ppmglobe(1) - Man pages

Ppmglobe User Manual(1) General Commands Manual Ppmglobe User Manual(1)

NAME
ppmglobe - generate strips to glue onto a sphere

SYNOPSIS
ppmglobe [-background=colorname] [-closeok] stripcount [filename]

Minimum unique abbreviation of option is acceptable. You may use double
hyphens instead of single hyphen to denote options. You may use white
space in place of the equals sign to separate an option name from its
value.

DESCRIPTION
This program is part of Netpbm(1).

ppmglobe does the inverse of a cylindrical projection of a sphere.
Starting with a cylindrical projection, it produces an image you can cut
up and glue onto a sphere to obtain the spherical image of which it is
the cylindrical projection.

What is a cylindrical projection? Imagine a map of the Earth on flat
paper. There are lots of different ways cartographers show the three
dimensional information in such a two dimensional map. The cylindrical
projection is one. You could make a cylindrical projection by tracing
as follows: wrap a rectangular sheet of paper around the globe, touching
the globe at the Equator. For each point of color on the globe, run a
horizontal line from the axis of the globe through that point and out to
the paper. Mark the same color on the paper there. Lay the paper out
flat and you have a cylindrical projection.

Here's where ppmglobe comes in: Pass the image on that paper through
ppmglobe and what comes out the other side looks something like this:

Example of map of the earth run through ppmglobe

You could cut out the strips and glue it onto a sphere and you'd have a
copy of the original globe.

Note that cylindrical projections are not what you normally see as maps
of the Earth. You're more likely to see a Mercator projection. In the
Mercator projection, the Earth gets stretched North-South as well as
East-West as you move away from the Equator. It was invented for use in
navigation, because you can draw straight compass courses on it, but is
used today because it is pretty.

You can find maps of planets at ]8;;http://space.jpl.nasa.gov\space.jpl.nasa.gov]8;;\ .

PARAMETERS
stripcount is the number of strips ppmglobe is to generate in the out-
put. More strips makes it easier to fit onto a sphere (less stretching,
tearing, and crumpling of paper), but makes you do more cutting out of
the strips.

The strips are all the same width. If the number of columns of pixels
in the image doesn't evenly divide by the number of strips, ppmglobe
truncates the image on the right to create nothing but whole strips. In
the pathological case that there are fewer columns of pixels than the
number of strips you asked for, ppmglobe fails.

Before Netpbm 10.32 (February 2006), instead of truncating the image on
the right, ppmglobe produces a fractional strip on the right.

filename is the name of the input file. If you don't specify this, ppm-
globe reads the image from Standard Input.

OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see ]8;;index.html#commonoptions\ Common Options]8;;\ ), ppmglobe recognizes the
following command line options:

-background=colorname
This specifies the color that goes between the strips.

Specify the color (color) as described for the ]8;;libnetpbm_image.html#colorname\argument of the
pnm_parsecolor() library routine]8;;\ .

The default is black.

This option was new in Netpbm 10.31 (December 2005). Before
that, the background is always black.

-closeok
This means it is OK if the background isn't exactly the color you
specify. Sometimes, it is impossible to represent a named color
exactly because of the precision (i.e. maxval) of the image's
color space. If you specify -closeok and ppmglobe can't repre-
sent the color you name exactly, it will use instead the closest
color to it that is possible. If you don't specify closeok, ppm-
globe fails in that situation.

This option was new in Netpbm 10.31 (December 2005).