quick-and-dirty implementation to find the overlap area of the convex hulls of vowel measurements

voach.R

library(phonR)  # convexHullArea
library(sp)     # SpatialPolygons, etc
library(rgeos)  # gIntersection
data(indoVowels)
female_two <- indo[indo$subj == "F02",]
by_vowel <- split(female_two, female_two$vowel)
hull_indices <- sapply(by_vowel, function(df) with(df, chull(f1, f2)))
hulls <- sapply(names(by_vowel), function(v) by_vowel[[v]][hull_indices[[v]],],
                simplify=FALSE)
matrices <- sapply(hulls, function(df) as.matrix(df[,c("f1", "f2")]))
closed_mats <- sapply(matrices, function(m) rbind(m, m[1,]))
polygons <- sapply(closed_mats, Polygon, hole=FALSE)
polygon_lists <- sapply(names(polygons), function(i) Polygons(polygons[i], ID=i))
spatial_polygons <- sapply(names(polygon_lists), function(i) SpatialPolygons(polygon_lists[i]))
cmbns <- combn(names(by_vowel), 2)
overlap <- apply(cmbns, 2, function(i) gIntersection(spatial_polygons[[i[1]]],
                                                     spatial_polygons[[i[2]]]))
names(overlap) <- apply(cmbns, 2, paste, collapse="-")
overlap_area <- sapply(overlap, function(i) if (is.null(i)) {0} else {i@polygons[[1]]@area})
hull_area <- with(indo, convexHullArea(f1, f2, group=vowel))
voach <- sapply(names(overlap_area), function(i) {
    v <- strsplit(i, '-', fixed=TRUE)[[1]]
    min(hull_area[v]) / overlap_area[[i]]
})

I don't think there is an out-of-the-box way to make that plot with phonR. This gist computes the complete overlap of the 2 larger hulls, whereas that figure shows the (smaller) hull defined by the vowel tokens that are within the overlap area. It would be possible to use phonR to plot the 2 larger hulls, then use this code to find the overlap region, then find which vowel tokens are inside that region (using splancs::inpip() I think?), then defining the new hull based on those points (using this gist again, up through line 10), and finally adding the new hull to the phonR plot using polygon(). I'm writing all this from memory so don't be surprised if I got something wrong... Give it a go and let me know if you can't get it to work.

Thanks for your replies. I will try it this weekend and hopefully report back my victory on Monday!