A panoramic projection is a method used to convert the panoramic view at a particular point in space into a planar (flat) image. We can imagine the panoramic view to be painted on the inside surface of a sphere where the camera is at the center of the sphere. A projection flattens the surface of the sphere to make the image. Interestingly, this is the same process used to make flat maps from the curved surface of the earth. The projections used in panoramic photography are relatives of the projections used in cartography (mapmaking).
Conventional cameras make photographs that are in perspective projection (also called rectilinear projection). Unfortunately, two perspective projections cannot be merged without creating discontinuities in the final image. Therefore, in order to merge your individual photographs into a seamless image, The Panorama Factory must change the projection to one that can be merged seamlessly.
The Panorama Factory works with several different types of projections. The stitcher works internally with images in either spherical projection (also called equirectangular projection) or cylindrical projection depending upon options you choose on Wizard step 3/9 – Describe your camera or on the Camera properties dialog box.
When exporting to QuickTime QTVR format, The Panorama Factory can automatically convert an image to cubic projection. Actually, The Panorama Factory supports three different variations of cubic projections.
Finally, The Panorama Factory can convert an image in spherical, cylindrical or cubic projection back to a conventional perspective projection. This is done through the Crop command (New image menu) because perspective projection cannot represent more than about 120-140 degrees field of view. You choose the particular portion of the panorama to convert by cropping the image.
Swing-lens cameras (e.g. Noblex, Widelux, Horizon) and rotational cameras (e.g. Cirkut, Roundshot, Panoscan) produce cylindrical projections. In these cameras, the film is effectively bent into a cylinder within the camera when the film is exposed.
Spherical (equirectangular) projections can't be produced in a physical camera because the film would need to be bent in two axes at the same time. That is, the film would have to be bent onto a section of a sphere.
Certain VR image viewers may require a particular type of projection or may accept more than one type. Each type of projection produces its own characteristic distortions. When preparing images for printing or for online display as an image file only, the choice of projection is largely an esthetic choice.
The following sections show examples of each type of projection supported by The Panorama Factory and explain the specific characteristics of each one that are important for panoramic photography.