godot-docs/tutorials/math/vector_math.rst

.. _doc_vector_math:

Vector math
===========

Introduction
~~~~~~~~~~~~

This tutorial is a short and practical introduction to linear algebra as it
applies to game development. Linear algebra is the study of vectors and their
uses. Vectors have many applications in both 2D and 3D development and Godot
uses them extensively. Developing a good understanding of vector math is
essential to becoming a strong game developer.

.. note:: This tutorial is **not** a formal textbook on linear algebra. We will
          only be looking at how it is applied to game development. For a
          broader look at the mathematics, see
          https://www.khanacademy.org/math/linear-algebra

Coordinate systems (2D)
~~~~~~~~~~~~~~~~~~~~~~~

In 2D space, coordinates are defined using a horizontal axis (``x``) and a
vertical axis (``y``). A particular position in 2D space is written as a pair of
values such as ``(4, 3)``.

.. image:: img/vector_axis1.png

.. note:: If you're new to computer graphics, it might seem odd that the
          positive ``y`` axis points **downwards** instead of upwards, as you
          probably learned in math class. However, this is common in most
          computer graphics applications.

Any position in the 2D plane can be identified by a pair of numbers in this way.
However, we can also think of the position ``(4, 3)`` as an **offset** from the
``(0, 0)`` point, or **origin**. Draw an arrow pointing from the origin to the
point:

.. image:: img/vector_xy1.png

This is a **vector**. A vector represents a lot of useful information. As well
as telling us that the point is at ``(4, 3)``, we can also think of it as an
angle ``θ`` and a length (or magnitude) ``m``. In this case, the arrow is a
**position vector** - it denotes a position in space, relative to the origin.

A very important point to consider about vectors is that they only represent
**relative** direction and magnitude. There is no concept of a vector's
position. The following two vectors are identical:

.. image:: img/vector_xy2.png

Both vectors represent a point 4 units to the right and 3 units below some
starting point. It does not matter where on the plane you draw the vector, it
always represents a relative direction and magnitude.

Vector operations
~~~~~~~~~~~~~~~~~

You can use either method (x and y coordinates or angle and magnitude) to refer
to a vector, but for convenience, programmers typically use the coordinate
notation. For example, in Godot, the origin is the top-left corner of the
screen, so to place a 2D node named ``Node2D`` 400 pixels to the right and 300
pixels down, use the following code:

.. tabs::
 .. code-tab:: gdscript GDScript

    $Node2D.position = Vector2(400, 300)

 .. code-tab:: csharp

    var node2D = GetNode<Node2D>("Node2D");
    node2D.Position = new Vector2(400, 300);

Godot supports both :ref:`Vector2 <class_Vector2>` and :ref:`Vector3
<class_Vector3>` for 2D and 3D usage, respectively. The same mathematical rules
discussed in this article apply to both types.

Member access
-------------

The individual components of the vector can be accessed directly by name.

.. tabs::
 .. code-tab:: gdscript GDScript

    # create a vector with coordinates (2, 5)
    var a = Vector2(2, 5)
    # create a vector and assign x and y manually
    var b = Vector2()
    b.x = 3
    b.y = 1

 .. code-tab:: csharp

    // create a vector with coordinates (2, 5)
    var a = new Vector2(2, 5);
    // create a vector and assign x and y manually
    var b = new Vector2();
    b.x = 3;
    b.y = 1;

Adding vectors
--------------

When adding or subtracting two vectors, the corresponding components are added:

.. tabs::
 .. code-tab:: gdscript GDScript

    var c = a + b  # (2, 5) + (3, 1) = (5, 6)

 .. code-tab:: csharp

    var c = a + b;  // (2, 5) + (3, 1) = (5, 6)

We can also see this visually by adding the second vector at the end of
the first:

.. image:: img/vector_add1.png

Note that adding ``a + b`` gives the same result as ``b + a``.

Scalar multiplication
---------------------

.. note:: Vectors represent both direction and magnitude. A value representing
          only magnitude is called a **scalar**.

A vector can be multiplied by a **scalar**:

.. tabs::
 .. code-tab:: gdscript GDScript

    var c = a * 2  # (2, 5) * 2 = (4, 10)
    var d = b / 3  # (3, 6) / 3 = (1, 2)

 .. code-tab:: csharp

    var c = a * 2;  // (2, 5) * 2 = (4, 10)
    var d = b / 3;  // (3, 6) / 3 = (1, 2)

.. image:: img/vector_mult1.png

.. note:: Multiplying a vector by a scalar does not change its direction, only
          its magnitude. This is how you **scale** a vector.

Practical applications
~~~~~~~~~~~~~~~~~~~~~~

Let's look at two common uses for vector addition and subtraction.

