Projection Matrix -- where does it come from?

1. What makes a projection matrix?
2. Is projection a matrix identity?
3. What is a projection matrix graphics?

What makes a projection matrix?

In linear algebra, a projection matrix is a matrix associated to a linear operator that maps vectors into their projections onto a subspace.

Is projection a matrix identity?

[A projection matrix is hardly ever invertible. However, when it is invertible, it is the identity matrix. In that rare case, the inverse is a projection matrix.] (e) A nonsingular projection matrix is the identity matrix.

What is a projection matrix graphics?

Projection describes the transformation of a three-dimensional point into a two-dimensional point. This transformation can be represented by a projection matrix, which may encode both perspective, like a camera's focal length, as well as the transformation to normalized device coordinates (NDC).