Panorama Viewers
The awareness and use of panoramic images is increasing. While, because of their unique aspect (width-to-height) ratio, they naturally make perfect web masthead banners, other popular applications for these photos and ways to view them are increasing. Realtors now use panoramic photos with viewers on their websites to advertise specialty properties. Prospective buyers visit the website and, using their own mouse, navigate a "viewer" to scan a panoramic view of a property or home in any direction they wish - adding a dimension of reality never before possible.
Panographers are interested in finding new ways to present the panoramic experience over the web. The most common approaches include viewers based on Java, Quicktime (QTVR), or Flash.
The most common type of image for panoramic viewing is either a rectilinear or a gnomonic projection of a spherical field of view into the viewer window. This compares to how non-fisheye camera lenses present images and it's the way we are most comfortable viewing images. The rectilinear projection differs from the gnomonic projection in that it only shows a horizontal, midline portion of the virtual sphere of vision. The gnomonic project encompasses the entire sphere and allows tilt up to the zenith (straight up) and down to the nadir (straight down). The overall goal, is to give the viewer the illusion of being "immersed" in the scene.
An important feature of all panorama viewers is to provide the viewer the ability to navigate throughout the scene. The viewer software needs to continually remap the source image into the current view as the user pans, tilts and also zooms in and out.
The source image is usually one three types: cylindrical, cubic, or equirectangular.
A cylindrical projection of points on a unit sphere centered its equator consists of projecting points from the spheres center until they intersect a cylinder tangent to the sphere at its equator.
A cubic projection maps the spherical image onto the inside of a cube, which has 6 facets. four in the horizontal pan directions, one at the zenith, and one at the nadir.
An equirectangular projection is a cylindrical equidistant projection, also called a rectangular projection, plane chart, plate carre, or unprojected map, in which the horizontal coordinate is the longitude and the vertical coordinate is the latitude.
Of the three, the cylindrical projection can't represent the entire spherical field of view, it ignores the zenith and the nadir, so it cannot provide the fully "immersive" pano experience where you can look up, down and all around. A cylindrical pano view allows only left and right panning navigation and often a little tilt. However, because it is relatively easy to implement a cylindrical pano viewer, these projections are very common.
The cubic projection is used in Quicktime VR. You can often see the cubic grid displayed as the image loads.
The equirectangular projection maps the entire sphere in a single rectangular image. Each unit of distance represents one degree of latitude or longitude. The image width represents the entire 360 degrees of longitude, and the height the 180 degrees of latitude, so the image aspect ratio is always 2:1. The entire top edge maps to the north pole and the entire bottom edge to the south pole.
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