--rotate-y

--rotate-y accepts a floating-point value in degrees and rotates every loaded object around its Y axis by that amount before slicing. The conversion from degrees to radians is performed internally; the caller always supplies degrees. There is no enforced range: negative values rotate in the opposite direction, and values beyond 360 wrap naturally through the full rotation. The default, when the flag is omitted, is 0 (no rotation applied).

The rotation is applied at the volume level, not the instance level. This is a meaningful distinction for 3MF inputs: the transform is baked directly into the object geometry rather than being recorded as a placement offset on an instance. If the 3MF already carries instance-level placement transforms, those are preserved and the volume-level rotation is compounded on top. This can produce unexpected orientations if the source 3MF was authored with non-identity instance offsets.

--rotate-y is one of three axis rotation flags, alongside --rotate (Z axis) and --rotate-x (X axis). When multiple transform flags appear together, they are applied in the order they appear on the command line. Y-then-X produces a different final orientation than X-then-Y for any non-axis-aligned geometry, so the sequence is significant when combining flags. After all transforms finish, --ensure-on-bed (if active) translates any object whose geometry has dropped below Z=0 back up to the bed surface, so a rotation that sinks part of the model below the bed plane does not itself cause a permanent problem.