I get it! re: fast rotate vs slow rotate. It's the 90 (right angle, N*PI/2) degrees. so the fast one is just transposing the image pixels where the slow one does all sorts of averaging and interpolations. (Ref: rotations primer from leptonica library ). Imager could either warn user for using the slow rotate for right angles or just ignore user's wishes and do it the fast way. thanks, very enlightening.