library(Rsamtools)
bcf_bcffilename = "subset.bcf"
bcf <- BcfFile(bcf_bcffilename, mode="rw")
rng <- GRanges(c("scaffold_1"), IRanges(1, 2000))
param <- ScanBcfParam(which=rng)
bcf <- scanBcf(bcf, param=param)