mandelbrot renderer in Rust

mandel.rs

extern crate num;
use num::complex::Complex64;
use num::traits::{Zero, One};
use std::env;
use std::io::{self, Write};
use std::fs::File;

fn viewport(width: usize, height: usize, size: f64, center: Complex64) -> (f64, f64, f64, f64)
{
    let (w, h, s) = (width as f64, height as f64, size * 0.5);
    let dist = if w > h { Complex64::new(s, s * h / w) } else { Complex64::new(s * w / h, s) };
    let (p0, p1) = (center - dist, center + dist);
    let dp = p1 - p0;
    (p0.re, p1.im, dp.re / (w - 1.), dp.im / (1. - h))
}

fn render<F>(width: usize, height: usize, size: f64, center: Complex64, f: F) -> Vec<u8>
    where F: Fn(Complex64) -> u8
{
    let mut buf = Vec::with_capacity(width * height);
    let (x0, y0, ix, iy) = viewport(width, height, size, center);
    let mut y = y0;
    for _ in 0..height
    {
        let mut x = x0;
        for _ in 0..width
        {
            buf.push(f(Complex64::new(x, y)));
            x += ix;
        }
        y += iy;
    }
    buf
}

fn mandel_dist(c: Complex64, max_it: u32) -> Option<f64>
{
    let mut z = Complex64::zero();
    let mut dz = Complex64::one();
    for _ in 0..max_it
    {
        dz = (z * dz).scale(2.) + Complex64::one();
        z = z * z + c;
        let zsq = z.norm_sqr();
        if zsq > 1024.
        {
            return Some((zsq / dz.norm_sqr()).sqrt() * zsq.ln() * 0.5)
        }
    }
    None
}

fn save_ppm<W: Write>(file: &mut W, img: &[u8], w: usize, h: usize) -> io::Result<()>
{
    try!(write!(file, "P5\n{} {}\n{}\n", w, h, std::u8::MAX));
    file.write_all(&img)
}

fn main()
{
    let mut file: Box<Write> = match env::args().nth(1) {
        Some(name) => Box::new(File::create(name).unwrap()),
        None => Box::new(io::stdout()),
    };
    let (w, h, size) = (640, 480, 3.5);
    let img = render(w, h, size, Complex64::new(-0.7, 0.), |c| {
        mandel_dist(c, 255).map(|dist| {
            let val = 1. - (dist * 8. / size).max(0.).min(1.).powf(0.125);
            (val * 255.) as u8
        }).unwrap_or(255)
    });
    save_ppm(&mut file, &img, w, h).unwrap();
}