First public commit.

lenser/pet/palette.py

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+"""Commodore PET / CBM monochrome display.
+
+The PET has no bitmap or colour -- a 40- or 80-column text screen of a fixed
+character ROM on a monochrome (green P1 phosphor) monitor.  Images are rendered
+with the PETSCII 2x2 quadrant-block graphics characters, giving an effective
+80x50 (40-col) or 160x50 (80-col) one-bit pixel grid.
+"""
+from __future__ import annotations
+
+import numpy as np
+
+from ..palette import srgb_to_lab
+
+# 0 = background (dark), 1 = foreground (lit phosphor).  Green to match the
+# classic PET monitor; the signal is one bit, so only luminance matters.
+PALETTE = np.array([(0, 0, 0), (0x40, 0xE0, 0x40)], dtype=np.float64)
+
+
+def get_palette() -> np.ndarray:
+    return PALETTE
+
+
+def palette_lab() -> np.ndarray:
+    return srgb_to_lab(PALETTE)
+
+
+# screen (poke) code for each 2x2 quadrant pattern, bits TL<<3|TR<<2|BL<<1|BR.
+# Derived from the PET character ROM: 8 blocks exist directly, the other 8 are
+# their reverse-video forms (screen code | $80).
+QUAD = [32, 108, 123, 98, 124, 225, 255, 254, 126, 127, 97, 252, 226, 251, 236, 160]