Triangle example

This tutorial explains how to use the Triangle class from Gazebo Math library.

Compile the code

Go to gz-math/examples and use cmake to compile the code:

git clone https://github.com/gazebosim/gz-math/ -b gz-math8
cd gz-math/examples
mkdir build
cd build
cmake ..
# Linux and macOS
make
# Windows
cd ..
cmake --build build --config Release

When the code is compiled, run:

# Linux and macOS
./triangle_example
# Windows
.\build\Release\triangle_example.exe

The ouput of the program:

Vertex 1: -1 0
Vertex 2: 0 1
Vertex 3: 1 0
Side 1: -1 0 0 1
Side 2: 0 1 1 0
Side 3: 1 0 -1 0
Perimeter=4.82843 Area=1
Triangle contains the point 0, 0.5
A line from (-2, 0.5) to (2, 0.5) intersects the triangle at the
following points:
     Pt1=-0.5 0.5
     Pt2=0.5 0.5

Code

Create a triangle with the following vertices:

  gz::math::Triangled tri(
      gz::math::Vector2d(-1, 0),
      gz::math::Vector2d(0, 1),
      gz::math::Vector2d(1, 0));

The individual vertices are accessible through the [] operator.

  std::cout << "Vertex 1: " << tri[0] << "\n"
            << "Vertex 2: " << tri[1] << "\n"
            << "Vertex 3: " << tri[2] << "\n";

Each side of the triangle is also accessible via the Side method. Each side consists of 2 vertices, the following code will print out the X and Y values of each vertex.

  std::cout << "Side 1: " << tri.Side(0) << "\n"
            << "Side 2: " << tri.Side(1) << "\n"
            << "Side 3: " << tri.Side(2) << "\n";

It's also possible to set each vertex individually or set all the vertices at once.

  tri.Set(0, gz::math::Vector2d(-10, 0));
  tri.Set(1, gz::math::Vector2d(0, 20));
  tri.Set(2, gz::math::Vector2d(10, 2));

  tri.Set(gz::math::Vector2d(-1, 0),
          gz::math::Vector2d(0, 1),
          gz::math::Vector2d(1, 0));

You can get the perimeter length and area of the triangle

  std::cout << "Perimeter=" << tri.Perimeter()
            << " Area=" << tri.Area() << "\n";

The Contains function checks if a line or point is inside the triangle.

  if (tri.Contains(gz::math::Vector2d(0, 0.5)))
    std::cout << "Triangle contains the point 0, 0.5\n";
  else
    std::cout << "Triangle does not contain the point 0, 0.5\n";

The Intersects function checks if a line segment intersects the triangle. It also returns the points of intersection.

  gz::math::Vector2d pt1, pt2;
  if (tri.Intersects(gz::math::Line2d(-2, 0.5, 2, 0.5), pt1, pt2))
  {
    std::cout << "A line from (-2, 0.5) to (2, 0.5) intersects "
              << "the triangle at the\nfollowing points:\n"
              << "\t Pt1=" << pt1 << "\n"
              << "\t Pt2=" << pt2 << "\n";
  }
  else
  {
    std::cout << "A line from (-2, 0.5) to (2, 0.5) does not intersect "
              << "the triangle\n";
  }

There are more functions in Triangle. Take a look at the API