A projection maps 3D camera-space points to a 2D image.

In orthographic projection, depth does not change apparent size. Parallel lines remain parallel, making the projection useful for technical views and measurements.

In a pinhole perspective projection, similar triangles give image coordinates proportional to

$$x' = f\frac{x}{z},\qquad y' = f\frac{y}{z},$$

up to the coordinate and sign convention used. Objects farther from the camera therefore appear smaller.

Graphics pipelines express perspective using a homogeneous projection matrix. After the matrix multiplication, coordinates are divided by the homogeneous component $w$ in the perspective divide.

A practical perspective camera also defines a field of view and near and far clipping distances. Together these bounds form a viewing frustum.

Projection changes representation rather than physically moving the object: modeling and view transforms place geometry in camera space; projection determines how that geometry appears on the image plane.