Show two images layered in one frame for a while, revealing a character's memories or thoughts or the relationship between two things. Unlike a dissolve, which overlaps briefly and moves on, it shows the overlapped state itself.
A memory rising over a character's face, dream or hallucination scenes, laying a map over a travel scene.
Generate two clips separately and overlap them in the edit. Make one clip a dark, simple background so the two images don't swallow each other. Match the resolution and frame rate of the two clips, then layer them with blend=all_mode=screen or blend=all_opacity=0.5, and keep the movement of both clips slow while they overlap.
Superimpose a slow memory over a face: generate the memory as a slow shot on a dark plain background, then screen-blend it over a face clip of the same size and frame rate: ffmpeg -i face.mp4 -i memory.mp4 -filter_complex '[0:v][1:v]blend=all_mode=screen:all_opacity=0.7' -an out.mp4. Keep movement slow in both clips.
Apocalypse Now (1979) — In the opening, palm trees and helicopter blades flow over the ceiling fan and the upside-down face of Captain Willard in a slow dissolve (No Film School)
Not verified