2D coordinate transformations in Lua using matrices
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affine.lua
readme.md

readme.md

Affine

Affine is simple 2D transformation library for Lua.

To load the library: a = require 'affine'

Transformations

All transformation functions return a table T that stores the 3x3 transformation matrix. Where i is the i-th row, and j is the j-th column, The value at (i,j) is stored in T[i][j]. The table T can be called as a function to transform a given point (x,y): x2,y2 = T(x,y)

Below each transformation function are the transformation formulas.


Translation

Return a table that stores the translation transformation.

T = a.trans(dx,dy)

x2 = x + dx
y2 = y + dy

Scaling

Return a table that stores the scaling transformation. Scaling is with respect to the origin.

T = a.scale(sx,sy)

x2 = x * sx
y2 = y * sy

Rotation

Return a table that stores the rotation transformation. Rotation is in radians with respect to the origin.

T = a.rotate(theta)

x2 = x*math.cos(theta) - y*math.sin(theta)
y2 = x*math.sin(theta) + y*math.cos(theta)

Shearing

Return a table that stores the shearing transformation.

T = a.shear(kx,ky)

x2 = x + kx*y
y2 = y + ky*x

Inversion

Return a table IT that has the inverse transformation of T.

IT = a.inverse(T)

Example:

T	= a.shear(kx,ky)
IT	= a.inverse(T)

x2,y2 = IT(T(x,y))

print(x2,y2) -- should be the same as x,y

Transformation Operators

You can compose transformations into a new transformation by using the *,/, or ^ operator. To get the inverse transformation, you can do: IT = T^-1 Negative, and non-integer powers are undefined. For division: A/B is the same as A*B^-1

Order of operation matters for multiplication and division!

Example:

T1 = a.trans(10,10)
T2 = a.scale(-1,-1)
T3 = T1 * T2

x2,y2 = T3(10,10)

print(x2,y2) --> 0 0

T3 = T2 * T1

x2,y2 = T3(10,10)

print(x2,y2) --> -20 -20 // order of operation matters!

Conversions

The following functions can be used to transform between polar and cartesian coordinates. Angles are in radians measured from the +x axis.

Convert from cartesian to polar coordinate:

r,theta	= a.polar(x,y)

Convert from polar to cartesian coordinate:

x,y = a.cart(r,theta)