Euclidean distance matrix performance between two shapes

Question

The problem I am having is that I have to calculate a Euclidean distance matrix between shapes that can range from 20,000 up to 60,000 points, which produces 10-20GB amounts of data. I have to run each of these calculates thousands of times so 20GB x 7,000 (each calculation is a different point cloud). The shapes can be either 2D or 3D.

EDITED (Updated questions)

1. Is there a more efficient way to calculate the forward and backward distances without using two separate nested loops?

   I know I could save the data matrix and calculate the minimum distances in each direction, but then there is a huge memory issue with large point clouds.

2. Is there a way to speed up this calculation and/or clean up the code to trim off time?

The irony is that I only need the matrix to calculate a very simple metric, but it requires the entire matrix to find that metric (Average Hausdorff distance).

Data example where each column represents a dimension of the shape and each row is a point in the shape:

first_configuration <- matrix(1:6,2,3)
second_configuration <- matrix(6:11,2,3)
colnames(first_configuration) <- c("x","y","z")
colnames(second_configuration) <- c("x","y","z")

This code calculates a Euclidean distance between between coordinates:

m <- nrow(first_configuration)
n <- nrow(second_configuration)

D <- sqrt(pmax(matrix(rep(apply(first_configuration * first_configuration, 1, sum), n), m, n, byrow = F) + matrix(rep(apply(second_configuration * second_configuration, 1, sum), m), m, n, byrow = T) - 2 * first_configuration %*% t(second_configuration), 0))
D

Output:

     [,1]      [,2]
[1,] 8.660254 10.392305
[2,] 6.928203  8.660254

EDIT: included hausdorff average code

d1 <- mean(apply(D, 1, min))
d2 <- mean(apply(D, 2, min))
average_hausdorff <- mean(d1, d2)

EDIT (Rcpp solution): Here is my attempt to implement it in Rcpp so the matrix is never saved to memory. Working now but very slow.

sourceCpp(code=
#include <Rcpp.h>
#include <limits>
using namespace Rcpp;

// [[Rcpp::export]]
double edist_rcpp(NumericVector x, NumericVector y){
    double d = sqrt( sum( pow(x - y, 2) ) );
    return d;
}


// [[Rcpp::export]]
double avg_hausdorff_rcpp(NumericMatrix x, NumericMatrix y){
    int nrowx = x.nrow();
    int nrowy = y.nrow();
    double new_low_x = std::numeric_limits<int>::max();
    double new_low_y = std::numeric_limits<int>::max();

    double mean_forward = 0;
    double mean_backward = 0;
    double mean_hd; 
    double td; 

    //forward
    for(int i = 0; i < nrowx; i++) {
        for(int j = 0; j < nrowy; j++) {
            NumericVector v1 = x.row(i);
            NumericVector v2 = y.row(j);
            td = edist_rcpp(v1, v2);
            if(td < new_low_x) {
                new_low_x = td;
            }
        }
        mean_forward = mean_forward + new_low_x;
        new_low_x = std::numeric_limits<int>::max();
    }

    //backward
    for(int i = 0; i < nrowy; i++) {
        for(int j = 0; j < nrowx; j++) {
            NumericVector v1 = y.row(i);
            NumericVector v2 = x.row(j);
            td = edist_rcpp(v1, v2);
            if(td < new_low_y) {
                new_low_y = td;
            }
        }
        mean_backward = mean_backward + new_low_y;
        new_low_y = std::numeric_limits<int>::max();
    }

    //hausdorff mean
    mean_hd = (mean_forward / nrowx + mean_backward / nrowy) / 2;

    return mean_hd;
}
)

EDIT (RcppParallel solution): Definitely faster than the serial Rcpp solution and most certainly the R solution. If anyone has tips on how to improve my RcppParallel code to trim off some extra time it would be much appreciated!

sourceCpp(code=
#include <Rcpp.h>
#include <RcppParallel.h>
#include <limits>

// [[Rcpp::depends(RcppParallel)]]
struct minimum_euclidean_distances : public RcppParallel::Worker {
    //Input
    const RcppParallel::RMatrix<double> a;
    const RcppParallel::RMatrix<double> b;

    //Output
    RcppParallel::RVector<double> medm;

    minimum_euclidean_distances(const Rcpp::NumericMatrix a, const Rcpp::NumericMatrix b, Rcpp::NumericVector medm) : a(a), b(b), medm(medm) {}

    void operator() (std::size_t begin, std::size_t end) {
        for(std::size_t i = begin; i < end; i++) {
            double new_low = std::numeric_limits<double>::max();
            for(std::size_t j = 0; j < b.nrow(); j++) {
                double dsum = 0;
                for(std::size_t z = 0; z < b.ncol(); z++) {
                    dsum = dsum + pow(a(i,z) - b(j,z), 2);
                }
                dsum = pow(dsum, 0.5);
                if(dsum < new_low) {
                    new_low = dsum;
                }
            }
            medm[i] = new_low;
        }
    }
};


