How to decode encoded polylines from OSRM and plotting route geometry?

Question

I'm using an instance of OSRM (OpenStreetMap Routing Machine) to evaluate distance and time from different points. Using the API, I can retrieve information that I want and need especially the real route as a polyline.

Until today, I have plotted straight lines between start and end point.

segments(
         lon_patient,lat_patient,lon_lieu,lat_lieu,col = transp_time,lwd = 3
         )

Now I want to plot the polylines. But it is encoded (https://github.com/Project-OSRM/osrm-backend/wiki/Server-api#response-2). How can I draw it?

Thanks!

Answer 1

One (quick) way to get this going is to download the polyline.js file from the mapbox github repo then use the V8 package to do the hard work for you:

library(V8)

ctx <- new_context()
ctx$source("polyline.js")
ctx$call("polyline.decode", "_p~iF~ps|U_ulLnnqC_mqNvxq`@")

##        [,1]     [,2]
## [1,] 38.500 -120.200
## [2,] 40.700 -120.950
## [3,] 43.252 -126.453

It returns a matrix of lat/lon pairs you should be able to work with.

A pure R/Rcpp answer would be better in the long run, though.

UPDATE

There is one! This came from: https://gist.github.com/diegovalle/916889 (I added the requires and combined some wordy 0 assignments):

DecodeLineR <- function(encoded) {
  require(bitops)
  require(stringr)
  len = str_length(encoded)
  encoded <- strsplit(encoded, NULL)[[1]]
  index = 1
  N <- 100000
  df.index <- 1
  array = matrix(nrow = N, ncol = 2)
  lat <- dlat <- lng <- dlnt <- b <- shift <- result <- 0

  while(index <= len) {

    shift <- result <- 0

    repeat {
      b = as.integer(charToRaw(encoded[index])) - 63
      index <- index + 1
      result = bitOr(result, bitShiftL(bitAnd(b, 0x1f), shift))
      shift = shift + 5
      if(b < 0x20) break
    }
    dlat = ifelse(bitAnd(result, 1),
                 -(result - (bitShiftR(result, 1))),
                 bitShiftR(result, 1))
    lat = lat + dlat;

    shift <- result <- b <- 0

    repeat {
      b = as.integer(charToRaw(encoded[index])) - 63
      index <- index + 1
      result = bitOr(result, bitShiftL(bitAnd(b, 0x1f), shift))
      shift = shift + 5
      if(b < 0x20) break
    }
    dlng = ifelse(bitAnd(result, 1),
                  -(result - (bitShiftR(result, 1))),
                  bitShiftR(result, 1))
    lng = lng + dlng

    array[df.index,] <- c(lat = lat * 1e-05, lng = lng * 1e-5)
    df.index <- df.index + 1
  }

  ret <- data.frame(array[1:df.index - 1,])
  names(ret) <- c("lat", "lng")
  return(ret)
}

DecodeLineR("_p~iF~ps|U_ulLnnqC_mqNvxq`@")

##      lat      lng
## 1 38.500 -120.200
## 2 40.700 -120.950
## 3 43.252 -126.453

That gets you a data frame vs a matrix. And is pure R. Not sure which one will be faster (if speed is a need).

UPDATE #2

There's another pure R implementation here: http://s4rdd.blogspot.com/2012/12/google-maps-api-decoding-polylines-for.html and it's much faster than the one above (see below for benchmarks).

decodeLine <- function(encoded){
  require(bitops)

  vlen <- nchar(encoded)
  vindex <- 0
  varray <- NULL
  vlat <- 0
  vlng <- 0

  while(vindex < vlen){
    vb <- NULL
    vshift <- 0
    vresult <- 0
    repeat{
      if(vindex + 1 <= vlen){
        vindex <- vindex + 1
        vb <- as.integer(charToRaw(substr(encoded, vindex, vindex))) - 63  
      }

      vresult <- bitOr(vresult, bitShiftL(bitAnd(vb, 31), vshift))
      vshift <- vshift + 5
      if(vb < 32) break
    }

    dlat <- ifelse(
      bitAnd(vresult, 1)
      , -(bitShiftR(vresult, 1)+1)
      , bitShiftR(vresult, 1)
    )
    vlat <- vlat + dlat

    vshift <- 0
    vresult <- 0
    repeat{
      if(vindex + 1 <= vlen) {
        vindex <- vindex+1
        vb <- as.integer(charToRaw(substr(encoded, vindex, vindex))) - 63        
      }

      vresult <- bitOr(vresult, bitShiftL(bitAnd(vb, 31), vshift))
      vshift <- vshift + 5
      if(vb < 32) break
    }

    dlng <- ifelse(
      bitAnd(vresult, 1)
      , -(bitShiftR(vresult, 1)+1)
      , bitShiftR(vresult, 1)
    )
    vlng <- vlng + dlng

    varray <- rbind(varray, c(vlat * 1e-5, vlng * 1e-5))
  }
  coords <- data.frame(varray)
  names(coords) <- c("lat", "lon")
  coords
}

Here's an Rcpp/C++11 version courtesy of https://mapzen.com/documentation/mobility/decoding/ :

#include <Rcpp.h>
#include <vector>

using namespace Rcpp;

// [[Rcpp::plugins(cpp11)]]

