This article was first posted on 360 Cities site in 2008 by David Martin, Jeffrey Martin and the 360 Cities team. The information is still interesting and relevant so let me repost it here. I’ve left most of the text exactly the same but I changed some of the links and inserted comments in italics where the information was not accurate anymore. -Jan
You are probably already familiar with medium of 360 degree panoramic images from either Google Street View or KML PhotoOverlays in Google Earth (from the Gigapxl, Gigapan, or 360 Cities layers). Tools for authoring panoramic images are getting very good, but the process of preparing and presenting such images on the web is still difficult. That is part of the motivation behind the creation of 360cities.net: panoramic photographers use our site to replace the otherwise difficult and time-consuming process of publishing their geotagged panoramas to the web.
You can turn on the 360 Cities gallery layer in Google Earth to view panoramas from our system (Update: 360 Cities is now a part of the default layer in Google Earth). The screenshots below show how these panoramas appear in Google Earth. From top left to bottom right, these images show (1) the 360 Cities icons marking the locations of panoramas; (2) the info window that opens when you click on one of those icons; (3) the floating sphere that appears when you get close to one of the images, and (4) the view from inside the spherical image. In this article we will demonstrate our technique of creating these spherical panoramic PhotoOverlays for Google Earth.
Google Earth Screenshots (click to view larger images)
What Is a Spherical Panoramic Image?
A panoramic image is the ultimate wide angle image. A normal photo shows you what the world looks like in an instant in time from a single viewpoint over some limited field of view. The widest wide angle lens, a fisheye lens, can show you up to a 180 degree field of view, which covers half the viewing sphere (Update: there are now lenses that can cover more than 180 degrees). A spherical panoramic image, on the other hand, covers the entire viewing sphere, so that you can look in any direction. A spherical panoramic image is not a 3D image: It is still a single viewpoint image, but its field of view is not limited. (Update: There are now so-called one shot cameras that cover the whole 360 degrees. They are great for taking spherical photos without any knowledge of stitching, but their image quality is still mediocre).
Because a panoramic image shows you what the world looks like from a single viewpoint, it is shaped like a sphere. Computers aren’t so good at representing spheres, however, so panoramic images are stored as regular rectangular images. One could use any cartographic mapping to unwrap the sphere onto a rectangle. The most common representation used for panoramic images is the equirectangular projection, which looks like this:
In an equirectangular projection, the equator and all lines of longitude are undistorted. You can picture the projection as follows: Prick a hole in the north and south poles of the sphere, and cut the date line. Now unwrap the sphere onto a flat surface by stretching at the poles without stretching the lines of longitude so that the image forms a rectangle. In an equirectangular projection, the lines of longitude and latitude form a grid of equal sized squares. The equator is the central horizontal line; the north pole is all along the top, and the south pole all along the bottom. Distortion is minimal at the equator, and infinite at the poles. The aspect ratio of an equirectangular image is 2:1, because the equator is twice the length of each line of longitude.
One does not typically view the equirectangular image, or “equirect”, directly because of the severe distortions in the polar regions. The equirect is simply a means of storing the underlying spherical image. Special software can render an equirectangular image into an immersive panoramic experience. Click on the image above or below to view it in the 360 Cities system. Make sure to click and drag on the image at 360 Cities to pan; shift and control keys zoom in and out.
Panoramic images provide a compelling immersive experience that cannot be matched by traditional imagery. There exist good tools for creating and rendering these images, but managing and publishing them yourself is difficult and tedious. 360 Cities is a platform that fills that crucial gap in the tool chain. Not only does 360 Cities make embedding high resolution panoramic images on the web a snap, the image may be viewed in Google Earth as well. Now you have an excuse to buy that cool fisheye lens…