Dither using linear RGB, not gamma-compressed

dither.py

@@ -1,46 +1,60 @@
-# coding: utf-8
+import numpy as np
 from PIL import Image
+import tqdm
+
 img = Image.open('peacock.jpg')
 resized = img.resize((128,128))
-hex_colors = """#000000
-#1D2B53
-#7E2553
-#008751
-#AB5236
-#5F574F
-#C2C3C7
-#FFF1E8
-#FF004D
-#FFA300
-#FFEC27
-#00E436
-#29ADFF
-#83769C
-#FF77A8
-#FFCCAA
-#291814
-#111D35
-#422136
-#125359
-#742F29
-#49333B
-#A28879
-#F3EF7D
-#BE1250
-#FF6C24
-#A8E72E
-#00B543
-#065AB5
-#754665
-#FF6E59
-#FF9D81"""
-pico_palette_32 = [x.strip() for x in hex_colors.splitlines()]
-def hex_to_rgb(h):
-    dec = lambda x: int(x, base=16)
-    return (dec(h[1:3]), dec(h[3:5]), dec(h[5:7]))
 
-pico_rgb = [hex_to_rgb(x) for x  in pico_palette_32]
-import numpy as np
+# sRGB values for the PICO-8 palette.
+# The first 16 entries correspond with the default palette (0-15).
+# The last 16 entries correspond with the "secret" palette (128-143).
+PICO_RGB = [
+    (0, 0, 0),
+    (29, 43, 83),
+    (126, 37, 83),
+    (0, 135, 81),
+    (171, 82, 54),
+    (95, 87, 79),
+    (194, 195, 199),
+    (255, 241, 232),
+    (255, 0, 77),
+    (255, 163, 0),
+    (255, 236, 39),
+    (0, 228, 54),
+    (41, 173, 255),
+    (131, 118, 156),
+    (255, 119, 168),
+    (255, 204, 170),
+    (41, 24, 20),
+    (17, 29, 53),
+    (66, 33, 54),
+    (18, 83, 89),
+    (116, 47, 41),
+    (73, 51, 59),
+    (162, 136, 121),
+    (243, 239, 125),
+    (190, 18, 80),
+    (255, 108, 36),
+    (168, 231, 46),
+    (0, 181, 67),
+    (6, 90, 181),
+    (117, 70, 101),
+    (255, 110, 89),
+    (255, 157, 129)
+]
+
+def component_to_linear(x):
+    x /= 255
+    if x <= 0.04045:
+        return x/12.92
+    else:
+        return ((x + 0.055)/1.055)**2.4
+
+def rgb_to_linear(rgb):
+    return tuple(component_to_linear(c) for c in rgb)
+
+PICO_RGB_LINEAR = [rgb_to_linear(x) for x in PICO_RGB]
+
 bayer = np.array([[0]])
 np.block([[4*bayer, 4*bayer+3],[4*bayer+2, 4*bayer+1]])
 bayer = np.block([[4*bayer, 4*bayer+3],[4*bayer+2, 4*bayer+1]])
@@ -48,7 +62,7 @@ bayer = np.block([[4*bayer, 4*bayer+3],[4*bayer+2, 4*bayer+1]])
 bayer = np.block([[4*bayer, 4*bayer+3],[4*bayer+2, 4*bayer+1]])
 def dither_pixel(img, xy, pal):
     global bayer
-    pal2 = [np.array(list(c)) for c in pal]
+    pal2 = [np.array(rgb_to_linear(c)) for c in pal]
     counts = [0]*len(pal)
     x, y = xy
     def diff(c1, c2):
@@ -56,8 +70,8 @@ def dither_pixel(img, xy, pal):
     def nearest(rgb):
         cost, index = min((diff(rgb,c), i) for i,c in enumerate(pal2))
         return index
-    color = np.array(list(img.getpixel(xy)))
-    err = np.array([0,0,0])
+    color = np.array(rgb_to_linear(img.getpixel(xy)))
+    err = np.array([0.0,0.0,0.0])
     for _ in range(64):
         err += color
         i = nearest(err)
@@ -70,10 +84,13 @@ def dither_pixel(img, xy, pal):
             return i
 
 dithered = Image.new('P', resized.size)
-dithered.putpalette([x for rgb in pico_rgb[:16] for x in rgb])
-import qtdm
-import tqdm
-for x in tqdm.tqdm(range(128)):
+dithered.putpalette([x for rgb in PICO_RGB[:16] for x in rgb])
+def do_it():
+    global dithered, resized
+    for x in tqdm.tqdm(range(128)):
         for y in range(128):
-        i = dither_pixel(resized, (x,y), pico_rgb[:16])
+            i = dither_pixel(resized, (x,y), PICO_RGB[:16])
             dithered.putpixel((x,y), i)
+# import cProfile
+# cProfile.run('do_it()')
+do_it()