What it is actually solving
A photograph records the result of light meeting a surface, and throws away almost everything about the arrangement that produced it. Relighting has to run that backwards. To move the key light from the right to the left it needs to know which parts of the face point which way, what is in front of what, and how rough each surface is, and none of that is in the file.
So every relighting system is really two systems. One estimates shape, usually as a depth map or a surface normal map. The other re-renders shading using that estimate plus whatever lighting you asked for. Quality tracks the shape estimate far more than it tracks the rendering, which is why relighting works well on faces, where models have strong priors about geometry, and badly on cluttered interiors where the depth guess falls apart.
What it can and cannot do
Reliable, in roughly descending order of success:
- Moving the key direction on a face or a single object. The most common request, and the one models handle best.
- Changing softness. Hard to soft is easier than soft to hard, because softening is a blur of shading whereas hardening has to invent shadow edges that were never there.
- Colour temperature and cast. Nearly always convincing, since it is closest to a graded change.
- Adding a rim or practical. Works well as long as the light source stays out of frame.
Unreliable, and worth knowing before you promise a fix:
- Recovering blown highlights or crushed blacks. There is nothing under them. A relight that swings the key toward a clipped area produces plausible fiction.
- Reflections and speculars on glass or metal. These depend on the whole environment, not just the surface.
- Hair, especially backlit hair. Fine strands need per-strand occlusion that a depth estimate does not have.
- Consistent results across a sequence. Each frame is estimated separately.
Prompting a relight
Treat it as a lighting brief for a gaffer rather than a wish. Two things matter most: naming the light precisely, and stating explicitly what must not change.
Name direction, quality, colour and falloff in that order, using the vocabulary a lighting plan would use: single soft key from high frame left at about 45 degrees, warm 3200K, deep falloff into shadow on the right cheek. Vague requests like better lighting or cinematic light get you a contrast bump, because the model has nothing spatial to act on.
Then lock everything else, because portrait relighting models drift on identity and wardrobe when they are re-rendering a face: keep the pose, expression, wardrobe and background exactly as they are. This single clause removes most of the unwanted edits.
Where a description would be laborious, use light transfer instead. Supplying a reference frame purely as a lighting example is faster than describing an unusual setup, and it is the right approach when the shot has to match a plate you already have.
For anything that needs precision, mask the region and relight inside it. A masked relight on one face in a group shot is both easier for the model and easier for you to check, and it protects the parts of the frame you were happy with.
Where it fits in a pipeline
Relighting is most valuable early, as a look-development tool: generate one image, relight it four ways, and choose the direction before committing to a full set of shots. Used that way it replaces a lighting test rather than a reshoot.
Used late, as a rescue for footage that was lit badly, it is far less dependable, and the deciding factor is exposure rather than the model. If the original held detail in both the highlights and the shadows, a relight has material to work with. If it did not, the honest options are a grade or a reshoot.