bitstruct: ashet.Color
Documentation
An 8-bit color value with a specialized encoding suitable for embedding a practical set of 256 colors.
The color encoding is basically a HSV (hue, saturation, value) color with 8 bits, using 3 bits for the hue, 3 bits for the value and 2 bits for the saturation.
Naively mapping out the values to the HSV values has two problems though:
1. A value of 0 maps all colors to black, meaning that we would have 64 different
types of blacks, which all would encode have the rgb value (0, 0, 0).
2. A saturation of 0 maps all colors to gray, effectively ignoring the hue.
This creates the situation that in addition to having 64 blacks, we would also
have each gray tone 8 times, wasting even more encoding space.
To address these two problems, the color scheme uses a modified mapping:
- hue is used without special interpretation.
- value maps to a range of [1:8] instead of [0:7], allowing 8 different
values that are all not black.
- saturation is used without special interpretation except for zero:
If the saturation field is zero, hue and value are interpreted together as a 6 bit
integer storing the brightness of gray.
This yields a color space which has the following properties:
- 64 true gray levels ranging from black to white.
- 8 different hues (red, yellow, lime, green, cyan, blue, purple, magenta).
- 3 different levels of saturation for each non-gray color.
- black maps to 0x00 (but white does not map to 0xFF).
This means we have all 256 colors mapped to a distinct, meaningful color that still allows programmatic conversion from and to the color without the need of a look-up table that would require searching the correct color.
Note:
This color encoding shall be referred to as "Ashet HSV".
Lore:
This color encoding was developed over the course of several days, playing around with many different encodings. The color encodings/palettes were tested on a diverse set of images, including game screenshots, photographs, artificial images, vector graphics and so on.
The "Ashet HSV" encoding showed the best visual matches for most pictures, allowing both visual fidelity on the color side, but also allowing both bright and dark images to work really well.
Fields
hue: u3The hue of the color, encoded as 0 = 0° (red), 7 = 315° (magenta).
value: u3The value of the color, with 0 = 12.5% brightness and 7 = 100% brightness.
saturation: u2The saturation of the color, encoded as 0 = desaturated, and 3 = fully saturated.
Note:
The value is encoded as the uppermost 2 bits, so a check if saturation is 0 can be performed by doing a less-than operation interpreting the color as an integer.