This vast ceiling fresco painted in the 1690's by Padre Andrea Pozzo on the nave vault of St. Ignazio, is considered the high point of illusionism, or "fool-the-eye" painting.
The ceiling celebtrates the "Missionary Work of the Jesuits Throughout the World" , (also sometimes called the "Entrance of St. Ignazio into Heaven").
The fresco is painted on a simple barrel vaulted ceiling, but its illusionism creates the effect of the ceiling soaring at least a story above the tops of the windows in the actual church interior.
If the visitor stands on a point marked with a bronze disk on the floor of the nave , the illusionism all comes together in a cohesive way. This image was shot from that viewpoint.

A dome was originally planned for the crossing of the church, but objections to its height blocking the light of neighboring buidings caused the project to be scrapped,
and a canvas cover on the ceiling is painted to look like the interior of the dome, again an illusionistic "trick".