Dissolve

One image fades down while the next fades up, and for a moment both are on screen together. The overlap says that time has passed, or that the two shots belong to one another.

It is the softest join a film has. Where a cut asserts, a dissolve suggests, which is why it settles over memory, sleep and long stretches of travel so easily.

The length sets the meaning. Twelve to twenty four frames reads as a gentle join. Two seconds or more reads as a passage of time, and the middle of a long dissolve becomes a composite image in its own right.

Both shots have to work when they are half transparent. A busy frame dissolving into another busy frame turns to mud in the middle, so at least one side should be simple.

Generate both shots and blend them in an edit. The overlap is an opacity ramp on a timeline, and no prompt produces it, because a model returns one continuous image rather than two layered ones.

Use it to pass time, to move into a memory or a dream, and to join two shots that should be read together rather than in sequence.

Avoid it inside a scene playing in real time. A dissolve tells the audience that time has moved, so one placed mid conversation makes the scene incoherent.

A fade goes through a solid colour, usually black, and only one image is visible at any moment. A dissolve overlaps the two and both are visible through the middle.

A wipe replaces one image with the next behind a hard travelling edge, and nothing is ever transparent. Dissolve blends. Wipe divides.

Francis Ford Coppola opens Apocalypse Now (1979) on a treeline, helicopter rotors and a ceiling fan laid over one another in a long dissolve with Willard's face, and the film holds the composite long enough that the jungle and the hotel room become one place.

A single run returns a camera move or a morph instead of a blend. Render the two shots separately and cross fade them on a timeline.

The second failure is a muddy middle. If both shots are busy, the overlap reads as noise, so make one of them simple, dark or close to empty in the area the other one fills.