sextants

Convert raster images (JPEG, PNG, …) to teletext / viewdata block-mosaic graphics — the 40×25 character page rendered by the SAA5050 and its relatives (BBC Micro Mode 7, Ceefax/Oracle teletext, Prestel viewdata, CEPT videotex).

The name comes from Unicode's own term for the 2×3 sub-pixel mosaic cell these displays draw — the sextants of the "Symbols for Legacy Computing" block (U+1FB00…). Each cell is six sub-pixels sharing a two-colour sub-palette, and a colour change is an in-band control code that costs a whole cell. Encoding an image well is therefore a constrained optimisation, not a simple resample — which is what this library does, row by row, with an optimal dynamic-programming solver.

Attribution

sextants is a derivative work of image2mode7 / image2teletext by Kieran Connell. The conversion algorithm at its heart — the per-row dynamic-programming solver, the sub-pixel error model, and the preprocessing pipeline — is Kieran's work, used here under its MIT licence. This project repackages that code as an installable, src-layout Python library with a split module structure, a Click CLI, and a test suite, and is the basis for further development. Please see LICENSE for the full notice.

Install

uv add sextants           # or: pip install sextants

Optional numba acceleration for the DP solver:

uv add "sextants[fast]"

Library usage

from pathlib import Path
from PIL import Image
from sextants import convert, convert_image, render_preview
from sextants import PreprocessOptions, SolveOptions

# Preset-aware entry point: a named bundle configures the whole pipeline.
page = convert(Image.open("photo.jpg"), preset="photo")   # 1000-byte teletext page
Path("photo.bin").write_bytes(page)                        # load at &7C00 on a BBC Micro
render_preview(page).save("preview.png")                   # what it looks like on-screen

# Or drive every knob directly with the two typed option groups.
page = convert_image(
    "logo.png",
    preprocess=PreprocessOptions(quant_colors=8, snap=40, saturation=2.0),
    solve=SolveOptions(edge_weight=3.0),
)

convert(image, preset=None, *, preprocess=None, solve=None) is the preset-aware entry point; convert_image(image, preprocess=None, solve=None) takes the options directly. Configuration lives in two frozen, validated dataclasses — PreprocessOptions (tone, colour, palette, resize) and SolveOptions (the per-row solver and its error metric). Passing preprocess= or solve= to convert replaces that whole stage's options; the preset's other stage still applies. Presets bundle sensible options for common sources:

Command line

sextants photo.jpg -o photo.bin --preset photo
sextants photo.jpg --preview preview.png --url    # also print an edit.tf URL
sextants --help

The output is a raw 1000-byte page (25 rows × 40 bytes of teletext character codes). It loads directly at &7C00 on a BBC Micro, or paste the --url into edit.tf.

Development

uv sync            # create the environment
uv run pytest      # run the test suite

Licence

MIT — see LICENSE. Original algorithm © 2020 Kieran Connell; packaging and modifications © 2026 Robert Smallshire.