Matrices and transforms¶
Before reading this tutorial, it is advised to read the previous one about Vector math as this one is a direct continuation.
This tutorial will be about transformations and will cover a little about matrices (but not in-depth).
Transformations are most of the time applied as translation, rotation and scale so they will be considered as priority here.
Oriented coordinate system (OCS)¶
Imagine we have a spaceship somewhere in space. In Godot this is easy, just move the ship somewhere and rotate it:
Ok, so in 2D this looks simple, a position and an angle for a rotation. But remember, we are grown ups here and don’t use angles (plus, angles are not even that useful when working in 3D).
We should realize that at some point, someone designed this spaceship. Be it for 2D in a drawing such as Paint.net, Gimp, Photoshop, etc. or in 3D through a 3D DCC tool such as Blender, Max, Maya, etc.
When it was designed, it was not rotated. It was designed in its own coordinate system.
This means that the tip of the ship has a coordinate, the fin has another, etc. Be it in pixels (2D) or vertices (3D).
So, let’s recall again that the ship was somewhere in space:
How did it get there? What moved it and rotated it from the place it was designed to its current position? The answer is… a transform, the ship was transformed from their original position to the new one. This allows the ship to be displayed where it is.
But transform is too generic of a term to describe this process. To solve this puzzle, we will superimpose the ship’s original design position at their current position:
So, we can see that the « design space » has been transformed too. How can we best represent this transformation? Let’s use 3 vectors for this (in 2D), a unit vector pointing towards X positive, a unit vector pointing towards Y positive and a translation.
Let’s call the 3 vectors « X », « Y » and « Origin », and let’s also superimpose them over the ship so it makes more sense:
Ok, this is nicer, but it still does not make sense. What do X,Y and Origin have to do with how the ship got there?
Well, let’s take the point from top tip of the ship as reference:
And let’s apply the following operation to it (and to all the points in the ship too, but we’ll track the top tip as our reference point):
Doing this to the selected point will move it back to the center:
This was expected, but then let’s do something more interesting. Use the dot product of X and the point, and add it to the dot product of Y and the point:
Then what we have is.. wait a minute, it’s the ship in its design position!
How did this black magic happen? The ship was lost in space, and now it’s back home!
It might seem strange, but it does have plenty of logic. Remember, as we have seen in the Vector math, what happened is that the distance to X axis, and the distance to Y axis were computed. Calculating distance in a direction or plane was one of the uses for the dot product. This was enough to obtain back the design coordinates for every point in the ship.
So, what we have been working with so far (with X, Y and Origin) is an Oriented Coordinate System. X an Y are the Basis, and Origin is the offset.
We know what the Origin is. It’s where the 0,0 (origin) of the design coordinate system ended up after being transformed to a new position. This is why it’s called Origin, But in practice, it’s just an offset to the new position.
The Basis is more interesting. The basis is the direction of X and Y in the OCS from the new, transformed location. It tells what has changed, in either 2D or 3D. The Origin (offset) and Basis (direction) communicate « Hey, the original X and Y axes of your design are right here, pointing towards these directions. »
So, let’s change the representation of the basis. Instead of 2 vectors, let’s use a matrix.
The vectors are up there in the matrix, horizontally. The next problem now is that.. what is this matrix thing? Well, we’ll assume you’ve never heard of a matrix.
Transforms in Godot¶
This tutorial will not explain matrix math (and their operations) in depth, only its practical use. There is plenty of material for that, which should be a lot simpler to understand after completing this tutorial. We’ll just explain how to use transforms.
class_Transform2D is a 3x2 matrix. It has 3 Vector2 elements and it’s used for 2D. The « X » axis is the element 0, « Y » axis is the element 1 and « Origin » is element 2. It’s not divided in basis/origin for convenience, due to its simplicity.
Most operations will be explained with this datatype (Transform2D), but the same logic applies to 3D.
