wan-scail-replacement

1# SCAIL-2 In-Video Character Replacement (WAN 2.1)
2 
3SCAIL-2 (zai-org, on WAN 2.1 14B) replaces the person in a driving video with a
4character you supply as a reference image, end-to-end, with no pose maps, and
5with multi-character support. It is the successor to WAN Animate / motion
6transfer for the "swap the subject, keep the motion" job. The official ComfyUI
7template is `video_wan21_scail2_character_replacement_int8`, built around
8`WanSCAILToVideo` (+ `SCAIL2ColoredMask`) with a SAM3 mask driving where the
9character goes.
10 
11This skill documents the non-obvious behaviours that cost a full multi-minute
12render to discover. It is not a from-scratch graph. Start from the official
13template and apply the guidance below.
14 
15## The one rule that costs a re-render: reference framing controls SCALE
16 
17**In `replacement_mode: true`, the reference image's FRAMING controls the output
18character's SIZE as well as its appearance.** The SAM3 mask controls where the
19character is placed; the reference image controls how large.
20 
21Measured on a 720x1280 driving clip where the subject occupied ~30% of frame
22height (subject bbox 363 px):
23 
24| Reference framing | Person bbox in output | vs driving subject |
25|---|---|---|
26| Full-bleed portrait (person ~93% of frame) | 621 px | **1.71x oversized** |
27| Reframed (person ~34% of frame) | 364 px | **1.003x — correct** |
28 
29After reframing, top/bottom registration matched the driving subject within 1 px
30…