First public commit.

lenser/atari/convert/gr15.py

@@ -0,0 +1,51 @@
+"""Atari GR.15 (ANTIC mode E): 160x192, 4 colours chosen globally from 256.
+
+No per-cell colour limit, so this is a clean 4-colour dithered image.
+"""
+
+from __future__ import annotations
+
+import numpy as np
+
+from ... import palette as c64pal       # for srgb_to_lab (shared)
+from ...convert.base import (Conversion, mean_error, perceptual_error,
+                             DIFFUSION_DITHERS)
+from .. import palette as apal
+from . import _common
+
+WIDTH, HEIGHT = 160, 192
+PIXEL_ASPECT = 2.0
+
+
+def convert(img_rgb, palette_name="ntsc", dither_mode="floyd",
+            intensive=False, base_color=None):
+    plab = apal.palette_lab(palette_name)
+    prgb = apal.get_palette(palette_name).astype(np.uint8)
+    img_lab = c64pal.srgb_to_lab(img_rgb)
+
+    # Dither-aware palette for error-diffusion modes: pick 4 colours whose blends
+    # span the image gamut (so dithering reproduces saturated/intermediate shades)
+    # instead of k-means centroids the dither can't reach.
+    if dither_mode in DIFFUSION_DITHERS:
+        colors = _common.choose_palette_dither(img_lab, plab, k=4)
+    else:
+        colors = _common.choose_palette(img_lab, plab, k=4)
+    colors.sort(key=lambda c: plab[c, 0])       # value 0 = darkest (background)
+
+    idx = _common.quantize_global(img_lab, plab, colors, dither_mode)
+    value_of = {c: v for v, c in enumerate(colors)}
+    val_image = np.vectorize(value_of.get)(idx).astype(np.uint8)
+
+    lines = _common.pack_2bpp(val_image)
+    data = _common.split_screen(lines) + bytes(colors)   # 4 colour regs at $6000
+
+    preview = np.repeat(prgb[idx], int(PIXEL_ASPECT), axis=1)
+
+    err = (perceptual_error if dither_mode in DIFFUSION_DITHERS else mean_error)
+    return Conversion(
+        mode="gr15", width=WIDTH, height=HEIGHT, pixel_aspect=PIXEL_ASPECT,
+        index_image=idx.astype(np.uint16), data=data, data_addr=_common.DATA_ADDR,
+        viewer="gr15", preview_rgb=preview,
+        error=err(idx, img_lab, plab),
+        meta={"palette": palette_name, "dither": dither_mode, "colors": colors},
+    )