SCAIL-2 In-Video Character Replacement (WAN 2.1)

SCAIL-2 (zai-org, on WAN 2.1 14B) replaces the person in a driving video with a character you supply as a reference image, end-to-end, with no pose maps, and with multi-character support. It is the successor to WAN Animate / motion transfer for the "swap the subject, keep the motion" job. The official ComfyUI template is video_wan21_scail2_character_replacement_int8, built around WanSCAILToVideo (+ SCAIL2ColoredMask) with a SAM3 mask driving where the character goes.

This skill documents the non-obvious behaviours that cost a full multi-minute render to discover. It is not a from-scratch graph. Start from the official template and apply the guidance below.

The one rule that costs a re-render: reference framing controls SCALE

In replacement_mode: true, the reference image's FRAMING controls the output character's SIZE as well as its appearance. The SAM3 mask controls where the character is placed; the reference image controls how large.

Measured on a 720x1280 driving clip where the subject occupied ~30% of frame height (subject bbox 363 px):

Reference framing	Person bbox in output	vs driving subject
Full-bleed portrait (person ~93% of frame)	621 px	1.71x oversized
Reframed (person ~34% of frame)	364 px	1.003x, correct

After reframing, top/bottom registration matched the driving subject within 1 px (the character stands on the same ground plane at the same height). Pose transfer was correct in both cases. Only the scale was wrong, which makes it easy to misread as "the model works" until you A/B against the source.

Guidance: pad or reframe the reference before you render.