CLI tool to convert images (and PDFs) to black-and-white optimized for cheap laserjet printing. Built with Babashka (Clojure) and ImageMagick.
Unlike a generic grayscale conversion, this tool is aimed at minimizing toner usage: it lightens midtones with gamma correction, optionally inverts dark backgrounds (so a black page becomes a white page), and picks a B&W rendering that matches the content type — local adaptive threshold for text, error-diffusion or halftone for photos. Multi-page PDFs are supported in and out. Each run prints a coverage report so you can see roughly how much toner you'll save.
The install script checks that all requirements are present before downloading.
curl -fsSL https://raw.githubusercontent.com/ampersanda/monojet/main/install.sh | bashThe following must be installed before running the install script:
- Babashka (bb)
- ImageMagick (magick)
- Ghostscript (gs) — required only if you want to convert PDFs
brew install borkdude/brew/babashka imagemagick ghostscriptThe script installs monojet to ~/.local/bin. Make sure it is in your PATH:
export PATH="${HOME}/.local/bin:${PATH}"Re-run the install command. It detects existing installs and updates in place:
curl -fsSL https://raw.githubusercontent.com/ampersanda/monojet/main/install.sh | bashrm ~/.local/bin/monojetIf installed via the install script:
# Convert a single image (default mode: text)
monojet screenshot.png -o printable.png
# Convert all images in a directory; outputs go to ./bw-output/
monojet -d screenshots/
# Photo-friendly Floyd-Steinberg dithering, with aggressive toner saving
monojet photo.jpg -m dither -T 3 -o photo-bw.png
# Auto-invert if the background is dark (e.g. dark-mode terminal screenshots)
monojet -i terminal.png -o terminal-bw.png
# Verbose output (per-image stats)
monojet -v -d screenshots/ -o output/
# PDF in -> B&W PDF out (multi-page preserved)
monojet zine.pdf -m text -o zine-bw.pdf
# Higher DPI rasterization for sharper PDF output
monojet zine.pdf -m text -D 300 -o zine-bw.pdfOr run directly with Babashka:
bb convert -- image.png -o out.png
bb convert -- -d screenshots/ -o output/
bb -m monojet.core image.png -o out.png| Flag | Description | Default |
|---|---|---|
-o, --output |
Output file (single input) or directory (multiple) | bw-<input> / ./bw-output/ |
-d, --dir |
Read all images from directory | |
-m, --mode |
text, threshold, dither, halftone, gray |
text |
-t, --threshold |
Threshold % (text/threshold modes, 0-100) | 50 |
-T, --toner-saving |
Toner-saving lightening: 0 (off) - 3 (aggressive) | 2 |
-b, --brightness |
Brightness adjustment (-100..100) | 0 |
-c, --contrast |
Contrast adjustment (-100..100) | 0 |
-i, --auto-invert |
Invert if dark background detected | off |
--invert |
Force invert | off |
-D, --density |
Rasterization DPI for PDF input (30-600) | 150 |
-v, --verbose |
Print progress info | |
-V, --version |
Show version | |
-h, --help |
Show help |
| Mode | When to use | What it does |
|---|---|---|
text |
Documents, screenshots, code | ImageMagick -lat 20x20 local adaptive threshold; keeps text crisp on uneven backgrounds |
threshold |
High-contrast B&W source | Hard global threshold; smallest output, no midtones |
dither |
Photos | Floyd-Steinberg error diffusion (-colors 2); fine detail, smooth gradients |
halftone |
Photos for newspaper-like look | Ordered dither (h6x6o); regular dot pattern |
gray |
Charts/diagrams | Lightened grayscale (no B&W conversion); use when toner-saving alone is enough |
Avoid
thresholdmode on images with large solid color regions (e.g. ID cards with a coloured photo background, brand banners). A flat mid-tone block will collapse to solid black, which is the opposite of toner saving. Useditherorhalftoneinstead — both reproduce solid colours as sparse dot patterns.
- Reads the image with ImageMagick and computes its mean luminance.
