First public commit.

lenser/atari/convert/gr9.py

@@ -0,0 +1,48 @@
+"""Atari GR.9 (GTIA): 80x192, 16 luminance shades of one hue.
+
+Excellent greyscale (hue 0) or tinted monochrome (any of 16 hues) -- 16 real
+shades, not just dithered.  ``base_color`` selects the hue (0..15); None = grey.
+"""
+
+from __future__ import annotations
+
+import numpy as np
+
+from ...convert.base import Conversion, perceptual_error
+from .. import palette as apal
+from . import _common
+
+WIDTH, HEIGHT = 80, 192
+PIXEL_ASPECT = 4.0
+
+
+def convert(img_rgb, palette_name="ntsc", dither_mode="floyd",
+            intensive=False, base_color=None):
+    hue = 0 if base_color is None else (int(base_color) & 0x0F)
+    plab = apal.palette_lab(palette_name)
+    prgb = apal.get_palette(palette_name).astype(np.uint8)
+
+    img_mono, plab_mono = _common.luminance_lab(img_rgb, plab)
+    ramp = apal.hue_ramp(hue)                       # 16 register values of this hue
+    idx = _common.quantize_global(img_mono, plab_mono, ramp, dither_mode)
+    val = (idx & 0x0F).astype(np.uint8)            # GR.9 pixel = 4-bit luminance
+
+    # GTIA mode 9 takes the hue from COLBK and the luminance from each pixel. A
+    # COLBK of exactly $00, though, blanks the whole playfield to black -- the
+    # register must be non-zero to enable the display. For a tinted hue that is
+    # automatic ((hue<<4) != 0); for greyscale (hue 0) force a non-zero luminance
+    # nibble, which the mode ignores for output (luminance still comes from the
+    # pixels) but which switches the 16-shade display on.
+    colbk = (hue & 0x0F) << 4
+    if colbk == 0:
+        colbk = 0x0E
+    data = _common.split_screen(_common.pack_4bpp(val)) + bytes([colbk])
+    preview = np.repeat(prgb[idx], int(PIXEL_ASPECT), axis=1)
+
+    return Conversion(
+        mode="gr9", width=WIDTH, height=HEIGHT, pixel_aspect=PIXEL_ASPECT,
+        index_image=idx.astype(np.uint16), data=data, data_addr=_common.DATA_ADDR,
+        viewer="gr9", preview_rgb=preview,
+        error=perceptual_error(idx, img_mono, plab_mono),
+        meta={"palette": palette_name, "dither": dither_mode, "hue": hue},
+    )