smooth.hexbin

Hexagon Bin Smoothing

Given a "hexbin" (hexagon bin) object, compute a discrete kernel smoother that covers seven cells, namely a center cell and its six neighbors. With two iterations the kernel effectively covers 1+6+12=19 cells.

Usage

smooth.hexbin(bin, wts=c(48,4,1))

Details

This discrete kernel smoother uses the center cell, immediate neighbors and second neighbors to smooth the counts. The counts for each resulting cell is a linear combination of previous cell counts and weights. The weights are ll{ 1 center cell, weight = wts[1] 6 immediate neighbors weight = wts[2] 12 second neighbors weight =wts[3] } If a cell, its immediate and second neighbors all have a value of max(cnt), the new maximum count would be max(cnt)*sum(wts). It is possible for the counts to overflow.

The domain for cells with positive counts increases. The hexbin slots xbins, xbnds, ybnds, and dimen all reflect this increase. Note that usually dimen[2] = xbins+1.

The intent was to provide a fast, iterated, immediate neighbor smoother. However, the current hexbin plotting routines only support shifting even numbered rows to the right. Future work can

(1) add a shift indicator to hexbin objects that indicates left or right shifting. (2) generalize plot.hexbin() and hexagons() (3) provide an iterated kernel. With wts[3]=0, the smoother only uses the immediate neighbors. With a shift indicator the domain could increase by 2 rows (one bottom and on top) and 2 columns (one left and one right). However the current implementation increases the domain by 4 rows and 4 columns, thus reducing plotting resolution.

References

see grid.hexagons and hexbin.

See Also

hexbin, erode.hexbin, hcell2xy, gplot.hexbin, hboxplot, grid.hexagons, grid.hexlegend.

Aliases

• smooth.hexbin
• smoothbin-class

Examples

x <- rnorm(10000)
y <- rnorm(10000)
bin <- hexbin(x,y)
# show the smooth counts in gray level
smbin  <- smooth.hexbin(bin)
plot(smbin, main = "smooth.hexbin(.)")

# Compare the smooth and the origin
smbin1 <- smbin
smbin1@count <- as.integer(ceiling(smbin@count/sum(smbin@wts)))
plot(smbin1)
smbin2 <- smooth.hexbin(bin,wts=c(1,0,0))  # expand the domain for comparability
plot(smbin2)