Image boundary after rotation


Hi Guys!
I am trying to find keypoints in a rotated and then subsampled image using fastcorners. My code:

tfm = Translation((r/2)-1,(c/2)-1) ∘ LinearMap(RotMatrix(-theta)) ∘ Translation(-((r/2)-1),-((c/2)-1))
uR = warp(img1, inv(tfm), indices(img1))
uT = subSam(uR, axes(uF)[1][1], axes(uF)[1][end], t) #subsampling in vertical direction. So resultant image is no longer a rectangle but a parallelogram
kpts = Keypoints(fastcorners(uT, 12, 0.5))

Now I want to remove keypoints at the boundary of the rotated and subsampled image(i.e. something like d distance from the boundary of the distorted image, parallelogram).
Can you guys suggest something how I can proceed? kpts is storing the cartesian coordinates of the keypoints in the distorted image.