"Little Planet" Photographs
From the University of Western Australia
Photography by Paul Bourke
May 2011
The making of ... and third place in the UWA Friends
of the Grounds Photographic Competition, 2011.
Appears on the cover of the UWA Friends of the Grounds Calendar, 2012
Centenary edition and 2013 calendar.
The making of ...
The images here are created using stereographic projections,
it is one of the many ways of mapping points on a sphere onto a plane.
Since a sphere and a plane are topologically different forms, the mapping cannot be performed
without some form of distortion. Stereographic projections, also called planisphere projections
have been employed by Hipparchus and documented by Ptolemy, it arises as a way of mapping
spherical data onto an image plane in a range of fields that include astronomy,
cartography, geology, and mathematics.
The first stage is the creation of a spherical projection, otherwise known as a
equirectangular projection of a sphere. There are many ways of doing this photographically,
they are mostly distinguished from each other by the final image resolution. The simplest is
with a 180 degree fisheye lens and SLR camera. Three images are captured each separated
by 120 degrees horizontally.
Since these three images capture the entire visual field, 360 degrees horizontally and 180
degrees vertically, they can be stitched and blended together to form a spherical
projection, as follows.
It is this image, mapped as a texture onto a sphere, that can be projected onto a
plane using a stereographic projection.
A stereographic projection involves selecting a focal point (normally along a vertical line
through the origin) and a plane which will become the projected image plane.
To determine where any point on the sphere maps to on the image plane, a ray is drawn
from the focal point through the point in question. Where this ray intersects the image
plane is the projected position.
In this case the focal point is the north pole of the sphere and the image plane is
tangential to the sphere and touches the south pole. This is illustrated below for two points
in the image, the top of the building roof and the yacht mast.
Stereographic projections preserve angles (conformal) but they do not preserve lengths (obvious
if one considers what happens to points towards the north pole) and it therefore follows
that it does not preserve area (isometric). The projection is smooth (no discontinuities), at
least for points on the sphere between the focal point and the image plane.
Written by Paul Bourke
June 2006
Fisheye images are generally created with a 180 degree field of view, that is,
the view with a hemisphere as the projection surface. However, the mathematics that describes
a fisheye can extend the angles to 360 (actually even further than that but it results
in a space replication).
The images below are the result of creating a fisheye projection with the camera looking
straight down and the fisheye angle is 360 degrees. The result is strangely compelling,
they can appear to be a planet on which extreme structures have been built.
Corresponding raw spherical panoramic image
The images here are captured using a 185 degree fisheye lens on a SLR camera. Three images
are captured each with the camera/lens rotated 120 degrees horizontally with respect to
the other images. These three fisheye images are stitched together to form a 360 degree
by 180 degree spherical projection. This image (also linked to for each example) is then
resampled to a 360 degree fisheye using locally developed software, namely sphere2fish.
Corresponding raw spherical panoramic image
There are some extreme distortions occurring, for example, the entire rim of the image
is in fact a single point corresponding to the north pole.
Corresponding raw spherical panoramic image
The images here are all from the University of Western Australia. The first is the South
side of Hackett hall, the second the North side, the last is the Reid Library.
Stereographic projection (Little Planet)
Another technique that gives similar results, but perhaps more striking, uses
stereographic projections.
Corresponding stereographic projections given below.
The utility that creates these converts spherical projections into stereographic
projections. The main variable is the size of the projection radius (t).
Usage: sphere2stereo [options] sphereimage
Options
w n width and height of the stereographic image, default = 512
t n stereographic radius, default = 4
a n antialiasing level, default = 2
o s output file name, name derived from input filename
A stereographic projection can be visualiased as a sphere, a line is drawn from
the north pole to each point on the sphere surface, for example P_{1} and
P_{2}. Where that line intersects a plane that touches the south pole, is the
position of the point on the stereographic projection plane, for example
P'_{1} and P'_{2}.
Rotomahana, New Zealand
