In our case, we will assume our painting is of the interior of an open square room with all sides parallel or perpendicular to the picture line. The top of the room is red, the back is green, and the bottom is yellow. The applet on the right allows you to change the position and size of the room as well as the artist's viewpoint. In addition to the image on the y-axis, another copy of the painting is shown on the right of the room.
Note that we assume the opening to the room lies on the y-axis - this is for our convenience only. If the opening was further back from the picture line, we could apply a linear transformation (treating the artist's viewpoint as the origin) scaling the object towards the artist's viewpoint until the opening was on the y-axis. See the picture above for an example. The resulting painting would be the same.