// [[Rcpp::export]]
DataFrame decode_polyline(const std::string& encoded) {
  size_t i = 0;     // what byte are we looking at

  constexpr double kPolylinePrecision = 1E6;
  constexpr double kInvPolylinePrecision = 1.0 / kPolylinePrecision;

  auto deserialize = [&encoded, &i](const int previous) {
    int byte, shift = 0, result = 0;
    do {
      byte = static_cast<int>(encoded[i++]) - 63;
      result |= (byte & 0x1f) << shift;
      shift += 5;
    } while (byte >= 0x20);
    return previous + (result & 1 ? ~(result >> 1) : (result >> 1));
  };

  std::vector<double> lonv, latv;
  int last_lon = 0, last_lat = 0;
  while (i < encoded.length()) {
    int lat = deserialize(last_lat);
    int lon = deserialize(last_lon);

    latv.emplace_back(static_cast<float>(static_cast<double>(lat) * kInvPolylinePrecision));
    lonv.emplace_back(static_cast<float>(static_cast<double>(lon) * kInvPolylinePrecision));

    last_lon = lon;
    last_lat = lat;
  }

  return DataFrame::create(_["lon"] = lonv, _["lat"] = latv);
}

Save that to polyline.cpp and just:

Rcpp::sourceCpp("polyline.cpp")

Then you can:

decode_polyline("_p~iF~ps|U_ulLnnqC_mqNvxq`@")
##        lon    lat
## 1 -120.200 38.500
## 2 -120.950 40.700
#3 3 -126.453 43.252

Benchmarks

I sourced the two R function into the global environment and did the js & C++ equivalents for the javascript and C++ implementations.

The max value is pretty "out there" for DecodeLineR no matter what microbenchmark parameters I use. The decodeLine() pure R version seems performant enough to not warrant incurring the V8 or Rcpp/C++11 dependency, but YMMV.

FINAL UPDATE (MOAR BENCHMARKS)

I incorporated the googleway::decode_pl() function into the new benchmarks and used a much longer polyline. Benchmark code is below and the new plot is below that.

library(microbenchmark)
library(Rcpp)
library(inline)
library(V8)
library(googleway)
library(ggplot2)

sourceCpp("polyline.cpp")

ctx <- v8()
ctx$source("polyline.js")

source("DecodeLineR.R")
source("decodeline.R")

line_str <- "{ae{HntiQtCcDzG_I|^uc@rFgHhC{CxAiA~AaA~BkAvB}A|F_G|AgBbBkCtAwCd@sA|BoIVw@Pc@|@gBt@}@|@y@lCwBvA_B`@k@~@aBt@iBlAaE~@oEp@sDX{BP_BJaDAcEIeCe@gHo@yMUaEk@uDm@iD]mCAwBNsDXyDL}@nByIZyCt@cLr@gNB_ABoEAkFJmDTkBVeAZ_Af@gAnDwF|@gBbAoChHgUPWlAT`@B|@GbE_@dAW`Cu@vBe@tDs@xD{@`Bg@bBq@hBaAtB}@dCi@bF}@jBg@pBeAj@SNE\\C^@\\DbAZ`Ah@~C`A\\H|ALzAFLA^Gl@UdBgAjBaBZSh@Qz@MjD_@`FoAtCa@j@Ez@DxE|@xF\\nBP~@TxHvBf@Tb@\\pBvC\\^`@XxAf@fBT|BDfAIr@MfBe@rBa@rBMvBYxBg@xA_@^Ir@@NF|@l@nBfAjAj@dBV`Bb@lBbAbB~ALPhC`FV`@n@z@^VNBX?LGZa@d@eAp@qAt@Sx@Cz@G\\IZOhCcBb@c@T]jA_CrE_HfEiFz@}@p@k@|@o@`C{A`A{@rBwBx@oAbByCp@wArAoDLWxA}BhAcBjAqAlAiB~AaDr@sBp@{CD[TkC^}FZyD^oCx@gF`@qAh@kAz@yAtAgBpD_E|JoKdDuEjBcCfC{ExCqGdAgBlBuBrAyBpEkIpEsI\\]^YbAg@|GaBzKeEfBe@lCW`AQr@U|A_AtAkAhDyCpAeA|Aq@`EeCrDgBvA{@tD}C`BmAzBm@t@QvAQxBOl@Q~Ai@~BsAlCcB"

microbenchmark(
  googleway = decode_pl(line_str),
  rcpp = decode(line_str),
  js = ctx$call("polyline_decode", line_str),
  DecodeLineR = DecodeLineR(line_str),
  decodeLine = decodeLine(line_str),
  control=list(warmup=50),
  times=1000
) -> mb

mb
## Unit: microseconds
##         expr      min         lq       mean    median        uq        max neval cld
##    googleway  404.322   471.8475   817.8312   526.270   579.095 135564.973  1000 a  
##         rcpp  253.062   302.9550   363.9325   359.765   401.054    831.699  1000 a  
##           js 2793.677  3099.3390  3359.6190  3219.312  3427.636  15580.609  1000  b 
##  DecodeLineR 9366.714  9656.4140 12029.3991 10128.676 12570.216 181536.940  1000   c
##   decodeLine 9907.869 10218.0465 11413.5732 10655.949 11491.305 150588.501  1000   c

update_geom_defaults("violin", list(fill="maroon"))

autoplot(mb) +
  scale_y_log10(name="Time [microseconds]", label=scales::comma) +
  hrbrmisc::theme_hrbrmstr(grid="X")