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Here's the plan
We want to be able to map any RGB color to an approximate "256 color"
In general, the 256 colors have three zones:
Zone 1 - Legacy colors (8 basic colors and 8 "bright" colors -- 16 colors)
Zone 2 - 6-bit RGB space (6x6x6 == 216 colors)
Zone 3 - Grayscale (24 colors)
ZONE 1
======
Zone 1, the legacy colors, are too varied to be mapped reliably.
So we won't be mapping any colors to it.
ZONE 2
======
In Zone 2, every color is represented as an RGB value with 6 values
rather than 256 values. To convert 256 values to 6 requires lossy
compression -- and the bands to be used are somewhat speculative.
I'm going to use the xterm bands
Value Normative Band
------ ---------- -----------
0 0 0 - 47
1 95 (5f) 48 - 114
2 135 (87) 115 - 154
3 175 (af) 155 - 194
4 215 (d7) 195 - 234
5 255 (ff) 235 - 255
Note there are 6 zone 2 colors that map to "grayscale colors" (which
will be discmissed more later)
color rgb tuple
----- --------
16 0
59 95
102 135
145 175
188 215
231 255
ZONE 3
======
In Zone 3, every number has RGB values approximately equal,
with 24 values starting with 8 + (10 * value) (to 238)
interposed with the 6 "gray" colors called out above.
A color is "approximately equal" if the largest difference
between the RGB values is 10, they are declared "equal"
Colors where some pair of RGB values are more than 10 apart,
then they will be mapped to a zone 2 color.
The "value" of a zone 3 color is (R+G+B/3)
Value Normative Band
----- ----------- ------------
<16> 0 0-4
0 (232) 8 5-13
1 (233) 18 14-23
2 (234) 28 24-33
3 (235) 38 34-43
4 (236) 48 44-53
5 (237) 58 54-63
6 (238) 68 64-73
7 (239) 78 74-83
8 (240) 88 84-91
<59> 95 92-96
9 (241) 98 97-103
10 (242) 108 104-113
11 (243) 118 114-123
12 (244) 128 124-131
<102> 135 132-136
13 (245) 138 137-143
14 (246) 148 144-153
15 (247) 158 154-163
16 (248) 168 164-171
<145> 175 172-176
17 (249) 178 177-183
18 (250) 188 184-193
19 (251) 198 194-203
20 (252) 208 204-211
<188> 215 212-216
21 (253) 218 217-223
22 (254) 228 224-233
23 (255) 238 234-246
<231> 255 247-255
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