An important transform is the « identity » matrix. This means:
- “X” Points right: Vector2(1,0)
- “Y” Points up (or down in pixels): Vector2(0,1)
- “Origin” is the origin Vector2(0,0)
It’s easy to guess that an identity matrix is just a matrix that aligns the transform to its parent coordinate system. It’s an OCS that hasn’t been translated, rotated or scaled.
Rotating Transform2D is done by using the « rotated » function:
There are two ways to translate a Transform2D, the first one is moving the origin:
This will always work in global coordinates.
If instead, translation is desired in local coordinates of the matrix (towards where the basis is oriented), there is the Transform2D.translated() method:
You could also transform the global coordinates to local coordinates manually:
But even better, there are helper functions for this as you can read in the next sections.
Local to global coordinates and vice versa¶
There are helper methods for converting between local and global coordinates.
There are Node2D.to_local() and Node2D.to_global() for 2D as well as Spatial.to_local() and Spatial.to_global() for 3D.
A matrix can be scaled too. Scaling will multiply the basis vectors by a vector (X vector by x component of the scale, Y vector by y component of the scale). It will leave the origin alone:
These kind of operations in matrices are accumulative. It means every one starts relative to the previous one. For those who have been living on this planet long enough, a good reference of how transform works is this:
A matrix is used similarly to a turtle. The turtle most likely had a matrix inside (and you are likely learning this many years after discovering Santa is not real).
Transform is the act of switching between coordinate systems. To convert a position (either 2D or 3D) from « designer » coordinate system to the OCS, the « xform » method is used.
And only for basis (no translation):
To do the opposite operation (what we did up there with the rocket), the « xform_inv » method is used:
Only for Basis:
However, if the matrix has been scaled (vectors are not unit length), or the basis vectors are not orthogonal (90°), the inverse transform will not work.
In other words, inverse transform is only valid in orthonormal matrices. For this, these cases an affine inverse must be computed.
The transform, or inverse transform of an identity matrix will return the position unchanged:
The affine inverse is a matrix that does the inverse operation of another matrix, no matter if the matrix has scale or the axis vectors are not orthogonal. The affine inverse is calculated with the affine_inverse() method:
If the matrix is orthonormal, then:
Matrices can be multiplied. Multiplication of two matrices « chains » (concatenates) their transforms.
However, as per convention, multiplication takes place in reverse order.
To make it a little clearer, this:
Is the same as:
However, this is not the same:
Because in matrix math, A * B is not the same as B * A.
Multiplication by inverse¶
Multiplying a matrix by its inverse, results in identity:
Multiplication by identity¶
Multiplying a matrix by identity, will result in the unchanged matrix:
When using a transform hierarchy, remember that matrix multiplication is reversed! To obtain the global transform for a hierarchy, do:
For 3 levels:
To make a matrix relative to the parent, use the affine inverse (or regular inverse for orthonormal matrices).
Revert it just like the example above:
OK, hopefully this should be enough! Let’s complete the tutorial by moving to 3D matrices.
Matrices & transforms in 3D¶
As mentioned before, for 3D, we deal with 3 Vector3 vectors for the rotation matrix, and an extra one for the origin.
Godot has a special type for a 3x3 matrix, named Basis. It can be used to represent a 3D rotation and scale. Sub vectors can be accessed as:
Or, alternatively as:
The Identity Basis has the following values:
And can be accessed like this:
Rotation in 3D¶
Rotation in 3D is more complex than in 2D (translation and scale are the same), because rotation is an implicit 2D operation. To rotate in 3D, an axis, must be picked. Rotation, then, happens around this axis.
The axis for the rotation must be a normal vector. As in, a vector that can point to any direction, but length must be one (1.0).
To add the final component to the mix, Godot provides the Transform type. Transform has two members:
- basis (of type Basis)
- origin (of type Vector3)
Any 3D transform can be represented with Transform, and the separation of basis and origin makes it easier to work translation and rotation separately.