Projection Models

Overview

Here we introduce the two major projection models used by PyTorch3D:

• The perspective projection

• The orthographic projection

The projection models

Projection models essentially describe yet another coordinate space transformation. They provide a complete picture of how we transform 3D world space objects into 2D image space renders. They dictate mathematically how light passes through the 3D world onto the image plane. We’ll talk primarily about perspective projection and orthographic projection.

The difference between perspective projection and orthographic projection essentially comes down to differences in the orientation of light rays that make up the image plane. In the perspective model, all rays of light that enter a camera cross over at the same point in 3D space, the optical center (recall that, in reality, they actually pass through a disk-shaped aperture rather than a single point). That means that the vector that passes through each ...