Blend Layer Alignment
blend-align
Positions the blend layer image within the free space of the picture. The default is centered.
Valid values are top, middle, bottom combined with left, center, right, separated by a comma. Two-letter shorthands such as tl are also accepted.
Aliases: blendalign, ba.
Use blend-align[N] to target a specific blend layer, where N is 0–3. Layer 0 is either the bare blend-align or blend-align[0]; if both are supplied, the bare name wins.
On video with fit=fill, alignment measures against the picture rather than the filled canvas.