2D Transformations
What is meant by a 2D geometric transformation?
- The following transformations can be represented (in 2D) using multiplication of a 2x2 matrix
- rotations (about the origin)
- reflections
- enlargements
- (horizontal) stretches parallel to the x-axis
- (vertical) stretches parallel to the y-axis
What are the matrices for geometric transformations?
- Rotation
- Anticlockwise (or counter-clockwise) through angle θ about the origin
- This is given in the formula booklet
- Clockwise through angle θ about the origin
- In both cases
- θ > 0
- θ may be measured in degrees or radians
- Anticlockwise (or counter-clockwise) through angle θ about the origin
- Reflection
- In the line
- This is given in the formula booklet
- θ may be measured in degrees or radians
- for a reflection in the x-axis, θ = 0° (0 radians)
- for a reflection in the y-axis, θ = 90° (π/2 radians)
- In the line
- Enlargement
- Scale factor k, centre of enlargement at the origin (0, 0)
- Scale factor k, centre of enlargement at the origin (0, 0)
- Horizontal stretch (or stretch parallel to the x-axis)
- Scale factor k
- Scale factor k
- Vertical stretch (or stretch parallel to the y-axis)
- Scale factor k
- Scale factor k
How do I find the matrix of a 2D transformation?
- Let the transformation matrix be
- The image of the point (x, y) after the transformation is (x', y') which can be found by:
- You can find the values of a, b, c, d by seeing where the points (1, 0) and (0, 1) are transformed
- (1, 0) is transformed to (a, c)
- (0, 1) is transformed to (b, d)
- (1, 0) is transformed to (a, c)
Worked example
Triangle PQR has coordinates P(-1, 4), Q(5, 4) and R(2, -1).
The transformation T is a reflection in the line .