First public commit.

lenser/nes/palette.py

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+"""Nintendo Entertainment System (2C02 PPU) master palette.
+
+The NES has no fixed RGB palette -- the PPU generates an NTSC signal.  We
+reproduce MAME's exact ``nespal_to_RGB`` (YUV->RGB) formula so the encoder
+matches what the emulator renders.  A palette value is a 6-bit index 0-63:
+high nibble = luminance (0-3), low nibble = hue (0-15, with 0/13/14/15 greyscale).
+The byte written to PPU palette RAM IS this index, so ``PALETTE``/``GREYS`` are
+indexed by the hardware value.
+"""
+from __future__ import annotations
+
+import math
+
+import numpy as np
+
+from ..palette import srgb_to_lab
+
+_TINT, _HUE = 0.22, 287.0
+_Kr, _Kb, _Ku, _Kv = 0.2989, 0.1145, 2.029, 1.140
+_BRIGHT = [[0.50, 0.75, 1.0, 1.0],
+           [0.29, 0.45, 0.73, 0.9],
+           [0.0, 0.24, 0.47, 0.77]]
+
+
+def _rgb(intensity: int, num: int):
+    if num == 0:
+        sat = rad = 0.0; y = _BRIGHT[0][intensity]
+    elif num == 13:
+        sat = rad = 0.0; y = _BRIGHT[2][intensity]
+    elif num in (14, 15):
+        sat = rad = y = 0.0
+    else:
+        sat = _TINT; rad = math.radians(num * 30 + _HUE); y = _BRIGHT[1][intensity]
+    u, v = sat * math.cos(rad), sat * math.sin(rad)
+    R = (y + _Kv * v) * 255.0
+    G = (y - (_Kb * _Ku * u + _Kr * _Kv * v) / (1 - _Kb - _Kr)) * 255.0
+    B = (y + _Ku * u) * 255.0
+    cl = lambda x: max(0, min(255, int(math.floor(x + 0.5))))
+    return (cl(R), cl(G), cl(B))
+
+
+PALETTE = np.array([_rgb(i >> 4, i & 0x0F) for i in range(64)], dtype=np.float64)
+
+# Distinct grey ramp (R==G==B), sorted dark->light, deduped by luminance.
+_grey = {}
+for _i in range(64):
+    r, g, b = PALETTE[_i]
+    if r == g == b:
+        _grey.setdefault(int(r), _i)
+GREYS = [_grey[k] for k in sorted(_grey)]          # e.g. $0F,$1D,$2D,$10,$20
+
+
+def get_palette() -> np.ndarray:
+    return PALETTE
+
+
+def palette_lab() -> np.ndarray:
+    return srgb_to_lab(PALETTE)