detail.h

#pragma once
 
#include <stdint.h>
 
 
inline uint32_t Adafruit_DotStar_ColorHSV(uint16_t hue, uint8_t sat, uint8_t val) {
 
    uint8_t r, g, b;
 
    // Remap 0-65535 to 0-1529. Pure red is CENTERED on the 64K rollover;
    // 0 is not the start of pure red, but the midpoint...a few values above
    // zero and a few below 65536 all yield pure red (similarly, 32768 is the
    // midpoint, not start, of pure cyan). The 8-bit RGB hexcone (256 values
    // each for red, green, blue) really only allows for 1530 distinct hues
    // (not 1536, more on that below), but the full unsigned 16-bit type was
    // chosen for hue so that one's code can easily handle a contiguous color
    // wheel by allowing hue to roll over in either direction.
    hue = (hue * 1530L + 32768) / 65536;
    // Because red is centered on the rollover point (the +32768 above,
    // essentially a fixed-point +0.5), the above actually yields 0 to 1530,
    // where 0 and 1530 would yield the same thing. Rather than apply a
    // costly modulo operator, 1530 is handled as a special case below.
 
    // So you'd think that the color "hexcone" (the thing that ramps from
    // pure red, to pure yellow, to pure green and so forth back to red,
    // yielding six slices), and with each color component having 256
    // possible values (0-255), might have 1536 possible items (6*256),
    // but in reality there's 1530. This is because the last element in
    // each 256-element slice is equal to the first element of the next
    // slice, and keeping those in there this would create small
    // discontinuities in the color wheel. So the last element of each
    // slice is dropped...we regard only elements 0-254, with item 255
    // being picked up as element 0 of the next slice. Like this:
    // Red to not-quite-pure-yellow is:        255,   0, 0 to 255, 254,   0
    // Pure yellow to not-quite-pure-green is: 255, 255, 0 to   1, 255,   0
    // Pure green to not-quite-pure-cyan is:     0, 255, 0 to   0, 255, 254
    // and so forth. Hence, 1530 distinct hues (0 to 1529), and hence why
    // the constants below are not the multiples of 256 you might expect.
 
    // Convert hue to R,G,B (nested ifs faster than divide+mod+switch):
    if (hue < 510) { // Red to Green-1
        b = 0;
        if (hue < 255) { //   Red to Yellow-1
            r = 255;
            g = hue;       //     g = 0 to 254
        } else {           //   Yellow to Green-1
            r = 510 - hue; //     r = 255 to 1
            g = 255;
        }
    } else if (hue < 1020) { // Green to Blue-1
        r = 0;
        if (hue < 765) { //   Green to Cyan-1
            g = 255;
            b = hue - 510;  //     b = 0 to 254
        } else {            //   Cyan to Blue-1
            g = 1020 - hue; //     g = 255 to 1
            b = 255;
        }
    } else if (hue < 1530) { // Blue to Red-1
        g = 0;
        if (hue < 1275) {   //   Blue to Magenta-1
            r = hue - 1020; //     r = 0 to 254
            b = 255;
        } else { //   Magenta to Red-1
            r = 255;
            b = 1530 - hue; //     b = 255 to 1
        }
    } else { // Last 0.5 Red (quicker than % operator)
        r = 255;
        g = b = 0;
    }
 
    // Apply saturation and value to R,G,B, pack into 32-bit result:
    uint32_t v1 = 1 + val;  // 1 to 256; allows >>8 instead of /255
    uint16_t s1 = 1 + sat;  // 1 to 256; same reason
    uint8_t s2 = 255 - sat; // 255 to 0
    return ((((((r * s1) >> 8) + s2) * v1) & 0xff00) << 8) |
           (((((g * s1) >> 8) + s2) * v1) & 0xff00) |
           (((((b * s1) >> 8) + s2) * v1) >> 8);
}