- If
-iis set and the image is mostly dark, the pipeline starts with-negateso the background becomes white (massive toner savings on dark-mode UIs). - Converts to grayscale, then applies a gamma curve (
-evaluate Pow) tuned by-Tto lighten midtones — gray boxes, light shadows and anti-aliased edges become lighter or disappear entirely. - Applies the chosen mode-specific operator (adaptive threshold, ordered dither, etc.).
- Re-measures luminance of the output to estimate ink coverage and report toner savings vs. the original.
A typical UI screenshot drops from ~40% ink coverage to ~5–10% after text mode with -T 2.
Both input and output PDFs are handled when Ghostscript is available. Multi-page input PDFs are rasterized page-by-page at -D DPI (default 150). If your output extension is .pdf, all pages are written into a single multi-page B&W PDF. The text mode's local-window size scales with -D so titles in large fonts don't render as outlines at higher DPI.
For text-heavy documents start with -m text -T 1. For zines or comics with photos, -m halftone -T 2 keeps photos legible while staying toner-cheap.
A small Flutter desktop app under flutter/ wraps the CLI for drag-and-drop use. It's built with ClojureDart + flutter95 for Win95-style chrome.
-
Make sure ImageMagick is installed (Ghostscript only needed for PDFs):
brew install imagemagick ghostscript
-
Download the latest monojet-desktop-mac-arm64.zip (Apple Silicon only), unzip it, and drag
monojet.appto/Applications.curl -L -o monojet.zip \ https://github.com/ampersanda/monojet/releases/latest/download/monojet-desktop-mac-arm64.zip unzip monojet.zip && mv monojet.app /Applications/ -
The app is not yet notarized, so the first launch needs Gatekeeper bypass: right-click → Open in Finder and confirm. Subsequent launches work normally.
If you'd rather build it yourself:
Highlights
- Drop a file (or click the Original pane) and conversion runs automatically — no Convert button to press.
- Automatic mode (default) runs every CLI mode in parallel and shows the result of whichever saved the most toner. A small "auto · dither" caption under the toner-saved % reveals which mode won.
- If a non-inverted run could be improved by inverting (e.g. dark-mode screenshot), a modal pops with the savings comparison and one click to apply.
- Threshold / brightness / contrast are sliders. Path/binary fields are tucked behind a Show advanced options toggle.
- Save as… to write the result anywhere; Print to send it to your default printer (with a confirmation modal).
- Every settings change re-runs conversion after a 400 ms debounce.
cd flutter
clj -M:cljd flutter -d macos # dev: compiles cljd → Dart and launches
flutter build macos # release: produces flutter/build/macos/Build/Products/Release/monojet.appThe Xcode build phase Bundle monojet runtime copies bb, monojet.bb, bb.edn, and src/ into the .app's Contents/Resources/monojet/ so the bundle ships the conversion logic. magick (and gs for PDFs) must still be installed on the target machine — install via brew install imagemagick ghostscript. macOS deployment target is 11.0; minimum window size is 880 × 540.
bb.edn # Project config (no external deps)
monojet.bb # Entry point for uberscript bundling
src/monojet/
core.clj # CLI parsing, orchestration, reporting
imagemagick.clj # ImageMagick interop (identify, analyze, convert)
flutter/
deps.edn # ClojureDart deps + :cljd/opts
pubspec.yaml # Flutter deps (desktop_drop, file_picker, flutter95, ...)
src/monojet_desktop/
core.cljd # Layout, subprocess plumbing, modals
state.cljd # Constants + the defonce app-state atom
widgets/
form.cljd # labeled-field, num-field, slider-field, …
buttons.cljd # icon-button, chevron-toggle (a11y-wrapped)
preview.cljd # tappable image/PDF preview tile
stat.cljd # big-number stat card
modal.cljd # reusable centered modal
lib/main.dart # Re-exports the cljd-compiled entry point
macos/ # Flutter macOS scaffold (Xcode project, entitlements, icons)
- ImageMagick: Local Adaptive Thresholding - used for the
textmode - ImageMagick: Quantization & Dithering - background on
-monochromeand-ordered-dither - HP toner-saving guidance - inspiration for the toner-saving gamma defaults
Assisted by Claude Code.
MIT