Found the perfect value

src/dithering.rs

@@ -1,7 +1,25 @@
 use image::{
-    DynamicImage, GrayImage,
+    DynamicImage, GrayImage, Luma,
-    imageops::{self, colorops},
+    imageops::{self, BiLevel, colorops, colorops::ColorMap, index_colors},
 };
+
+struct CustomLevel {
+    threshold: u8,
+}
+
+impl ColorMap for CustomLevel {
+    type Color = Luma<u8>;
+
+    fn index_of(&self, color: &Self::Color) -> usize {
+        let v = color.0[0];
+        if v >= self.threshold { 1 } else { 0 }
+    }
+
+    fn map_color(&self, color: &mut Self::Color) {
+        color.0[0] = if color.0[0] >= self.threshold { 255 } else { 0 };
+    }
+}
+
 pub fn auto_brighten(gray: &mut GrayImage) -> () {
     let avg = gray.pixels().map(|p| p.0[0] as u32).sum::<u32>() / gray.len() as u32;
 
@@ -14,7 +32,8 @@ pub fn auto_brighten(gray: &mut GrayImage) -> () {
 }
 
 pub fn dither(gray: &mut GrayImage) -> Vec<u8> {
-    imageops::dither(gray, &imageops::colorops::BiLevel);
+    let level = CustomLevel { threshold: 175 };
+    let bw: image::ImageBuffer<Luma<u8>, Vec<u8>> = index_colors(&gray, &level);
 
     let (w, h) = (gray.width() as usize, gray.height() as usize);
     let pitch = (w + 7) >> 3;
@@ -24,7 +43,7 @@ pub fn dither(gray: &mut GrayImage) -> Vec<u8> {
         for x in 0..w {
             let byte = y * pitch + (x >> 3);
             let bit = 7 - (x & 7);
-            if gray[(x as u32, y as u32)].0[0] == 0 {
+            if bw[(x as u32, y as u32)].0[0] == 0 {
                 bits[byte] |= 1 << bit;
             }
         }

src/escpos.rs

@@ -30,7 +30,7 @@ pub fn escpos_raster(
     let img = img.resize(w, h, FilterType::Triangle);
 
     let mut gray = img.to_luma8();
-    auto_brighten(&mut gray);
+    // auto_brighten(&mut gray);
 
     let mono = dither(&mut gray);
     let (width_px, height_px) = (img.width() as u16, img.height() as u16);