahalanobis(X, center = X.wmean, cov = X.wcov))
    plot(1:nrow(X),rwd,pch = 16, cex = 0.9,
         ylab = "robust distance", xlab = "Index",
         main = "Wrapping-based robust distances", xlim = c(0, nrow(X)),
         ylim = c(0, max(rwd, cutoff) + 0.2), col = "black")
    abline(h = cutoff, lwd = 1.5, col = "red")

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    # Many outliers are visible, the same as in Maronna-Yohai 1995.

    # Distance-distance plot:
    #
    plot(md, rwd, pch = 16, cex = 0.9, ylab = "robust distance",
         xlab = "classical distance", main = "distance-distance plot",
         xlim = c(0, max(md, cutoff) + 0.2),
         ylim = c(0, max(rwd, cutoff) + 0.2), col = "black")
    abline(v = cutoff, lwd = 1.5, col = "red")
    abline(h = cutoff, lwd = 1.5, col = "red")
    abline(0, 1, lwd = 1.5, lty = 2)

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    # only the robust distances detect the outliers

    # Classical and wrapped tolerance ellipses:
    #
    coordgrid <- expand.grid(1:dim(X)[2], 1:dim(X)[2])
    coordgrid <- coordgrid[-(1 + (1:dim(X)[2] - 1) * (dim(X)[2] + 1)), ]
    i = 1
    pairs(X, panel = function(x, y) {
            points(x, y)
            inds <- as.numeric(coordgrid[i, ])
            ell1 <- ellipse::ellipse(X.cov[inds, inds], centre = X.mean[inds],
                                     t = cutoff, npoints = 120)
            ell2 <- ellipse::ellipse(X.wcov[inds, inds], centre = X.wmean[inds],
                                     t = cutoff, npoints = 120)
            points(ell1, type = "l", lwd = 1.5, col = "red")
            points(ell2, type = "l", lwd = 1.5, col = "blue")
            i <<- i + 1
          })

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# Dog walker video example

    data(data_dogWalker)
    # keep a copy of the data for later:
    X <- data_dogWalker

    # Combine all pixels of an image in a row:
    dims <- dim(X); dims # 54 144 180   3

    ## [1]  54 144 180   3

    dim(X) <- c(dims[1], prod(dims[2:4]))
    dim(X) #  54 77760

    ## [1]    54 77760

    # Estimate location and scale using estLocScale:
    locScaleX <- estLocScale(X, precScale = 1e-12)

    ## Warning in estLocScale(X, precScale = 1e-12): 85  out of  77760  variables have an estimated scale <=
    ## "precScale" =  1e-12 .

    # Preprocess the data to deal with zeroscales:
    zeropix <- which(locScaleX$scale < 1e-12)
    nzero <- length(zeropix)
    filler <- 2 * ((1:dims[1]) - (dims[1] + 1) / 2) / (dims[1] - 1)
    filler <- 0.9 / 255 * matrix(rep(filler, nzero), ncol = nzero, byrow = F)
    centers <- apply(X[, zeropix], 2, FUN = median)
    centers <- pmax(0, pmin(1, centers));
    filler <- sweep(filler, 2, centers, "+")
    X[, zeropix] <- filler
    locScaleX$scale[zeropix] <- estLocScale(X[, zeropix])$scale

    # Wrap the data
    Xw <- wrap(X, locScaleX$loc, locScaleX$scale)$Xw

    # center the wrapped data
    aveXw <- colMeans(Xw)
    XwC <- sweep(Xw, 2, aveXw)

    # Compute singular value decomposition of centered wrapped data
    prcXwz <- svd::propack.svd(XwC, neig = 3)
    rloadings <- prcXwz$v; dim(rloadings)

    ## [1] 77760     3

    Xwscores <- XwC %*% rloadings; dim(Xwscores)

    ## [1] 54  3

    ## Project original data on robust loadings:
    XC <- sweep(X, 2, aveXw)
    robscores <- XC %*% rloadings

    # Compute the fitted frames and residuals:
    Xznew.rfitted <- robscores %*% t(rloadings[, 1:3])
    X.rfitted <- sweep(Xznew.rfitted, 2, aveXw, "+")
    X.rresidual <- X - X.rfitted

    # Plot the first loading
    rim1 <- pmax(0, pmin(rloadings[, 1] + locScaleX$loc, 1))
    dim(rim1) <- dims[2:4]; grid::grid.newpage()
    grid::grid.raster(rim1, interpolate = FALSE)

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    # Compute location and scale of the residuals
    res.med <- median(X.rresidual)
    res.mad <-  mad(X.rresidual)
    res <- (X.rresidual - res.med) / res.mad

    # Make plots
    frameinds <- c(10, 20, 30, 40)
    cutoff <- 200
    for (i in 1:4) {
      index <- frameinds[i]
      restemp <- res[index,];
      dim(restemp) <- c(prod(dims[2:3]), dims[4]);
      res2 <- restemp[, 1]^2 + restemp[, 2]^2 + restemp[, 3]^2
      mask <- rep(0, prod(dims[2:3])); mask[res2 > cutoff] <- 1
      mask <- cbind(mask,mask,mask);   dim(mask) <- dims[2:4]
      realim <- data_dogWalker[index, , , ];
      realim <- pmax(0, pmin(realim, 1)); dim(realim) <- dims[2:4]
      masked <- realim * mask; dim(masked)
      masked <- pmax(0, pmin(masked, 1)); dim(masked) <- dims[2:4]
      masked[masked == 0] <- 0.5
      finalim <- array(0, dim = c(dims[2], 2*dims[3], dims[4]))
      finalim[, 1:dims[3], ] <- realim
      finalim[, (dims[3] + 1):(2 * dims[3]), ] <- masked
      grid::grid.newpage()
      grid::grid.raster(finalim,interpolate = FALSE)
    }

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