saitoha/libsixel perception-test-2026-08

Colour perception test scripts (256 vs 128 / 64 / 32 / 16)

on GitHub

Self-contained SIXEL test scripts for the perceptual study discussed in #196.

Each script runs 100 two-alternative forced-choice trials. Every trial shows the same
photograph twice, side by side: once encoded with a 256-colour palette and once with a
reduced one. Press 1 for the left image or 2 for the right one, whichever looks
better. A summary is printed at the end.

Run them as files — they read their own image payload from $0, so piping into bash
will not work:

curl -LO <asset url>
bash test-p256-vs-p128-commons.bash

Requirements: a terminal with SIXEL support, at least 256 colour registers, and
per-image (private) colour registers; a window of at least 1280x496 px; bash,
base64 -d, gunzip. Ctrl-C restores the terminal.

Encoding, on the develop branch:

img2sixel --threads=2 --diffusion=lso2 \
          --quantize-model=kmeans:inittype=pca:binning=none:merge=ward \
          --clustering-colorspace=oklab --working-colorspace=cielab \
          --colors=<256|128|64|32|16>

Licensing. The shell code is part of libsixel and is under libsixel's own licence
(MIT). The 100 embedded photographs are separately licensed works from the Wikimedia
Commons "Featured pictures" collection, included as a collection rather than as an
adaptation of the code. Every image has been modified: centre-cropped to 640x480 at 1:1
(no resampling) and re-encoded with a reduced colour palette. The modified versions are
distributed under the licence of the original — CC BY-SA adaptations under CC BY-SA 4.0,
CC BY adaptations under the same CC BY licence, CC0 and public-domain images without
restriction. Per-image attribution is embedded in the header of every script and is also
listed in CREDITS-images.md.