Movement
--------

A vector can represent **any** quantity with a magnitude and direction. Typical
examples are: position, velocity, acceleration, and force. In this image, the
spaceship at step 1 has a position vector of ``(1,3)`` and a velocity vector of
``(2,1)``. The velocity vector represents how far the ship moves each step. We
can find the position for step 2 by adding the velocity to the current position.

.. image:: img/vector_movement1.png

.. tip:: Velocity measures the **change** in position per unit of time. The new
         position is found by adding velocity to the previous position.

Pointing toward a target
------------------------

In this scenario, you have a tank that wishes to point its turret at a robot.
Subtracting the tank's position from the robot's position gives the vector
pointing from the tank to the robot.

.. image:: img/vector_subtract2.png

.. tip:: To find a vector pointing from ``A`` to ``B`` use ``B - A``.

Unit vectors
~~~~~~~~~~~~

A vector with **magnitude** of ``1`` is called a **unit vector**. They are also
sometimes referred to as **direction vectors** or **normals**. Unit vectors are
helpful when you need to keep track of a direction.

Normalization
-------------

**Normalizing** a vector means reducing its length to ``1`` while preserving its
direction. This is done by dividing each of its components by its magnitude.
Because this is such a common operation, ``Vector2`` and ``Vector3`` provide a
method for normalizing:

.. tabs::
 .. code-tab:: gdscript GDScript

    a = a.normalized()

 .. code-tab:: csharp

    a = a.Normalized();


.. warning:: Because normalization involves dividing by the vector's length, you
             cannot normalize a vector of length ``0``. Attempting to do so
             would normally result in an error. In GDScript though, trying to
             call the ``normalized()`` method on a ``Vector2`` or ``Vector3`` of
             length 0 leaves the value untouched and avoids the error for you.

Reflection
----------

A common use of unit vectors is to indicate **normals**. Normal vectors are unit
vectors aligned perpendicularly to a surface, defining its direction. They are
commonly used for lighting, collisions, and other operations involving surfaces.

For example, imagine we have a moving ball that we want to bounce off a wall or
other object:

.. image:: img/vector_reflect1.png

The surface normal has a value of ``(0, -1)`` because this is a horizontal
surface. When the ball collides, we take its remaining motion (the amount left
over when it hits the surface) and reflect it using the normal. In Godot, the
:ref:`Vector2 <class_Vector2>` class has a ``bounce()`` method to handle this.
Here is a GDScript example of the diagram above using a :ref:`KinematicBody2D
<class_KinematicBody2D>`:


.. tabs::
 .. code-tab:: gdscript GDScript

    # object "collision" contains information about the collision
    var collision = move_and_collide(velocity * delta)
    if collision:
        var reflect = collision.remainder.bounce(collision.normal)
        velocity = velocity.bounce(collision.normal)
        move_and_collide(reflect)

 .. code-tab:: csharp

    // KinematicCollision2D contains information about the collision
    KinematicCollision2D collision = MoveAndCollide(_velocity * delta);
    if (collision != null)
    {
        var reflect = collision.Remainder.Bounce(collision.Normal);
        _velocity = _velocity.Bounce(collision.Normal);
        MoveAndCollide(reflect);
    }

Dot product
~~~~~~~~~~~

The **dot product** is one of the most important concepts in vector math, but is
often misunderstood. Dot product is an operation on two vectors that returns a
**scalar**. Unlike a vector, which contains both magnitude and direction, a
scalar value has only magnitude.

The formula for dot product takes two common forms:

.. image:: img/vector_dot1.png

and

.. image:: img/vector_dot2.png

However, in most cases it is easiest to use the built-in method. Note that the
order of the two vectors does not matter:

.. tabs::
 .. code-tab:: gdscript GDScript

    var c = a.dot(b)
    var d = b.dot(a) # These are equivalent.

