Working Python version for Commodore.

c64view/palette.py

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+"""Commodore 64 (VIC-II) 16-colour palettes and colour-space helpers.
+
+All colour distance work in the converter happens in CIELAB, which is far more
+perceptually uniform than RGB, so the per-cell colour choices and dithering land
+much closer to what a human eye judges as "the same colour".
+"""
+
+from __future__ import annotations
+
+import numpy as np
+
+# 16 fixed VIC-II colours, in canonical index order:
+#  0 black      4 purple     8 orange     12 grey (medium)
+#  1 white      5 green      9 brown      13 light green
+#  2 red        6 blue      10 light red  14 light blue
+#  3 cyan       7 yellow    11 dark grey  15 light grey
+
+# "Colodore" (pepto's reworked, calibrated values) -- the modern default.
+COLODORE = np.array([
+    (0x00, 0x00, 0x00),
+    (0xff, 0xff, 0xff),
+    (0x81, 0x33, 0x38),
+    (0x75, 0xce, 0xc8),
+    (0x8e, 0x3c, 0x97),
+    (0x56, 0xac, 0x4d),
+    (0x2e, 0x2c, 0x9b),
+    (0xed, 0xf1, 0x71),
+    (0x8e, 0x50, 0x29),
+    (0x55, 0x38, 0x00),
+    (0xc4, 0x6c, 0x71),
+    (0x4a, 0x4a, 0x4a),
+    (0x7b, 0x7b, 0x7b),
+    (0xa9, 0xff, 0x9f),
+    (0x70, 0x6d, 0xeb),
+    (0xb2, 0xb2, 0xb2),
+], dtype=np.float64)
+
+# "Pepto" (PAL) -- classic reference values, slightly more saturated.
+PEPTO = np.array([
+    (0x00, 0x00, 0x00),
+    (0xff, 0xff, 0xff),
+    (0x68, 0x37, 0x2b),
+    (0x70, 0xa4, 0xb2),
+    (0x6f, 0x3d, 0x86),
+    (0x58, 0x8d, 0x43),
+    (0x35, 0x28, 0x79),
+    (0xb8, 0xc7, 0x6f),
+    (0x6f, 0x4f, 0x25),
+    (0x43, 0x39, 0x00),
+    (0x9a, 0x67, 0x59),
+    (0x44, 0x44, 0x44),
+    (0x6c, 0x6c, 0x6c),
+    (0x9a, 0xd2, 0x84),
+    (0x6c, 0x5e, 0xb5),
+    (0x95, 0x95, 0x95),
+], dtype=np.float64)
+
+PALETTES = {"colodore": COLODORE, "pepto": PEPTO}
+
+COLOR_NAMES = [
+    "black", "white", "red", "cyan", "purple", "green", "blue", "yellow",
+    "orange", "brown", "light red", "dark grey", "grey", "light green",
+    "light blue", "light grey",
+]
+
+
+def srgb_to_linear(rgb: np.ndarray) -> np.ndarray:
+    """sRGB (0..255) -> linear-light (0..1)."""
+    c = rgb.astype(np.float64) / 255.0
+    return np.where(c <= 0.04045, c / 12.92, ((c + 0.055) / 1.055) ** 2.4)
+
+
+def linear_to_srgb(lin: np.ndarray) -> np.ndarray:
+    """linear-light (0..1) -> sRGB (0..255)."""
+    c = np.clip(lin, 0.0, 1.0)
+    s = np.where(c <= 0.0031308, c * 12.92, 1.055 * (c ** (1 / 2.4)) - 0.055)
+    return np.clip(s * 255.0 + 0.5, 0, 255).astype(np.uint8)
+
+
+# D65 reference white.
+_XYZ_FROM_LIN = np.array([
+    [0.4124564, 0.3575761, 0.1804375],
+    [0.2126729, 0.7151522, 0.0721750],
+    [0.0193339, 0.1191920, 0.9503041],
+])
+_WHITE = np.array([0.95047, 1.0, 1.08883])
+
+
+def srgb_to_lab(rgb: np.ndarray) -> np.ndarray:
+    """sRGB (0..255, last axis = RGB) -> CIELAB. Shape preserved except last axis."""
+    lin = srgb_to_linear(rgb)
+    xyz = lin @ _XYZ_FROM_LIN.T
+    xyz = xyz / _WHITE
+    eps = 216 / 24389
+    kappa = 24389 / 27
+    f = np.where(xyz > eps, np.cbrt(xyz), (kappa * xyz + 16) / 116)
+    fx, fy, fz = f[..., 0], f[..., 1], f[..., 2]
+    L = 116 * fy - 16
+    a = 500 * (fx - fy)
+    b = 200 * (fy - fz)
+    return np.stack([L, a, b], axis=-1)
+
+
+def get_palette(name: str = "colodore") -> np.ndarray:
+    """Return the 16x3 sRGB palette (float64, 0..255)."""
+    return PALETTES[name]
+
+
+def palette_lab(name: str = "colodore") -> np.ndarray:
+    """Return the 16 palette colours in CIELAB (16x3)."""
+    return srgb_to_lab(get_palette(name))