First public commit.

lenser/coco/convert/pmode4.py

@@ -0,0 +1,41 @@
+"""CoCo PMODE 4: 256x192, 1 bit/pixel, black & white (buff on black).
+
+The CoCo's high-resolution monochrome mode -- 256x192 is exactly 4:3, so square
+pixels.  Tone is carried by dithering, like Apple HGR mono.
+"""
+
+from __future__ import annotations
+
+import numpy as np
+
+from ... import dither
+from ...convert.base import Conversion, perceptual_error
+from .. import palette as cpal
+
+WIDTH, HEIGHT = 256, 192
+PIXEL_ASPECT = 1.0
+
+
+def convert(img_rgb, palette_name="mc6847", dither_mode="floyd",
+            intensive=False, base_color=None):
+    from ...palette import srgb_to_lab
+    plab = cpal.mono_lab()
+    L = srgb_to_lab(img_rgb)[..., 0]
+    img_mono = np.zeros((HEIGHT, WIDTH, 3))
+    img_mono[..., 0] = L
+    plab_mono = np.zeros((2, 3))
+    plab_mono[:, 0] = plab[:, 0]
+
+    allowed = np.tile(np.array([0, 1]), (HEIGHT, WIDTH, 1))
+    idx = dither.quantize(img_mono, allowed, plab_mono, dither_mode).astype(np.uint8)
+
+    data = cpal.pack_pmode4(idx)                 # 6144-byte video buffer
+    preview = cpal.MONO.astype(np.uint8)[idx]
+
+    return Conversion(
+        mode="pmode4", width=WIDTH, height=HEIGHT, pixel_aspect=PIXEL_ASPECT,
+        index_image=idx.astype(np.uint16), data=data, data_addr=0,
+        viewer="pmode4", preview_rgb=preview,
+        error=perceptual_error(idx, img_mono, plab_mono),
+        meta={"palette": "mc6847", "dither": dither_mode, "vdg": 0xF8},
+    )