rgn

Minimal screen region selector for macOS, designed for shell scripting workflows. It lets you interactively select a region and outputs coordinates directly usable in tools like ffmpeg.

output.mp4

Features

• Click-and-drag region selection (move and resize to adjust)
• Outputs region coordinates in different format: X Y WIDTH HEIGHT
• Pixel-perfect (Retina-aware)
• Keyboard to cancel ESC, <C-c> and <C-[>
• Customizable appearance: --color, --thickness, --fill, --no-dim, etc
• No dependencies beyond macOS AppKit

Installation

Using mise:

mise install github:soifou/rgn

Or download a prebuilt binary from the Releases page:

rgn-<version>-darwin_arm64.tar.gz (Apple Silicon)
rgn-<version>-darwin_amd64.tar.gz (Intel)
tar -xzf rgn-*.tar.gz
chmod +x rgn
./rgn

Usage

rgn

1. Click and drag to select a region
2. (Optional) Drag inside the region to move, drag the corners to resize
3. Double click: coordinates are printed to stdout

Note

Use --no-confirm to prints coordinates on release instead.

The coordinates are in the form: X Y WIDTH HEIGHT. You can either print coordinates as pixels or points, controlled by --mode:

• --mode pixel (default): Values in pixels (Retina-aware). Relative to the selected screen’s bottom-left. Good for ffmpeg’s crop filter.
• --mode point: Values in points. In global CoreGraphics space, origin at top-left of the main display. Good for screencapture -R.

Options

Option	Description
--color <hex>	Set border color in hex format
--style <style>	Set border style: solid (default), dashed, dotted, double
--thickness <px>	Set border thickness in pixels
--fill	Fill selection area with transparent color
--alpha <float>	Set transparency level for the overlay
--no-dim	Disable dark background overlay
--no-crosshair	Hide crosshair
--no-confirm	Print to stdout on mouse release instead of double-click
--format <format>	Output format: text (default), json
--mode <mode>	Coordinate mode: pixel (default), point
-V, --version	Display version information and exit
-h, --help	Show help

Examples

Screen recording with ffmpeg

coords=$(rgn --mode pixel)
set -- $coords
X=$1 Y=$2 W=$3 H=$4

ffmpeg \
    -f avfoundation \
    -framerate 30 \
    -i "1:none" \
    -vf "crop=${W}:${H}:${X}:${Y}" \
    -pix_fmt yuv420p \
    out.mp4

Screenshot with screencapture

coords=$(rgn --mode point)
set -- $coords
X=$1 Y=$2 W=$3 H=$4

screencapture -x -R"$X,$Y,$W,$H" out.png

Motivation

On Linux, tools like slop or hacksaw provide simple ways to select a screen region and pipe coordinates into scripts.

On macOS, this workflow seem to be missing:

• screencapture supports region selection, but only for screenshots
• no CLI tool exists to select a region and reuse it for video recording
• GUI tools don't integrate well with shell pipelines

rgn fills that gap: a small, script-friendly region picker for macOS.

Build

Requirements:

• macOS (tested on modern versions, i.e. Tahoe)
• Swift toolchain (swiftc)
• AppKit framework (included with macOS)

make

Contributing

Issues and ideas are welcome.

Feel free to open a PR or discussion if you have improvements or feature requests.

License

MIT