mandelbrot renderer - my first Rust program

mandel.rs

extern crate image;
extern crate num_complex;

use std::fs::File;

use image::{ImageBuffer, Luma};
use num_complex::Complex;

fn main() {
    // for a 3 x 2 image: (-2,-1) to (1,1)
    const PPU: u32 = 500; // pixels per unit
    const X_SIZE: u32 = 3 * PPU;
    const Y_SIZE: u32 = 2 * PPU;
    const X_ORIGIN: i32 = 2 * PPU as i32;
    const Y_ORIGIN: i32 = PPU as i32;

    let mut buffer = ImageBuffer::new(X_SIZE, Y_SIZE);

    let bailout_norm_sqr = 2.0 * 2.0;
    let bailout_cycles = 255;

    for (x, y, pixel) in buffer.enumerate_pixels_mut() {
        let x_re = (x as i32 - X_ORIGIN) as f64 / PPU as f64;
        let y_im = (y as i32 - Y_ORIGIN) as f64 / PPU as f64;
        let c = Complex::new(x_re, y_im);

        let mut z = Complex::new(0.0, 0.0);
        let mut escaped = 0;
        for cycle in 0..bailout_cycles {
            if z.norm_sqr() > bailout_norm_sqr { break }
            z = z * z + c;
            escaped = bailout_cycles - cycle;
        }
        *pixel = Luma([escaped as u8]);
    }

    let ref mut outfile = File::create("mandel.png").unwrap();
    image::ImageLuma8(buffer).save(outfile, image::PNG).unwrap();
}