The Identity Matrix
You know that you can multiply any number by 1 and get the original number. The number 1 is called the multiplicative identity for that reason. Well, the identity matrix is like the number 1, but for matrices. If you multiply any matrix or tuple by the identity matrix, you get back the matrix or tuple you started with.
This may sound utterly pointless right now, but consider this: if multiplying by the identity matrix just returns the original value, it means you can use it as the default transformation for any object in your scene. You don’t need any special cases to tell the difference between a shape with a transformation and a shape without. This is, in fact, exactly what you’ll use it for when you get to Chapter 5, ...
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