 .. code-tab:: csharp

    float c = a.Dot(b);
    float d = b.Dot(a); // These are equivalent.

The dot product is most useful when used with unit vectors, making the first
formula reduce to just ``cosθ``. This means we can use the dot product to tell
us something about the angle between two vectors:

.. image:: img/vector_dot3.png

When using unit vectors, the result will always be between ``-1`` (180°) and
``1`` (0°).

Facing
------

We can use this fact to detect whether an object is facing toward another
object. In the diagram below, the player ``P`` is trying to avoid the zombies
``A`` and ``B``. Assuming a zombie's field of view is **180°**, can they see the
player?

.. image:: img/vector_facing2.png

The green arrows ``fA`` and ``fB`` are **unit vectors** representing the
zombies' facing directions and the blue semicircle represents its field of view.
For zombie ``A``, we find the direction vector ``AP`` pointing to the player
using ``P - A`` and normalize it, however, Godot has a helper method to do this
called ``direction_to``. If the angle between this vector and the facing vector
is less than 90°, then the zombie can see the player.

In code it would look like this:

.. tabs::
 .. code-tab:: gdscript GDScript

    var AP = A.direction_to(P)
    if AP.dot(fA) > 0:
        print("A sees P!")

 .. code-tab:: csharp

    var AP = A.DirectionTo(P);
    if (AP.Dot(fA) > 0)
    {
        GD.Print("A sees P!");
    }

Cross product
~~~~~~~~~~~~~

Like the dot product, the **cross product** is an operation on two vectors.
However, the result of the cross product is a vector with a direction that is
perpendicular to both. Its magnitude depends on their relative angle. If two
vectors are parallel, the result of their cross product will be a null vector.

.. image:: img/vector_cross1.png

.. image:: img/vector_cross2.png

The cross product is calculated like this:

.. tabs::
 .. code-tab:: gdscript GDScript

    var c = Vector3()
    c.x = (a.y * b.z) - (a.z * b.y)
    c.y = (a.z * b.x) - (a.x * b.z)
    c.z = (a.x * b.y) - (a.y * b.x)

 .. code-tab:: csharp

    var c = new Vector3();
    c.x = (a.y * b.z) - (a.z * b.y);
    c.y = (a.z * b.x) - (a.x * b.z);
    c.z = (a.x * b.y) - (a.y * b.x);



With Godot, you can use the built-in method:

.. tabs::
 .. code-tab:: gdscript GDScript

    var c = a.cross(b)

 .. code-tab:: csharp

    var c = a.Cross(b);

.. note:: In the cross product, order matters. ``a.cross(b)`` does not give the
          same result as ``b.cross(a)``. The resulting vectors point in
          **opposite** directions.

Calculating normals
-------------------

One common use of cross products is to find the surface normal of a plane or
surface in 3D space. If we have the triangle ``ABC`` we can use vector
subtraction to find two edges ``AB`` and ``AC``. Using the cross product, ``AB x
AC`` produces a vector perpendicular to both: the surface normal.

Here is a function to calculate a triangle's normal:

.. tabs::
 .. code-tab:: gdscript GDScript

    func get_triangle_normal(a, b, c):
        # find the surface normal given 3 vertices
        var side1 = b - a
        var side2 = c - a
        var normal = side1.cross(side2)
        return normal

 .. code-tab:: csharp

    Vector3 GetTriangleNormal(Vector3 a, Vector3 b, Vector3 c)
    {
        // find the surface normal given 3 vertices
        var side1 = b - a;
        var side2 = c - a;
        var normal = side1.Cross(side2);
        return normal;
    }

Pointing to a target
--------------------

In the dot product section above, we saw how it could be used to find the angle
between two vectors. However, in 3D, this is not enough information. We also
need to know what axis to rotate around. We can find that by calculating the
cross product of the current facing direction and the target direction. The
resulting perpendicular vector is the axis of rotation.

More information
~~~~~~~~~~~~~~~~

For more information on using vector math in Godot, see the following articles:

- :ref:`doc_vectors_advanced`
- :ref:`doc_matrices_and_transforms`