// [[Rcpp::export]]
double mean_directional_hausdorff_rcpp(Rcpp::NumericMatrix a, Rcpp::NumericMatrix b){
    Rcpp::NumericVector medm(a.nrow());
    minimum_euclidean_distances minimum_euclidean_distances(a, b, medm);
    RcppParallel::parallelFor(0, a.nrow(), minimum_euclidean_distances);    
    double results = Rcpp::sum(medm);
    results = results / a.nrow();
    return results;
}


// [[Rcpp::export]]
double max_directional_hausdorff_rcpp(Rcpp::NumericMatrix a, Rcpp::NumericMatrix b){
    Rcpp::NumericVector medm(a.nrow());
    minimum_euclidean_distances minimum_euclidean_distances(a, b, medm);
    RcppParallel::parallelFor(0, a.nrow(), minimum_euclidean_distances);    
    double results = Rcpp::max(medm);
    return results;
}
)

Benchmarks using large point clouds of sizes 37,775 and 36,659:

//Rcpp serial solution
system.time(avg_hausdorff_rcpp(ll,rr))
   user  system elapsed 
409.143   0.000 409.105 

//RcppParallel solution
system.time(mean(mean_directional_hausdorff_rcpp(ll,rr), mean_directional_hausdorff_rcpp(rr,ll)))
   user  system elapsed 
260.712   0.000  33.265 

Answer 1

I try to use JuliaCall to do the calculation for the average Hausdorff distance. JuliaCall embeds Julia in R.

I only try a serial solution in JuliaCall . It seems to be faster than the RcppParallel and the Rcpp serial solution in the question, but I don't have the benchmark data. Since ability for parallel computation is built in Julia. A parallel computation version in Julia should be written without much difficulty. I will update my answer after finding that out.

Below is the julia file I wrote:

# Calculate the min distance from the k-th point in as to the points in bs
function min_dist(k, as, bs)
    n = size(bs, 1)
    p = size(bs, 2)
    dist = Inf
    for i in 1:n
        r = 0.0
        for j in 1:p
            r += (as[k, j] - bs[i, j]) ^ 2
            ## if r is already greater than the upper bound, 
            ## then there is no need to continue doing the calculation
            if r > dist
                continue
            end
        end
        if r < dist
            dist = r
        end
    end
    sqrt(dist)
end

function avg_min_dist_from(as, bs)
    distsum = 0.0
    n1 = size(as, 1)
    for k in 1:n1
        distsum += min_dist_from(k, as, bs)
    end
    distsum / n1
end

function hausdorff_avg_dist(as, bs)
    (avg_min_dist_from(as, bs) + avg_min_dist_from(bs, as)) / 2
end

And this is the R code to use the julia function:

first_configuration <- matrix(1:6,2,3)
second_configuration <- matrix(6:11,2,3)
colnames(first_configuration) <- c("x","y","z")
colnames(second_configuration) <- c("x","y","z")

m <- nrow(first_configuration)
n <- nrow(second_configuration)

D <- sqrt(matrix(rep(apply(first_configuration * first_configuration, 1, sum), n), m, n, byrow = F) + matrix(rep(apply(second_configuration * second_configuration, 1, sum), m), m, n, byrow = T) - 2 * first_configuration %*% t(second_configuration))
D

d1 <- mean(apply(D, 1, min))
d2 <- mean(apply(D, 2, min))
average_hausdorff <- mean(d1, d2)

library(JuliaCall)
## the first time of julia_setup could be quite time consuming
julia_setup()
## source the julia file which has our hausdorff_avg_dist function
julia_source("hausdorff.jl")

## check if the julia function is correct with the example
average_hausdorff_julia <- julia_call("hausdauff_avg_dist",
                                      first_configuration,
                                      second_configuration)
## generate some large random point clouds
n1 <- 37775
n2 <- 36659
as <- matrix(rnorm(n1 * 3), n1, 3)
bs <- matrix(rnorm(n2 * 3), n2, 3)

system.time(julia_call("hausdauff_avg_dist", as, bs))

The time on my laptop was less than 20 seconds, note this is performance of the serial version of JuliaCall ! I used the same data to test RCpp serial solution in the question, which took more than 10 minutes to run. I don't have RCpp parallel on my laptop now so I can't try that. And as I said, Julia has built-in ability to do parallel computation.