Wan Scail Replacement

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 — not just 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/reframe the reference before you render:

> Pad/reframe the reference image onto a canvas at the working resolution so the

> person occupies roughly the same fraction of frame height as the subject in the

> driving video. A full-bleed portrait reference against a wide-shot driving clip

> renders the character oversized in proportion to the framing mismatch.

Why it's a trap: this isn't stated in the template's on-canvas notes, and it does

not surface in the popular Civitai motion-transfer workflows — those run

*animation* mode, where the reference legitimately fills the frame.

Tuning trade-off: distill LoRA strength / shift leaks driving-subject detail

Running the lightx2v distill LoRA at 0.8 with ModelSamplingSD3 shift 5 (the

settings the popular Civitai workflow uses) improves colour and detail versus

1.0 / shift 8 — but it also increases adherence to the driving video enough

that original-subject details bleed onto the replacement character. In one

run the original golfer's neon-yellow shoe appeared on a replacement character

who wears white shoes in the reference (same seed, same reference, only those two

params changed). If you see source details you didn't ask for, raise the LoRA

strength / shift back toward 1.0 / shift 8.

Don't "fix" colour with post-hoc compositing — it's a regression

The raw SCAIL-2 output shows a measurable colour error (background ~-5 per channel

from the VAE round-trip; the character loses red ~2.4x faster than green, which

reads as a slight green cast). It is tempting to composite the generated character

back over the original plate through the SAM3 mask to "correct" it. Don't —

it measures better but looks worse:

• The mask boundary produces an obvious halo.
• Layering the original plate's props (e.g. a golf club) over generated hands

severs the grip relationship the model had solved coherently.

SCAIL-2 resolves colour, edges, grip and occlusion *jointly*; correcting any one

of them in isolation breaks the others. Leave the raw output alone.

Models (reference set)

The template's int8 build was exercised with:

• wan2.1_14B_SCAIL_2_int8_convrot — SCAIL-2 model
• wan2.1_SCAIL_2_DPO_lora_bf16 — DPO LoRA
• lightx2v_I2V_14B_480p_cfg_step_distill_rank64_bf16 — distill LoRA (see tuning note)
• Wan2_1_VAE_bf16
• umt5_xxl_fp8_e4m3fn_scaled — text encoder
• clip_vision_h
• sam3.1_multiplex_fp16 — SAM3 mask model

VRAM: ~22.2 GB peak at 576x1024 / 81-frame chunks on a 24 GB card.

See also

• wan-t2v-video, wan-flf-video — other WAN 2.x video pipelines
• director — multi-shot scene direction for video pipelines