lightness_to_pwm.c

#include "heivs/lightness_to_pwm.h"

/**
 * \brief Table for converting percentage of lightness# into PWM ratio
 *
 * #lightness = human eye perceived luminance
 *
 * Here we use the formula provided in CIE 1931.
 *
 * Y = (L* / 902.3)           if Y ≤ 8
 * Y = ((L* + 16) / 116)^3    if Y > 8
 *    where L is lightness (0..100) and Y is luminance (0..1)
 *
 * The table below corresponds to :
 *  * lightness 0..100
 *  * luminance 0..255
 *
 *  This table has 101 element, covering 0%..100%
 */
static const uint8_t cie1931[101] =
{
      0,   0,   1,   1,  1,    1,   2,   2,   2,   3,
      3,   3,   4,   4,  4,    5,   5,   6,   6,   7,
      8,   8,   9,  10,  10,  11,  12,  13,  14,  15,
     16,  17,  18,  19,  20,  22,  23,  24,  26,  27,
     29,  30,  32,  34,  35,  37,  39,  41,  43,  45,
     47,  49,  51,  54,  56,  58,  61,  64,  66,  69,
     72,  75,  78,  81,  84,  87,  90,  93,  97, 100,
    104, 108, 111, 115, 119, 123, 127, 131, 136, 140,
    145, 149, 154, 159, 163, 168, 173, 179, 184, 189,
    195, 200, 206, 212, 217, 223, 230, 236, 242, 248,
    255,
};

#if LIGHTNESS_PWM_STEP != 255
#error this cie1931 array supports only 255 steps, feel free to implement your own
#endif

uint8_t lightness_to_pwm(uint8_t percentage)
{
    percentage = min(percentage, ARRAY_SIZE(cie1931) - 1);

    return cie1931[percentage];
}