Reflect the real edge into the space outside it

Stretching makes every object wider because the original frame is resampled to the target. Reflection leaves the photographed area alone and manufactures only the missing strips. A face in the middle keeps its proportions, text keeps its letterforms and the source remains pixel-for-pixel sharp.

The trade is symmetry. A mirror produces a continuation with the right colour and local texture but the wrong world geometry. That is almost invisible in ripples, leaves and soft architecture; it is immediately obvious in words, faces and repeated poles.

Look first at long straight lines. A horizon arriving at the edge returns in the opposite direction, making a shallow V. Next inspect unique objects close to the boundary. If either failure is visible, move the anchor so a quieter edge supplies more of the added area, choose blur, or use the on-device continuation model for a genuinely new edge.

Keep the feather close to zero when the boundary already joins cleanly. Increase it only until the seam stops catching the eye; the control is a repair for the join, not a style filter.

What part of the photograph is reflected?

Only the strips beside the new space. The original stays in the centre at its original dimensions; the left edge is turned outward into the new left area, the right edge into the right area, and the same rule applies above and below.

Why does a repeated face sometimes appear?

Reflection cannot know which edge pixels are background and which belong to the subject. If a face, hand, letter or logo touches the boundary, that object is mirrored too. Mirror fill is strongest when the outer strips contain texture without a recognisable object.

What does the feather control change?

It softens the line where the reflected strip meets the real photograph. A small feather hides compression noise and tiny colour steps; a large one can make a halo. It does not blur the whole new area.

When is mirror fill better than blur fill?

When the edge contains directional detail that should continue to look crisp: water, sand, foliage, curtains, walls or an abstract texture. Blur is safer around people and lettering because it does not create a recognisable duplicate.