Mutability and immutability
According to the coding conventions section of the Kotlin guide, we should prefer using immutable to mutable data structures and declare local variables and properties as val rather than var if they are not modified after initialization.
To learn more about this topic, please take a look at the hello world, basic types and collections sections of the Kotlin Tour.
This should help you familiarize with the idea of mutable and immutable data in Kotlin.
OPENRNDR data structures
Vectors and Matrices
OPENRNDR makes extensive use of data types like Vector2, Vector3, Vector4, Matrix2x2, Matrix3x3, Matrix4x4 and Matrix5x5 for creating and drawing 2D and 3D shapes
Such data structures are immutable in OPENRNDR. That means that the following would not work:
val v = Vector2.ZERO
v.x += 0.1 // error: x is immutable
We could switch from val to var:
var v = Vector2.ZERO
but we still can’t modify the x component of that vector, because x is still immutable. The solution is to not try changing the components independently, but to assign a new value to the variable:
var v = Vector2.ZERO
v += Vector2(0.1, 0.0)
The same idea applies to matrices. For example:
var m = Matrix4x4.IDENTITY // One of the existing Matrix4x4 "presets"
m = m * 2.0 + 1.0 // Assign a new matrix with all components multiplied by 2.0 and incremented by 1.0
In a nutshell, don’t try changing individual properties of OPENRNDR objects but assign a new value to the variable instead.
The drawer state is mutable
One area in the framework that does have a mutable state is the drawer.
We can alter properties like drawer.fill, drawer.stroke, drawer.strokeWeight and doing so will affect any following shapes we draw.
One detail: drawer.fill holds a ColorRGBa variable. This variable has r, g, b and a components. But the same way we could not modify the x component of a Vector2, we cannot modify the r component of a ColorRGBa. We just need to assign a new value to the fill, instead of trying to modify the components of the color.
Mutating other OPENRNDR objects
A ShapeContour is a core OPENRNDR class that holds an open or closed contour made out of straight or curved segments.
Let’s imagine we have a circular contour made out of four curved segments. Could we mutate it? Could we animate the position of one of its vertices? Again, the same rule applies here. Contours, segments and vertices are all immutable.
If we want to animate and deform the circle, we need to recreate the contour on every animation frame. We could do this based on the seconds variable, based on the previous version of the contour, or based on some other changing data.
OPENRNDR provides many powerful tools to construct and deform contours. We only need to get used to the idea of working with immutable data.