FA-79062 / Image orientation metadata / Member archive

Lossless mirror pads the width instead of trimming it · case 02

Mirrored JPEGs gain a strip of garbage pixels on the left edge.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Input [w, h, tag, sampling]. A lossless JPEG transform that bakes the orientation can only move whole MCUs (444: 8x8, 422: 16 wide x 8 high, 420: 16x16, 440: 8 wide x 16 high), so any partial MCU column or row that would move to the leading edge is trimmed. The tag decomposes into stored-space flips (fx, fy) followed by an optional transpose; fx trims the width and fy trims the height to a multiple of the MCU size. Invalid tags are treated as 1. Return the displayed [width, height], "refuse" when a trimmed side becomes 0, or "unknown-sampling".

Why this case matters

Camera, phone and scanner images carry an orientation hint separately from the stored pixels; galleries, thumbnailers, editors and upload pipelines must interpret it consistently or photos appear sideways, mirrored or doubly rotated.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: partial MCU trimming direction 0	`[2944, 16]`	`"refuse"`	Failed

MEMBER ARCHIVE

The complete case is available to members.

This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

Member access is invitation-based. Sign in with your invited account to inspect the sources.