Equirectangulars are a format used for representing full spherical panos. They're also known as cubics or 360°x180° panos. In essence, you can look straight up and down as well as all around. The kind of pano that has 360° coverage in yaw, but doesn't have a "top" or "bottom" is usually known as a "cylindrical", as the flat plane of the pano image wraps around like a cylinder in space.
An equirectangular remaps a sphere to a flat plane by simply using the longitude as the x-coordinate, and the latitude as the y-coordinate. You end up with a 2x1 rectangle that represents a sphere:
To get this kind of coverage, it's more convenient to use very wide-angle lenses. Most people who shoot these types of panos use circular fisheye lenses, which can give you a 180° field of view, so you can cover an entire sphere with only four images handheld. Or, if image quality is a priority, diagonal fisheyes or ultrawides that can cover the sphere in 8-20 shots. Here's an example of the member shots I made for a pano I took in the doorway of a used bookstore with my Canon XT and the Sigma 8mm circular fisheye:
I used a tripod and
Nodal Ninja 2 panohead to minimize parallax error, and rotated the six portrait shots at 60° intervals. Then took the zenith (straight up) and a handheld nadir (straight down) shot without the tripod and panohead for patching.
I stitched the six rotated shots and zenith shot in
PTMac since I'm on a mac (it is not, however, compatible with Snow Leopard, so I'd actually recommend looking at
PTGui or
Hugin instead if you want to get into this). And formed an equirectangular, with the tripod in the bottom of the shot. I then remapped the equirectangular out to six cube faces with
CubicConverter (
Pano2VR can do the same thing on all platforms), and patched the missing "hole" in the nadir with the handheld shot using layers and warping. In the end I had this:
This can then be used to create a QTVR Cubic, or uploaded to Flickr and
viewed through the Spi-V player. the Spi-V viewer can recognize and handle cylindrical and equirectangular panos and display them accordingly.
I can also remap the sphere another way (in this case, using a "mirrorball" remapping via
Flexify), and get a "little planet" image out of it.
