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The affine camera becomes a weak-perspective camera when the
rows of form a uniformly scaled rotation matrix. The
simplest form is
This is simply the perspective equation with individual point
depths replaced by an average constant depth
The weak-perspective model is valid when the average variation of the
depth of the object along the line of sight is small
compared to the and the field of view is small.
We see this as follows.
Expanding the perspective projection equation using a Taylor series,
only the zero-order term remains
giving the weak-perspective projection. The error in image position
showing that a small focal length , small field of view
and ) and small depth variation
contribute to the validity of the model.