Moving pixels, and the straight line that gives it away

What a warp actually is

Liquify, Puppet Warp, Free Transform’s Warp and Perspective Warp all do one thing: they build a displacement map, a grid saying where each pixel should move to, and then resample the image through it.

No pixels are created and none are destroyed; they are relocated and interpolated between. That is why a warp softens whatever it stretches (the stretched region is interpolated across more pixels than it originally occupied) and why an area compressed by a warp becomes slightly sharper and can alias.

The tools, and what each is for

Liquify (Filter > Liquify) gives you brushes that push the mesh around.

• Forward Warp pushes pixels in the direction you drag. The main tool.
• Pucker and Bloat contract and expand around the brush centre.
• Freeze Mask paints a region the warp cannot touch. This is the control that makes Liquify usable: freeze the background before you move anything near it, and the straight lines behind your subject survive.
• Reconstruct brings a region back towards the original.

Apply Liquify to a smart object and it becomes a re-editable smart filter: reopen it a week later with your mesh intact. GIMP’s equivalent is the Warp Transform tool (Tools > Transform > Warp Transform), which has the same brushes including a Freeze-like erase mode. Krita has a Liquify mode inside its Transform tool. Photopea has Liquify.

Puppet Warp places pins and lets you move them, with the mesh deforming smoothly between. Correct for bending an arm, straightening a drooping flower, adjusting the curve of a strap. Pin the parts that should not move first; most Puppet Warp disasters are from too few pins.

Perspective Warp lets you define quads on planes in the image and then adjust them, which is how you change the apparent camera angle on a box or straighten a building face.

Free Transform > Warp gives a simple grid, good for wrapping a label round a bottle or fitting artwork to a curved surface.

The uses nobody argues about

• Straightening a label that was applied crooked on the sample.
• Fixing the stretch at the frame edge. A wide lens stretches anything near the corner: a face photographed at the edge of a group shot is wider than it is. Correcting that restores the person’s real shape rather than altering it.
• Adjusting a fold of fabric so a garment hangs as it would on a person.
• Closing a gap where a badly fitted lid or a twisted strap shows on a sample.
• Matching two composited elements whose perspectives disagree slightly.
• Straightening a horizon that bends after a heavy distortion correction.

The tell: warping bends the background

The technical point that makes this lesson practical. A displacement mesh affects everything inside its influence, including whatever is behind the subject.

So a body reshaped against a tiled wall leaves the tile grout curving. A waist narrowed in front of a door frame leaves the frame bowed. A head slimmed against a window leaves the mullion bent. This is how retouching is spotted, every time, and it is why the Freeze Mask exists.

The practice that avoids it:

1. Freeze the background before you begin, or duplicate the subject onto its own layer and warp only that.
2. After warping, look only at the straight lines in the frame at 100 per cent. Walk every edge, every horizon, every architectural line.
3. Toggle the layer off and on. Any line that moves is a line you have bent.

It is also how you check somebody else’s work, which comes up more than you might expect.

The line, restated for shape

Earlier in the course the test was: would the viewer be surprised to learn what you did? Applied to shape:

• Correcting a distortion the lens introduced restores what was there. Nobody is misled.
• Changing what the subject’s body is shaped like asserts something about a person that is not true. Several countries require commercial images to be labelled where body shape has been altered, and the direction of legislation has been one way for over a decade.
• Changing a product’s proportions is the consumer protection issue from the product lesson.
• Changing a scene’s geometry (moving a building, straightening a road) in a documentary photograph is the same category as removing an object from one.

The practical rule for commercial portrait and fashion work: ask, and write down the answer. “Do you want any reshaping?” is one question. If the answer is no, you have a record. If the answer is yes, you have consent, which is what distinguishes a service from an imposition.

For your own work, the honest observation is that liquify is habit-forming. It is easy, invisible while you are doing it, and there is no natural stopping point. Working on a duplicate layer whose opacity you reduce at the end, exactly as with retouching, is the most effective brake anybody has found.

The technical limits

Warping softens. Heavy stretching interpolates, and the stretched region will not match the sharpness of its surroundings. Sharpen locally afterwards or keep the warp small.

Warping repeats noise. Stretched grain becomes elongated grain, which is visible against unstretched grain nearby. On a noisy file this is a real tell.

Warping cannot fix occlusion. If an arm is in front of a bag, no warp reveals the bag. That is a reconstruction job.

A mesh has a resolution. Liquify’s mesh is coarser than the pixels, so very fine, very local moves come out smeared rather than precise. For a small precise move, a selection, a nudge and a masked blend is cleaner than a warp.