[R] Filled vector contours

[David Winsemius]

On Aug 24, 2014, at 1:30 PM, Jan Tosovsky wrote:

> On 2014-08-24 Prof Brian Ripley wrote:
>> On 24/08/2014 08:51, Jan Tosovsky wrote:
>>> 
>>> I am trying to create vector output (SVG) of filled contours.
>>> 
>>> I've found two approaches so far:
>>> 
>>> (1)
>>> library('lattice')
>>> svg("D:/test.svg")
>>> filled.contour(volcano)
>>> #levelplot(volcano, panel=panel.levelplot.raster)
>>> dev.off()
>>> 
>>> (2)
>>> library("raster")
>>> svg("D:/test.svg")
>>> rr <- raster(t(volcano))
>>> rc <- cut(rr, breaks= 10)
>>> pols <- rasterToPolygons(rc, dissolve=T)
>>> spplot(pols)
>>> dev.off()
>>> 
>>> But I'd like to get smooth closed polygons not broken into small
>>> cells.
>> 
>> How can a polygon be 'smooth'?
> 
> This was related to the result of example (2), which is more coarse
> comparing it to the result of (1).
> 
>> 
>> But the region between two contours is not in general a closed polygon.
> 
> I appologize for not being precise here.
> 
> My goal is to get something like this:
> http://drifted.in/other/contours/composition.svg
> 
> which is a composition of level plots, e.g.
> http://drifted.in/other/contours/level_plot.svg
> 
> By 'level plot' I mean the cut of the data at certain level. It is always a
> closed polygon (or multiple polygons) consisting of contour lines connected,
> if required, by border lines. 
> 
> For me it is preferred way over 'isoband', which doesn't include areas
> hidden by upper levels:
> http://drifted.in/other/contours/isoband.svg
> 
> Please note I am not sure if this terminology (isoband, level plot, etc) is
> correct.
> 

Most people with any R experience would have thought that "levelplot" referred to something like:
(Example 6.9 from Lattice, by Deepayan Sarkar to which I have only added the contour line parameter which it appears is what you were seeking.)

env <- environmental 
env$ozone <- env$ozone^(1/3) 
env$Radiation <- equal.count(env$radiation, 4)
fm1.env <- lm(ozone ~ radiation * temperature * wind, env) 
fm2.env <- loess(ozone ~ wind * temperature * radiation, env, span = 0.75, degree = 1) 
fm3.env <- loess(ozone ~ wind * temperature * radiation, env, parametric = c("radiation", "wind"), span = 0.75, degree = 2) 
library("locfit") ; library(lattice)
fm4.env <- locfit(ozone ~ wind * temperature * radiation, env) 
w.mesh <- with(env, do.breaks(range(wind), 50)) 
t.mesh <- with(env, do.breaks(range(temperature), 50)) 
r.mesh <- with(env, do.breaks(range(radiation), 3)) 
grid <- expand.grid(wind = w.mesh, temperature = t.mesh, radiation = r.mesh) 
grid[["fit.linear"]] <- predict(fm1.env, newdata = grid) 
grid[["fit.loess.1"]] <- as.vector(predict(fm2.env, newdata = grid)) 
grid[["fit.loess.2"]] <- as.vector(predict(fm3.env, newdata = grid)) 
grid[["fit.locfit"]] <- predict(fm4.env, newdata = grid)

png()
print(levelplot(fit.linear + fit.loess.1 + fit.loess.2 + fit.locfit ~ wind * temperature | radiation, data = grid, contour=TRUE))
dev.off()



-- 
David.

>>> Are there better methods in R ?
>> 
>> Neither of those are 'in R'!
>> But you could get a long way by looking at the following
>> 
>> ?contourLines
>> ?polygon
>> ?grid::grid.polygon
>> 
>> all of which are part of R.  R is a fully-fledged programming 
>> language and you could write SVG directly from R: using the svg device 
>> is limited by the steps to the R graphics primitives (see the 'R 
>> Internals' manual) and cairographics.
>> 
> 
> Thanks a lot for detailed explanation. Now I understand it more.
> 
> Most of contouring libraries create the list of line fragments. And this
> seems to be the case also of R's 'contourLines'.
> 
> But filling the areas requires a further post-processing (joining line
> fragments, joining contours, set the correct direction to preserve holes).
> And if I understand correctly, this is let to R 'libraries'. Unfortunately,
> none of these two is optimal for my use case.
> 
> If there is any hidden option or another library (implementing e.g. Marching
> squares) which could produce output similar to that linked SVG file, I am
> one big ear.
> 
> Thanks, Jan
> 
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David Winsemius
Alameda, CA, USA