filterPointsByPolygon

Filters a set of 2D points according to their spatial relation to a polygon.
You can keep or discard points that fall inside, outside, or exactly on the polygon boundary.

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Function Signature

 std::vector<Eigen::RowVector2d> filterPointsByPolygon(
    const Eigen::MatrixX2d& polygonVertices,
    const std::vector<Eigen::RowVector2d>& points,
    bool removeBoundary = true,
    bool removeInside = true,
    bool needClosePolygon = true
);

Parameters

Parameter	Description
polygonVertices	Eigen::MatrixX2d — Each row is a vertex. If needClosePolygon is true, the vertices may be unordered or open; the function will sort them counter-clockwise and append the first vertex to close the loop.
points	std::vector<Eigen::RowVector2d> — the input points to filter.
removeBoundary	bool (default: true) — if true, removes points exactly on the polygon boundary; otherwise keeps them.
removeInside	bool (default: true) — if true, removes points strictly inside the polygon; otherwise keeps them.
needClosePolygon	bool (default: true) — true → auto-order & close polygonVertices; false → assume vertices are already counter-clockwise and closed.

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Return Value

• A std::vector<Eigen::RowVector2d> containing only those points that remain after filtering.

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Behavior

removeInside	removeBoundary	Kept Points
false	false	Keep inside & boundary; remove outside
false	true	Keep only inside; remove outside & boundary
true	false	Keep outside & boundary; remove inside
true	true	Keep only outside; remove inside & boundary

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Example Usage

The following demo uses randomPoints.csv from the repository.

1. Data Preparation

std::vector<Eigen::RowVector2d> testPoints2D;
testPoints2D.reserve(200);
std::ifstream file("./testData/randomPoints.csv");
std::string line;
while (getline(file, line)) {
    std::istringstream lineStream(line);
    std::string c1, c2;
    if (std::getline(lineStream, c1, ',') &&
        std::getline(lineStream, c2, ',')) {

        Eigen::RowVector2d vec2d;
        vec2d << stod(c1), stod(c2);
        testPoints2D.emplace_back(vec2d);

    }
}

// A simple quadrilateral
Eigen::Matrix<double, 4, 2> polygonVertices;
polygonVertices << 20.785733063246152, 76.974749045767595,
    79.748658377982537, 77.104254429716875,
    81.118930088650117, 18.361256017841775,
    22.15600477391374, 18.231750633892496;

×

2. Executing the Filter

// Remove only boundary points, keep inside and outside:
std::vector<Eigen::RowVector2d> insidePoints2D = filterPointsByPolygon(
    polygonVertices, testPoints2D,
    /*removeBoundary=*/true,
    /*removeInside=*/false,
    /*needClosePolygon=*/true
    );
std::cout << "\nNumber of points outside the polygon: " << insidePoints2D.size() << "\n";

// Remove inside and boundary, keep only outside:
std::vector<Eigen::RowVector2d> outsidePoints2D = filterPointsByPolygon(
    polygonVertices, testPoints2D,
    /*removeBoundary=*/true,
    /*removeInside=*/true,
    /*needClosePolygon=*/true
    );
std::cout << "\nNumber of points inside the polygon: " << outsidePoints2D.size() << "\n";

3. Results

×

Complete Implementation

#include "LiteGeometry.h"

int main() {

    std::vector<Eigen::RowVector2d> testPoints2D;
    testPoints2D.reserve(200);
    std::ifstream file("./testData/randomPoints.csv");
    std::string line;
    while (getline(file, line)) {
        std::istringstream lineStream(line);
        std::string c1, c2;
        if (std::getline(lineStream, c1, ',') &&
            std::getline(lineStream, c2, ',')) {

            Eigen::RowVector2d vec2d;
            vec2d << stod(c1), stod(c2);
            testPoints2D.emplace_back(vec2d);

        }
    }

    Eigen::Matrix<double, 4, 2> polygonVertices;
    polygonVertices << 20.785733063246152, 76.974749045767595,
        79.748658377982537, 77.104254429716875,
        81.118930088650117, 18.361256017841775,
        22.15600477391374, 18.231750633892496;

    std::vector<Eigen::RowVector2d> insidePoints2D = filterPointsByPolygon(
        polygonVertices, testPoints2D,
        /*removeBoundary=*/true,
        /*removeInside=*/false,
        /*needClosePolygon=*/true
        );
    std::cout << "\nNumber of points outside the polygon: " << insidePoints2D.size() << "\n";

    std::vector<Eigen::RowVector2d> outsidePoints2D = filterPointsByPolygon(
        polygonVertices, testPoints2D,
        /*removeBoundary=*/true,
        /*removeInside=*/true,
        /*needClosePolygon=*/true
        );
    std::cout << "\nNumber of points inside the polygon: " << outsidePoints2D.size() << "\n";

    return 0;
}