Linear coordinate transformations (§3.5)

A linear coordinate transformation of a function is a composite of that function with one or more non-constant linear functions.

For example, if f(x) = x² for all x (that is, f is the squaring function) and g(x) = x + 1 (that is, g is the linear function with rate of change and initial value both 1), then both f ∘ g and g ∘ f are linear coordinate transformations of f.

In particular, (f ∘ g)(x) = (x + 1)²; this is called a passive or inside coordinate transformation. On the other hand, (g ∘ f)(x) = x² + 1; this is called an active or outside coordinate transformation.

More generally, instead of a non-constant linear function, we could use any invertible function with a sufficiently large domain or range. (A non-constant linear function is always invertible, and its domain and range always consist of all real numbers.)

Graphs of transformed functions

Starting from a graph of the original function, it's easy to graph a linear coordinate transformation of it. The key principles are these:

A coordinate transformation outside the function acts vertically, while a coordinate transformation inside the function acts horizontally;
Adding and subtracting shift the graph, while multiplying and dividing change the scale;
Anything inside (horizontal) acts backwards.

More concretely, consider these examples:

Coordinate transformation of f:	Effect on the graph:
f(x) + 1,	Shift 1 unit upwards;
f(x) − 1,	Shift 1 unit downwards;
2f(x),	Stretch vertically by a factor of 2;
f(x)/2,	Compress vertically by a factor of 2;
−f(x),	Flip vertically across the horizontal axis;
−2f(x),	Flip and stretch vertically;
2f(x) + 1,	Stretch vertically and then shift upwards;
f(x + 1),	Shift 1 unit to the left;
f(x − 1),	Shift 1 unit to the right;
f(2x),	Compress horizontally by a factor of 2;
f(x/2),	Stretch horizontally by a factor of 2;
f(−x),	Flip horizontally across the vertical axis;
f(−2x),	Flip and compress horizontally;
f(2x + 1),	Shift to the left and then compress horizontally;
2f(x + 1),	Stretch vertically and shift to the left, in either order.

Go back to the course homepage.

This web page was written between 2010 and 2015 by Toby Bartels, last edited on 2015 October 30. Toby reserves no legal rights to it.

The permanent URI of this web page is http://tobybartels.name/MATH-1150/2016SP/transformations/.