rembg Operations — cutouts for flat illustration & sticker art

Turn opaque, flat-background illustration (avatars, stickers, cartoon/vector art, generated character portraits) into clean transparent PNGs — without regenerating the source. rembg's ML models are the default engine; the value this skill adds is the deterministic fallback ladder for the cases pure ML gets wrong on flat-colour art.

The one thing to know

rembg is a semantic model, not a chroma keyer. isnet-anime (the model tuned for outlined cartoon art) segments the salient character and ignores background colour — so it cannot be "helped" with a greenscreen, and it fails in two predictable ways on flat art:

1. Pale / low-salience subjects get dropped (a near-white moon, a mint UFO on a light background read as "not the subject").
2. Baked drop-shadows survive as a grey ghost or an opaque offset blob.

Both are fixed deterministically because flat sticker art always has one flat background colour + a sealing outline. That geometry makes a corner flood-fill colour-key exact, regardless of how pale the subject is. The axis that matters is shadow-vs-no-shadow, never greenscreen-vs-not.

Quickstart

# default: rembg isnet-anime, auto-fallback to colour-key if the subject collapses
python scripts/cutout.py "avatars/*.png" --out cutouts/ --contact-sheet

The --contact-sheet renders every cutout on a checkerboard so bad mattes jump out. Then triage failures with the ladder below. --json | jq '.data[] | select(.coverage < 0.12)' flags collapsed subjects programmatically.

Method ladder — escalate only for the images that need it

Symptom on the contact sheet	Fix	Flag
Clean die-cut	— (rembg isnet-anime)	default
Subject gone / faint ghost (pale subject)	flat-bg colour-key	--method colorkey
Translucent subject reads see-through (glass, clear cup)	colour-key forces the sealed interior opaque	--method colorkey
Semi-transparent grey shadow ghost	binarise alpha — drops the ~50% shadow, keeps the 100% subject	--flatten-alpha 170
Opaque baked offset shadow (survives binarise)	colour-aware: drop dark pixels not hugging a coloured fill	--strip-offset-shadow

auto (the default) already does rembg→colour-key when the silhouette collapses (--min-cov, default 0.10). The post-processing flags are opt-in per image because detecting a shadow ghost automatically is unreliable — eyeball the sheet, escalate the few that need it. See references/cutout-methods.md for how each method works and why.

Why no regeneration

Every method here alpha-masks the original pixels — it never re-renders the subject. That's what lets you cut out AI-generated character art (or any hand-drawn asset) without changing how the character looks. If your generator is text-to-image only (no img2img / transparent-output flag, e.g. most portrait CLIs), local background removal is the only way to get transparency without a character-drifting re-roll.

The intended downstream shape

Cut once to transparent PNG, then composite in the app: transparent PNG over a CSS background colour + a CSS hard-offset shadow (drawn in CSS, not baked into the image). This is what makes a colour-picker possible — one asset, any background. Never bake the shadow back into the PNG.

Gotchas

• isnet-anime vs u2net: the default u2net model leaves a grey ghost of a baked shadow and softer edges on cartoon art — always pass --model isnet-anime (this skill's default) for flat/outlined work. u2net is fine for photos.
• First rembg run downloads a ~170 MB model (cached after). Fully offline work → --method colorkey.
• Colour-key keeps bg-coloured regions inside the subject (they don't touch a border) — a subject with a same-as-background patch stays intact. It only removes background reachable from the image edge.
• Disconnected decorations (sparkle stars floating on the background) are kept by colour-key as islands — usually what you want for a sticker; crop them out first if not.
• --strip-offset-shadow assumes a dark shadow + coloured subject fills. On an all-dark subject it will over-eat; use --flatten-alpha there instead.

Dependencies

pillow + numpy (required); scipy (optional, faster flood-fill); rembg (only for the rembg/auto ML path — colorkey and the post-processors are pure Pillow/numpy and need no model download).

uv pip install pillow numpy scipy rembg   # rembg optional