First public commit.

lenser/iigs/palette.py

@@ -0,0 +1,31 @@
+"""Apple IIGS Super Hi-Res colour helpers.
+
+SHR colours are 12-bit ``$0RGB`` -- 4 bits per channel, each level scaled x17
+(0->0, 15->255), exactly as MAME's apple2gs palette.  There is no fixed master
+palette: each of the 16 line-palettes holds 16 freely chosen 12-bit colours, so
+the encoder works in the continuous 4096-colour space and quantises to 12-bit.
+"""
+from __future__ import annotations
+
+import numpy as np
+
+
+def quantize12(rgb):
+    """Snap an RGB array (..,3) float 0-255 to the nearest 12-bit SHR colour."""
+    q4 = np.clip(np.rint(np.asarray(rgb) / 17.0), 0, 15).astype(np.int64)
+    return q4 * 17, q4                                # (rgb 0..255 in x17 steps, 4-bit)
+
+
+def color_word(r4: int, g4: int, b4: int) -> bytes:
+    """16-bit little-endian SHR palette entry: low = (G<<4)|B, high = $0R."""
+    lo = ((g4 & 0x0F) << 4) | (b4 & 0x0F)
+    hi = r4 & 0x0F
+    return bytes([lo, hi])
+
+
+def greys12(n: int = 16):
+    """``n`` evenly spaced 12-bit greys, black..white (RGB array, 4-bit array)."""
+    lv4 = np.clip(np.rint(np.linspace(0, 15, n)), 0, 15).astype(np.int64)
+    rgb = np.stack([lv4 * 17] * 3, axis=1).astype(np.float64)
+    q4 = np.stack([lv4] * 3, axis=1)
+    return rgb, q4