Outline

Outline is an image background removal tool with flexible mask processing options.

It is written in Rust, powered by ONNX Runtime (ort) and VTracer, and works with U2-Net, BiRefNet, and other ONNX models with a compatible input/output shape.

This project is still in early development. Breaking changes may occur in future releases.

Installation

CLI

cargo install outline-core --features cli

For the CLI, you can also install with --features fetch-model to enable one-command model downloading:

cargo install outline-core --features "cli fetch-model"
outline fetch-model

Library

cargo add outline-core
# or use this if VtracerSvgVectorizer is needed:
cargo add outline-core --features vectorizer-vtracer

Usage

Outline works as both a library and a CLI.

Before using Outline, specify your ONNX model path:

• CLI flag: -m, --model <path>
• Library API: Outline::new(<path>)
• Environment variable: set OUTLINE_MODEL_PATH

If you don't have a model file yet, download one first: silueta.onnx.

Resolution order: user value > environment variable > cached model (CLI only with fetch-model enabled) > default (model.onnx).

CLI Usage

Use outline <COMMAND> --help to inspect the options of each subcommand.

Examples

# Remove the background and export a foreground PNG
outline cut input.jpg -o subject.png

# Export a foreground PNG with feathered edges
outline cut input.jpg --blur -o subject-soft-alpha.png

# Export the raw matte PNG
outline mask input.jpg -o subject-matte.png

# Export a processed binary mask png
outline mask input.jpg --binary -o subject-mask.png

# Generate an SVG outline from the raw matte
outline trace input.jpg -o subject-raw-mask.svg

# Generate an SVG outline with sticker-style processing
outline trace input.jpg -o subject.svg \
	--dilate 50.0 --fill-holes --blur 20.0

Detailed CLI Reference

Subcommands

• cut: Primary background-removal workflow. Produces a foreground PNG, optionally saves the raw matte and the processed mask, and lets you choose the alpha source.
• mask: Exports only the mask. It saves the raw matte by default and switches to the processed binary mask when --binary is provided.
• trace: Generates an SVG outline using the same mask-processing pipeline. Exposes VTracer color modes, hierarchy selection, path precision, and other options.

Global Options (shared by all subcommands)

• -m, --model <path>: Path to the ONNX model (defaults to model.onnx).
• --intra-threads <n>: ORT intra-op thread count. Omit to let ORT decide.
• --input-resample-filter {nearest,triangle,catmull-rom,gaussian,lanczos3}: Resampling filter used when scaling the input down to the model resolution.
• --output-resample-filter {nearest,triangle,catmull-rom,gaussian,lanczos3}: Filter used to resize the matte back to the original image size.

Shared Mask-Processing Options

The following switches can be used in mask, cut, and trace:

• --blur [sigma]: Apply Gaussian blur before thresholding (defaults to 6.0 when no value is provided).
• --mask-threshold <0-255 | 0.0-1.0>: Threshold applied to the matte (default 120) to produce a binary mask.
• --binary [enabled|disabled|auto]: Control thresholding in the processed pipeline (use the flag without a value to force a binary mask, or pass --binary disabled to keep soft edges; the default auto preserves the raw matte unless other processing steps require a hard mask).
• --dilate [radius]: Enable dilation after thresholding (defaults to 5.0 when no value is provided).
• --fill-holes: Fill enclosed holes before exporting/tracing.

The raw matte (soft mask) preserves the grayscale alpha predicted by the model. The processed mask (binary mask) goes through the blur/threshold/dilate/fill pipeline, producing a clean silhouette for tracing or foreground compositing.

cut Command

• -o, --output <path>: Foreground PNG output path (default <name>-foreground.png).
• --export-matte [path]: Additionally save the raw matte (default <name>-matte.png).
• --export-mask [path]: Save the processed mask (default <name>-mask.png); add --binary for hard edges.
• --alpha-source {raw|processed|auto}: Choose which mask becomes the PNG alpha (default auto). auto keeps the raw matte unless any mask-processing options are provided, in which case it uses the processed mask.

mask Command

• -o, --output <path>: Output path (default <name>-matte.png or <name>-mask.png depending on processing flags).
• --mask-source {raw|processed|auto}: Choose which mask to export. auto (default) exports the raw matte unless any mask-processing options (binary/blur/dilate/fill-holes or a custom threshold) are provided, in which case it exports the processed mask.

trace Command

• -o, --output <path>: SVG output path (default is the input name with .svg).
• --mask-source {raw|processed|auto}: Choose the mask used for tracing. auto (default) uses the raw matte unless any mask-processing options are enabled, in which case it uses the processed mask.
• --color-mode {color,binary}: Color mode (default binary).
• --hierarchy {stacked,cutout}: Hierarchy strategy (default stacked).
• --mode {none,polygon,spline}: Path simplification mode (default spline).
• --invert-svg: Invert foreground/background in the SVG output.

Other VTracer related options

• --filter-speckle <usize>: Speckle filter size (default 4).
• --color-precision <i32> / --layer-difference <i32> / --corner-threshold <i32> / --length-threshold <float> / --max-iterations <usize> / --splice-threshold <i32>: Fine-tune the remaining VTracer parameters (defaults: 6, 16, 60, 4.0, 10, 45).
• --path-precision <u32>: Decimal precision for path coordinates (default 2).
• --no-path-precision: Clear the explicit path precision override and defer to VTracer's internal behaviour.

Library Usage

use outline::{MaskProcessingOptions, Outline};

fn generate_assets() -> outline::OutlineResult<()> {
	let outline = Outline::new("model.onnx") // or: Outline::try_from_env()
		.with_default_mask_processing(MaskProcessingOptions::default());
	let session = outline.for_image("input.png")?;
	let matte = session.matte();

	// Compose the foreground directly from the raw matte (soft edges)
	let foreground = matte.foreground()?;
	foreground.save("input-foreground.png")?;

	// Declare the desired mask steps, then apply them with `processed`.
	let mask = matte
		.clone()
		.blur_with(6.0)
		.threshold_with(120)
		.processed()?;
	mask.save("input-mask.png")?;

	// Compose the foreground with processed mask (hard edges)
	let foreground_processed = mask.foreground()?;
	foreground_processed.save("input-foreground-processed.png")?;

	Ok(())
}

Optional SVG Tracing

Enable vectorizer-vtracer if you want to trace masks into SVG:

cargo add outline-core --features vectorizer-vtracer

use outline::{Outline, TraceOptions, VtracerSvgVectorizer};

fn trace_mask() -> outline::OutlineResult<()> {
	let outline = Outline::new("model.onnx");
	let session = outline.for_image("input.png")?;
	let mask = session.matte().blur().threshold().processed()?;

	let vectorizer = VtracerSvgVectorizer;
	let svg = mask.trace(&vectorizer, &TraceOptions::default())?;
	std::fs::write("input.svg", svg)?;
	Ok(())
}

You can also avoid depending on VTracer directly by implementing the MaskVectorizer trait with your own vectorizer.

Next Steps

• Add detailed documentation for library API
• Expose a WASM version of library API
• Improve the CLI syntax for better usability and expressiveness
• Provide a lightweight GUI for easier use.

Changelog

See CHANGELOG.md for a list of changes.

License

Distributed under the MIT License. See LICENSE for details.