Analysis of the -NADIR-SCENE-PXPYD[-NULLCAM] dumps (graveyard ref
1763232420_274928) showed the transformFromVirtual() refinement output is
accurate to <0.3 px in the interior, but produces garbage coordinates
(5..100 px off) in a rim band at the edge of the valid region, where the
interpolation works one-sided over NaN-filled disparity. Rendered, those
tiles put wrong content in a border strip: COLMAP fit it as fake barrel
distortion (f=1000, k1=-0.38) and image-based rotation measurements were
contaminated (apparent ~0.6x roll).

A correct scene-frame grid position is the tile center + small ERS
displacement, so tiles straying more than 4 px are nulled and render as
NaN instead of wrong content, slightly shrinking the usable FOV at the rim.
Co-authored-by: Claude <claude@elphel.com>