eoc4: implement suggestion at the end of chapter 4

rtchallenge/examples/eoc4.rs

@@ -0,0 +1,48 @@
+use std::f32::consts::PI;
+
+use anyhow::Result;
+
+use rtchallenge::{
+    canvas::Canvas,
+    matrices::Matrix4x4,
+    tuples::{Color, Tuple},
+};
+
+fn draw_dot(c: &mut Canvas, x: usize, y: usize) {
+    let red = Color::new(1., 0., 0.);
+    c.set(x.saturating_sub(1), y.saturating_sub(1), red);
+    c.set(x, y.saturating_sub(1), red);
+    c.set(x + 1, y.saturating_sub(1), red);
+    c.set(x.saturating_sub(1), y, red);
+    c.set(x, y, red);
+    c.set(x + 1, y, red);
+    c.set(x.saturating_sub(1), y + 1, red);
+    c.set(x, y + 1, red);
+    c.set(x + 1, y + 1, red);
+}
+fn main() -> Result<()> {
+    let w = 200;
+    let h = w;
+    let mut c = Canvas::new(w, h);
+    let t = Matrix4x4::translate(0., 0.75, 0.);
+    let p = Tuple::point(0., 0., 0.);
+    let rot_hour = Matrix4x4::rotation_z(-PI / 6.);
+
+    let mut p = t * p;
+    let w = w as f32;
+    let h = h as f32;
+    let h_w = w / 2.0;
+    let h_h = h / 2.0;
+    for _ in 0..12 {
+        let x = (h_w + h_w * p.x) as usize;
+        // Y-axis is 0 at the top, and h at the bottom, flip it.
+        let y = (h - (h_h + h_h * p.y)) as usize;
+        draw_dot(&mut c, x, y);
+        p = rot_hour * p;
+    }
+
+    let path = "/tmp/eoc4.png";
+    println!("saving output to {}", path);
+    c.write_to_file(path)?;
+    Ok(())
+}