Map DHGR colours to RGB values (for now using the BMP2DHR default

NTSC palette), and compute the matrix of pairwise perceptual
differences between these colours, using CIE2000 delta values.

transcoder/palette.py

@@ -0,0 +1,45 @@
+import colormath.color_objects
+import colormath.color_diff
+import colormath.color_conversions
+import numpy as np
+
+from colours import DHGRColours
+
+
+def rgb(r, g, b):
+    return colormath.color_objects.sRGBColor(r, g, b, is_upscaled=True)
+
+
+# Palette RGB values taken from BMP2DHGR's default NTSC palette
+# TODO: support other palettes as well, e.g. //gs RGB
+palette = {
+    DHGRColours.BLACK: rgb(0, 0, 0),
+    DHGRColours.MAGENTA: rgb(148, 12, 125),
+    DHGRColours.BROWN: rgb(99, 77, 0),
+    DHGRColours.ORANGE: rgb(249, 86, 29),
+    DHGRColours.DARK_GREEN: rgb(51, 111, 0),
+    DHGRColours.GREY1: rgb(126, 126, 126),
+    DHGRColours.GREEN: rgb(67, 200, 0),
+    DHGRColours.YELLOW: rgb(221, 206, 23),
+    DHGRColours.DARK_BLUE: rgb(32, 54, 212),
+    DHGRColours.VIOLET: rgb(188, 55, 255),
+    DHGRColours.GREY2: rgb(126, 126, 126),
+    DHGRColours.PINK: rgb(255, 129, 236),
+    DHGRColours.MED_BLUE: rgb(7, 168, 225),
+    DHGRColours.LIGHT_BLUE: rgb(158, 172, 255),
+    DHGRColours.AQUA: rgb(93, 248, 133),
+    DHGRColours.WHITE: rgb(255, 255, 255)
+}
+
+
+# Compute matrix of CIE2000 delta values for this palette, representing
+# perceptual distance between colours.
+diff_matrix = np.ndarray(shape=(16, 16), dtype=np.int)
+for colour1, a in palette.items():
+    alab = colormath.color_conversions.convert_color(
+        a, colormath.color_objects.LabColor)
+    for colour2, b in palette.items():
+        blab = colormath.color_conversions.convert_color(
+            b, colormath.color_objects.LabColor)
+        diff_matrix[colour1.value, colour2.value] = int(
+            colormath.color_diff.delta_e_cie2000(alab, blab))