Here's my thoughts on SPHERICAL mapping:
The spec for SPHERICAL says:
"V is given by the angle between the vector formed by points 1 and 2 and the vector formed by point 1 and a world point that has been projected to the plane P2."
Note that P2 is the plane that goes through the north and south poles and through the line of longitude in the centre of the image.
I want to take an example world point and see how it maps to UV, given this definition.
Take a world point just above the equator at the edge of the hemisphere. It will be projected onto P2 just above the centre of the sphere, giving a vertical vector from point 1, which is at right angles to the vector formed by points 1 and 2.
In this way all points around the rim of the hemisphere have V=0 (above the equator) or V=1 (below the equator), according to the specs.
This makes the SPHERICAL mapping unsuitable for a full globe, which is unfortunate, but that seems to be what the specs say.
SPHERICAL mapping would be useful for smaller patches, e.g. an image on the surface of an old CRT television that has curved edges.
The spec for SPHERICAL says:
"V is given by the angle between the vector formed by points 1 and 2 and the vector formed by point 1 and a world point that has been projected to the plane P2."
Note that P2 is the plane that goes through the north and south poles and through the line of longitude in the centre of the image.
I want to take an example world point and see how it maps to UV, given this definition.
Take a world point just above the equator at the edge of the hemisphere. It will be projected onto P2 just above the centre of the sphere, giving a vertical vector from point 1, which is at right angles to the vector formed by points 1 and 2.
In this way all points around the rim of the hemisphere have V=0 (above the equator) or V=1 (below the equator), according to the specs.
This makes the SPHERICAL mapping unsuitable for a full globe, which is unfortunate, but that seems to be what the specs say.
SPHERICAL mapping would be useful for smaller patches, e.g. an image on the surface of an old CRT television that has curved edges.