Affine transformation - under which circumstances, is it applicableto photo images

Hello,

Under which circumstances, would affine transformation be applicable
to undistort an image?
I know that projective transformation is the correct approach to
undistort an image from a pinhole camera. However, I find that affine
transformation is frequently used.
Affine transformation, preserves parrallel lines which avoids
correcting projective distortions that are usually seen in an
photograph image (e.g. parallel lines meet at the horizon).
Would the approximation be applicable if, e.g. the perspective
distortion is samll (e.g. the horizon is located far from the photo's
boundaries)?
What are other conditions that make the approximation of affine
transformation applicable?

Regards,
Avi

 

You can use an affine transformation (actually, just a simple
rotation) to correct tilt, and of course to crop and/or scale the
image.

Also, a projective transformation can be locally approximated by an
affine one, so I suppose one could use an affine transformation to
correct perspective distortion in a small part of the image without
significant errors. However, the situations where this would actually
be useful seem rather limited to me: in most cases either the error
would be noticeable or the approximate transform itself would be close
to identity. Easier to just do a proper projective transformation,
IMO.