Types of panoramic images
There are various types of panoramic image and various buzz-words used to describe them and the image formats used.
Planar or flat
|A planar or flat panorama|
Conventional photographs may be described as "flat" or "planar", meaning that they are intended to be viewed as is without any perspective correction.
|A cylindrical panorama|
A cylindrical panorama is intended to be viewed as if curved around the inside of a cylinder. If you view the image flat, you will see curves in the horizontal axis.
There are many Java applets and other viewers for displaying panoramic images on the Internet which are designed to display cylindrical panoramas as flat, i.e. without correcting perspective by displaying the image as if you are looking at it inside a cylinder. These viewers display the horizontal curves.
|A spherical panorama|
Spherical images are typically represented in equirectangular format image files, which represent exactly 360 degrees on the horizontal axis and 180 degrees on the vertical. These images are intended to be viewed as if projected using a special slide projector onto the inside of a sphere. Like cylindrical images, horizontal curves can be seen in the flat image, but unlike cylindrical images, the very top and bottom of the image will seem squashed. For panoramas with a large vertical field of view, the equirectangular (spherical) image format is far more efficient than cylindrical, but if you intend to print your images out, cylindrical images look better.
In practice any panoramic image can be stored with spherical projection in mind, meaning that it is intended to be displayed on the inside of a sphere... but the panoramic image does not need to be 180 degrees tall or 360 degrees wide. However if the image is less than 360 degrees wide, either the image must be padded with empty pixels, or the actual horizontal field of view needs to be noted down and provided to the display software so that the image is displayed correctly. ISeeMedia's PhotoVista software is an example of panoramic software that uses the spherical format for partial panoramic images.
|A cubic panorama|
Cubic images were made popular by QuickTime 5, which introduced the possibility of cubic panoramas in which the viewer can look straight up and straight down at the floor. (Of course you can do exactly that with spherical panoramas too.) Unlike cylindrical and spherical images, cubic images contain 6 separate cube faces, which are in themselves planar images, which look normal to the human eye (no horizontal curves or perspective distortion). Cubic is the most efficient format for panoramic images with a vertical field of view in excess of 140 degrees (see below), and because each of the cube faces looks "normal" these images can be easier to edit or modify with image editing software.
Panoramic image type and file size
A spherical panorama can be represented using an equirectangular image to create a sphere, or the same thing can be achieved if the image is converted to a cube and displayed as a cube... but the cube is more efficient for storing the image information (assuming the same image compression is used). This and a comparison with the cylindrical format is shown below. The image size given is for an image that is 360 degrees wide and has a horizontal dimension of 3600 pixels.
|Image file sizes depending on image projection|
Note that spherical/equirectangular has an advantage over cylindrical from quite a small vertical field of view upwards, assuming the image is cropped to the visible vertical field of view (it is in most panoramic software). Since cubic involves storing all 6 faces of a cube, no matter what size the field of view is, the image size is constant.
This chart doesn't show is the effect of compression on the image. QuickTime 4 allows only for cylindrical images to be projected, but it supports a large number of image and video codecs. Most other formats use JPEG image compression. The real image size is as much a matter of cleverly compressing the image as it is choosing the most appropriate image format.