Repalette

Repalette is an image colorizer written in C and Rust. It is available as both a CLI utility and a web application.

Features

• Recolor images using nearest neighbor search.
• Optional dithering with multisampling for smoother results.
• Palette extraction from any image
• SIMD accelerated palette search and dithering.
• Built-in palette presets bundled into the binary at compile time.
• Optimized encoding for small-palette PNG export.

Web-version:

• WASM-powered web application (mostly for mobile usage)
• Support for both text-based and color-picker-based palette editing
• Responsive layout and adaptive light/dark theme

Usage

input.jpg	output-no-dither.png	output-burkes.png

CLI version

Examples:

# a built-in preset, with 2x multisampling
repalette apply input.jpg output.png -p nord -m 2x

# a manual palette (6-digit hex, no '#')
repalette apply input.jpg output-no-dither.png -c 000000,ff0000,ffffff --dither none
repalette apply input.jpg output-burkes.png -c 000000,ff0000,ffffff --dither burkes

# recolor using a palette extracted from the image itself (compression)
repalette apply input.jpg output-extracted.png -k 32

# recolor using a palette extracted from a different reference image
repalette apply input.jpg output-transfer.png --palette-from reference.jpg

# browse the presets
repalette palette list
repalette palette show gruvbox-dark

# extract an N-color palette from an image
repalette extract input.jpg -k 16

Run repalette --help to learn more

Web version

Host the website with any server or go to https://ziap.github.io/repalette.

The user interface:

Building

You need:

• Both: Rust
• For native build: Any C compiler that supports C99.
• For WASM build: Clang, LLVM, and LLD.

Build the native version

cargo build --release

Build the WASM version

build.sh

Benchmark

Recoloring to the 13-color nord palette. Times are wall-clock means from hyperfine; quality is measured against the source (higher PSNR/SSIM = closer to the original). Absolute numbers are hardware dependent, so read them as relative.

Dithering speed on full 4K (3840x2160), repalette vs ImageMagick -remap:

Tool	Command	Time
repalette	-d none	198 ms
repalette	-d fs	297 ms
repalette	-d jjn	346 ms
ImageMagick	-dither None -remap	1050 ms
ImageMagick	-dither FloydSteinberg -remap	571 ms
ImageMagick	-dither Riemersma -remap	1256 ms

NOTE: ImageMagick's -dither FloydSteinberg -remap uses approximate nearest-neighbor search, hence it's faster than undithered remap.

Repalette uses exact search, but is still ~1.9x faster thanks to SIMD acceleration.

Multisample quality on 1080p, Floyd–Steinberg:

Method	PSNR (dB)	SSIM
-d none	19.79	0.836
-d fs	18.47	0.754
-d fs -m 2x	20.02	0.857
-d fs -m 4x	20.00	0.864

Error diffusion trades an artifact (color banding) for another (grains). Both has imperfect quality, but dithering is worse because it directly affect the image structural fidelity (SSIM).

Multisample mode combines colorization with image resampling/filtering to smooth out the grains, while keeping the band-free look. This recovers the quality and improves beyond the no-dither baseline.

Multisample performance on 1080p, Floyd–Steinberg.

Method	Time	Memory
repalette -d none	57 ms	18 MB
repalette -d fs	82 ms	18 MB
repalette -d fs -m 2x	206 ms	18 MB
repalette -d fs -m 4x	695 ms	18 MB
IM chain, 2x (Triangle)	663 ms	193 MB
IM chain, 4x (Catrom, direct)	2430 ms	595 MB
IM chain, 4x (Triangle, stacked 2x)	2534 ms	691 MB

Multisample mode uses a fused algorithm with buffered streaming to dramatically reduce memory usage, beating sequential processing pipelines: ~3x faster and ~10-30x less memory.

License

This project is licensed under the AGPL-3.0 license.