pixel interpolation and correct for y flip

src/main.rs

-use anyhow::bail;
-use image::{GenericImageView, ImageReader};
+use image::{GenericImageView, ImageReader, Rgba};
 
-const TOAST_TIME_SECONDS: u64 = 30;
+const TOAST_TIME_SECONDS: u64 = 10;
 const CENTER_X: f64 = 100.0;
 const CENTER_Y: f64 = 100.0;
-const BREAD_HEIGHT: f64 = 0.2;
+const BREAD_HEIGHT: f64 = 0.3;
 const IDLE_HEIGHT: f64 = BREAD_HEIGHT + 8.0;
-const PIXEL_SIZE_MM: f64 = 3.0;
+const PIXEL_SIZE_MM: f64 = 1.5;
 
 fn main() -> anyhow::Result<()> {
     let img = ImageReader::open("/home/stephen/Downloads/scd31-qr.png")?.decode()?;
 
     preamble();
 
-    for x in 0..img.width() {
-        for y in 0..img.height() {
-            let p = img.get_pixel(x, y);
-
-            match p.0 {
-                [0, 0, 0, 255] => {
-                    toast(
-                        x as f64 - (img.width() as f64 / 2.0),
-                        y as f64 - (img.height() as f64 / 2.0),
-                    );
-                }
-                [255, 255, 255, 255] => {
-                    // white
-                }
-                _ => bail!("Pixel was not entirely black nor entirely white: {:?}", p),
+    // 2x oversampling
+    for x in 0..(img.width() * 2 - 1) {
+        for y in 0..(img.height() * 2 - 1) {
+            let black = is_black(img.get_pixel(x / 2, y / 2))
+                && is_black(img.get_pixel(x / 2 + x % 2, y / 2))
+                && is_black(img.get_pixel(x / 2, y / 2 + y % 2))
+                && is_black(img.get_pixel(x / 2 + x % 2, y / 2 + y % 2));
+
+            if black {
+                toast(
+                    x as f64 - (img.width() as f64 / 2.0),
+                    y as f64 + (img.height() as f64 / 2.0), // correct for reflection when printing
+                );
             }
         }
     }
@@ -35,6 +32,14 @@ fn main() -> anyhow::Result<()> {
     Ok(())
 }
 
+fn is_black(p: Rgba<u8>) -> bool {
+    match p.0 {
+        [0, 0, 0, 255] => true,
+        [255, 255, 255, 255] => false,
+        _ => panic!("Pixel was not entirely black nor entirely white: {:?}", p),
+    }
+}
+
 fn preamble() {
     println!(
         r#"