Make a symmetrical image from one half

Mirror-half symmetry is the same reflection matrix applied to a region rather than a frame. You choose the axis the line runs along, which half of the picture is the source, where the line sits, and how wide a blend to run across the join. The kept half is drawn untouched; the other half is that half reflected about the line.

Four combinations exist and all four are here: an upright line keeping the left, an upright line keeping the right, a horizontal line keeping the top, and a horizontal line keeping the bottom. Nothing about the line is snapped to the centre, and the readout under the controls always states the frame the export will actually have, because a symmetry that changes the dimensions of the file is a surprise worth removing.

1. 1Choose the line and the half. Upright or across, then which side is the source. This page opens in symmetry mode with an upright line down the centre, keeping the left.
2. 2Move the line. The slider drives the line as a percentage of the frame. Dragging it redraws the reduced preview on every frame; letting go is what records the position and announces the new size.
3. 3Feather if the join shows. Zero to eight pixels. Start at zero and only add feather if you can see the seam at full size, because a blend on a hard edge just looks like a smear.
4. 4Pick a frame and save. As opened, or twice the kept half. Then Save mirrored file.

The intake is the same on every page of this site, and so is the export. What is different here is that the output frame can legitimately differ from the input frame, so an export in twice-the-half mode is a genuinely new size — the readout says so before you press save, and the saved-file note repeats the dimensions afterwards.

• A symmetrical face is a different face, not a truer one. This is the most common use of the operation and the one most likely to be misread. A mirrored portrait is an invented person built from one side of a real one. It is fine as an image and worthless as a measurement.
• The join is only hidden, never solved. Feathering blends the reflected pixels into the real ones across a few pixels. It does not reconstruct anything, and on a subject with directional texture — hair, brickwork, brushed metal — the reversal of that direction stays visible on both sides of the blend however wide you make it.

Why does a mirrored face look wrong?

Because it is not the face. Human faces are asymmetric by a few millimetres in every feature, and a mirrored half doubles whichever set of those deviations it kept while deleting the other. The result is a plausible face that nobody has, and the effect is stronger the better you know the person. It is a legitimate thing to make and a poor way to answer the question of whether someone is symmetrical.

Does the mirror line have to be in the middle?

No. The line can sit anywhere across the frame, and moving it is usually the difference between a composition and a curiosity. A subject that was never centred does not become centred by being reflected about the centre of its frame.

What does the feather control actually do?

It cross-fades the reflected half into the real pixels over up to eight pixels at the join. With the feather at zero the two halves meet exactly and the values match at the seam itself; what gives the seam away is the sudden reversal of texture direction on either side of it. A few pixels of blend hides that on fabric, foliage and skin, and is worth nothing at all on a hard edge.

What happens on the far side when the line is off centre?

In the frame you opened, the reflection covers the same distance on the far side as the kept half occupies on the near side. If the line sits well off centre there is a strip the reflection cannot reach, and the original pixels stay there. That is deliberate. Repeating the reflection to fill the strip would produce a pattern rather than a symmetry, and pattern-making is not this tool.

Which output frame should I pick?

As opened keeps the dimensions the file arrived with, which is what you want if the picture has to drop into a layout that already exists. Twice the half produces a frame exactly two of the kept halves wide, which is what you want if the symmetry is the picture and the original crop was incidental.