colorsys 0.7.3

A module for color conversion and mutation. Works with RGB(a)( as hexadecimal too), HSL(a), CMYK color models and with ANSI color codes
Documentation 
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use crate::ColorTuple;

use super::Rgb;

#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub enum GrayScaleMethod {
  Average,
  AverageProminent,
  Luminance,
  Rec709,
  Rec2100,
}

static R_YUV_FACTOR: f64 = 0.299;
static G_YUV_FACTOR: f64 = 0.587;
static B_YUV_FACTOR: f64 = 0.114;
static YUV_FACTORS: ColorTuple = (R_YUV_FACTOR, G_YUV_FACTOR, B_YUV_FACTOR);

static R_REC709_FACTOR: f64 = 0.2126;
static G_REC709_FACTOR: f64 = 0.7152;
static B_REC709_FACTOR: f64 = 0.0722;
static REC709_FACTORS: ColorTuple =
  (R_REC709_FACTOR, G_REC709_FACTOR, B_REC709_FACTOR);

static R_REC2100_FACTOR: f64 = 0.2627;
static G_REC2100_FACTOR: f64 = 0.6780;
static B_REC2100_FACTOR: f64 = 0.0593;
static REC2100_FACTORS: ColorTuple =
  (R_REC2100_FACTOR, G_REC2100_FACTOR, B_REC2100_FACTOR);

fn mul(rgb: &mut Rgb, factors: ColorTuple) {
  let vals = &mut rgb.units.list;
  vals[0].value *= factors.0;
  vals[1].value *= factors.1;
  vals[2].value *= factors.2;
}

fn rgb_to_grayscale_lum(rgb: &mut Rgb) {
  mul(rgb, YUV_FACTORS)
}

fn rgb_to_grayscale_rec709(rgb: &mut Rgb) {
  mul(rgb, REC709_FACTORS);
}

fn rgb_to_grayscale_rec2100(rgb: &mut Rgb) {
  mul(rgb, REC2100_FACTORS);
}

fn rgb_to_grayscale_avg(rgb: &mut Rgb) {
  let vals = &mut rgb.units.list;
  let y = (vals[0].value + vals[1].value + vals[2].value) / 3.0;
  vals[0].value = y;
  vals[1].value = y;
  vals[2].value = y;
}

fn rgb_to_grayscale_avg_prom(rgb: &mut Rgb) {
  let max = rgb.units.max();
  let min = rgb.units.min();
  let y = (max.0 + min.0) / 2.0;
  let vals = &mut rgb.units.list;
  vals[0].value = y;
  vals[1].value = y;
  vals[2].value = y;
}

pub fn rgb_grayscale(rgb: &mut Rgb, method: GrayScaleMethod) {
  match method {
    GrayScaleMethod::Average => rgb_to_grayscale_avg(rgb),
    GrayScaleMethod::AverageProminent => rgb_to_grayscale_avg_prom(rgb),
    GrayScaleMethod::Luminance => rgb_to_grayscale_lum(rgb),
    GrayScaleMethod::Rec709 => rgb_to_grayscale_rec709(rgb),
    GrayScaleMethod::Rec2100 => rgb_to_grayscale_rec2100(rgb),
  }
}