L' unico "aiuto" che ho trovato sono queste risposte....
a)If the transformed point, whose expression is x' = Hx coincides with x, then, since we are in homogeneous coordinates, kx = Hx. That is an equation at eigenvectors and eigenvalues. Then the fixed points – the invariants for the homography H – are H’s eigenvectors. Since H is 3x3 square, there are in general 3 eigenvectors, that is, 3 invariants, that is 3 fixed points.
b) A planar isometry is a rotation around a specific center of rotation. Then the center of rotation is invariant. The other two invariants are the circular points (they belong to the infinite line).
c) L = THT-1
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d) Since the center or rotation and the circular points do not change because of the rigid displacement H, then also their images do not change. Since the two circular points define the infinite line, the infinite line and its image are invariant as well (but note that other points on the infinite line are not invariant as they are transformed to different points on the infinite line). This can also be found analytically.