# Ignition Math

## API Reference

6.9.3~pre2
Triangle example

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

## Compile the code

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

git clone https://github.com/ignitionrobotics/ign-math/ -b ign-math6
cd ign-math/examples
mkdir build
cd build
cmake ..
make

When the code is compiled, run:

./triangle_example

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:

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

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(ignition::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.

if (tri.Intersects(ignition::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