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A Note on “Parallel” versus “Central” Projection

According to Euclid, and, I suspect, to most people, absolute sizes do exist, and may be propagated by something called, “parallel projection”,

but,

if we accept the Caley/Klein proposal that

parallels are those co-planar lines that meet in a point at infinity
,

we see that
parallel projection is central projection still
—because it is projection from a point,
albeit at infinity.

So, in general, Projection of points by lines is Central,
with Parallel Projection just a special case of it.

Parallel projection is Central Projection from Infinity.

Geometries based on this speciality are called, “Affine”.

### It appears to work for angular measure too.

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