# 8.1.6. 将绘制分形的线程分派到线程池

mpsc::channel 信道接收作业，Receiver::recv 接收器则检索作业。ImageBuffer::put_pixel 处理数据，设置像素颜色。最后，ImageBuffer::save 将图像存储为 output.png

use error_chain::error_chain;
use std::sync::mpsc::{channel, RecvError};
use num::complex::Complex;
use image::{ImageBuffer, Pixel, Rgb};

error_chain! {
MpscRecv(RecvError);
Io(std::io::Error);
}
}

// 将强度值转换为 RGB 值的函数
// 基于 http://www.efg2.com/Lab/ScienceAndEngineering/Spectra.htm
fn wavelength_to_rgb(wavelength: u32) -> Rgb<u8> {
let wave = wavelength as f32;

let (r, g, b) = match wavelength {
380..=439 => ((440. - wave) / (440. - 380.), 0.0, 1.0),
440..=489 => (0.0, (wave - 440.) / (490. - 440.), 1.0),
490..=509 => (0.0, 1.0, (510. - wave) / (510. - 490.)),
510..=579 => ((wave - 510.) / (580. - 510.), 1.0, 0.0),
580..=644 => (1.0, (645. - wave) / (645. - 580.), 0.0),
645..=780 => (1.0, 0.0, 0.0),
_ => (0.0, 0.0, 0.0),
};

let factor = match wavelength {
380..=419 => 0.3 + 0.7 * (wave - 380.) / (420. - 380.),
701..=780 => 0.3 + 0.7 * (780. - wave) / (780. - 700.),
_ => 1.0,
};

let (r, g, b) = (normalize(r, factor), normalize(g, factor), normalize(b, factor));
Rgb::from_channels(r, g, b, 0)
}

// 将茱莉亚集距离映射为强度值
fn julia(c: Complex<f32>, x: u32, y: u32, width: u32, height: u32, max_iter: u32) -> u32 {
let width = width as f32;
let height = height as f32;

let mut z = Complex {
// scale and translate the point to image coordinates
re: 3.0 * (x as f32 - 0.5 * width) / width,
im: 2.0 * (y as f32 - 0.5 * height) / height,
};

let mut i = 0;
for t in 0..max_iter {
if z.norm() >= 2.0 {
break;
}
z = z * z + c;
i = t;
}
i
}

// 规格 RGB 颜色值范围内的强度值
fn normalize(color: f32, factor: f32) -> u8 {
((color * factor).powf(0.8) * 255.) as u8
}

fn main() -> Result<()> {
let (width, height) = (1920, 1080);
let mut img = ImageBuffer::new(width, height);
let iterations = 300;

let c = Complex::new(-0.8, 0.156);

let (tx, rx) = channel();

for y in 0..height {
let tx = tx.clone();
pool.execute(move || for x in 0..width {
let i = julia(c, x, y, width, height, iterations);
let pixel = wavelength_to_rgb(380 + i * 400 / iterations);
tx.send((x, y, pixel)).expect("Could not send data!");
});
}

for _ in 0..(width * height) {
let (x, y, pixel) = rx.recv()?;
img.put_pixel(x, y, pixel);
}
let _ = img.save("output.png")?;
Ok(())
}