initial

.gitignore

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+/target
+.devenv
+*.jpg
+*.png

Cargo.toml

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+[package]
+name = "floyd-steinberg"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+image = "0.25.6"

src/main.rs

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+use std::{env, error::Error};
+
+use image::{DynamicImage, ImageReader, Luma};
+
+#[allow(clippy::cast_sign_loss, clippy::cast_possible_truncation)]
+fn main() -> Result<(), Box<dyn Error>> {
+    let kernel: &[(i64, i64, f32)] = &[
+        (1, 0, 7. / 16.),
+        (-1, 1, 3. / 16.),
+        (0, 1, 5. / 16.),
+        (1, 1, 1. / 16.),
+    ];
+
+    let src = env::args().nth(1).ok_or("i need a source path")?;
+    let out_path = env::args().nth(2).ok_or("i need an output path")?;
+    let mut img = ImageReader::open(&src)?.decode()?.to_luma32f();
+    let mut out = img.clone();
+    for y in 0..img.height() {
+        for x in 0..img.width() {
+            let old_pixel = img.get_pixel(x, y);
+            let old_luma = old_pixel.0[0];
+            let luma = if old_luma < 0.5 { 0. } else { 1. };
+
+            out.put_pixel(x, y, Luma([luma]));
+
+            let error = old_luma - luma;
+            for (dx, dy, quant) in kernel {
+                let x = (i64::from(x) + dx) as u32;
+                let y = (i64::from(y) + dy) as u32;
+
+                if x >= img.width() || y >= img.height() {
+                    continue;
+                }
+
+                let error = error * quant;
+
+                let luma = img.get_pixel(x, y).0[0];
+                let corrected = luma + error;
+                img.put_pixel(x, y, Luma([corrected]));
+            }
+        }
+    }
+    DynamicImage::from(out).into_luma8().save(out_path)?;
+    Ok(())
+}