Audio Mark Fade In

Fades the audio overlay in from silence over the specified duration at the start of its window, in seconds.

audio-mark-fade-in defaults to 0.0 (no fade-in). The fade is applied relative to the overlay’s window start (audio-mark-start). A fade-in of 2 seconds means the overlay ramps from silence to full volume over the first 2 seconds of its window.

If the combined fade-in and audio-mark-fade-out durations would exceed the window length, each is clamped to half the window so the result is never silent throughout.

Each layer takes its own [N] suffix: audio-mark-fade-in[1]=1.