diff --git a/src/bw.rs b/src/bw.rs
new file mode 100644
index 0000000..079fc41
--- /dev/null
+++ b/src/bw.rs
@@ -0,0 +1,61 @@
+use image::RgbaImage;
+use palette::{Hsl, Hsv, IntoColor, Lab, LinSrgb, Oklab, Srgb};
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub enum Method {
+    SrgbAverage,
+    LinSrgbAverage,
+    Hsl,
+    Hsv,
+    Cielab,
+    Oklab,
+}
+
+impl Method {
+    fn to_bw(self, pixel: Srgb) -> Srgb {
+        match self {
+            Self::SrgbAverage => {
+                let value = (pixel.red + pixel.green + pixel.blue) / 3.0;
+                Srgb::new(value, value, value)
+            }
+            Self::LinSrgbAverage => {
+                let pixel: LinSrgb = pixel.into_color();
+                let value = (pixel.red + pixel.green + pixel.blue) / 3.0;
+                LinSrgb::new(value, value, value).into_color()
+            }
+            Self::Hsl => {
+                let mut pixel: Hsl = pixel.into_color();
+                pixel.saturation = 0.0;
+                pixel.into_color()
+            }
+            Self::Hsv => {
+                let mut pixel: Hsv = pixel.into_color();
+                pixel.saturation = 0.0;
+                pixel.into_color()
+            }
+            Self::Cielab => {
+                let mut pixel: Lab = pixel.into_color();
+                pixel.a = 0.5;
+                pixel.b = 0.5;
+                pixel.into_color()
+            }
+            Self::Oklab => {
+                let mut pixel: Oklab = pixel.into_color();
+                pixel.a = 0.0;
+                pixel.b = 0.0;
+                pixel.into_color()
+            }
+        }
+    }
+}
+
+pub fn bw(image: &mut RgbaImage, method: Method) {
+    for pixel in image.pixels_mut() {
+        let [r, g, b, _] = pixel.0;
+        let srgb = Srgb::new(r, g, b).into_format::<f32>();
+        let srgb = method.to_bw(srgb).into_format::<u8>();
+        pixel.0[0] = srgb.red;
+        pixel.0[1] = srgb.green;
+        pixel.0[2] = srgb.blue;
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index de43bab..6637eee 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,61 +1 @@
-use image::RgbaImage;
-use palette::{Hsl, Hsv, IntoColor, Lab, LinSrgb, Oklab, Srgb};
-
-#[derive(Clone, Copy, PartialEq, Eq)]
-pub enum BwMethod {
-    SrgbAverage,
-    LinSrgbAverage,
-    Hsl,
-    Hsv,
-    Cielab,
-    Oklab,
-}
-
-impl BwMethod {
-    fn to_bw(self, pixel: Srgb) -> Srgb {
-        match self {
-            Self::SrgbAverage => {
-                let value = (pixel.red + pixel.green + pixel.blue) / 3.0;
-                Srgb::new(value, value, value)
-            }
-            Self::LinSrgbAverage => {
-                let pixel: LinSrgb = pixel.into_color();
-                let value = (pixel.red + pixel.green + pixel.blue) / 3.0;
-                LinSrgb::new(value, value, value).into_color()
-            }
-            Self::Hsl => {
-                let mut pixel: Hsl = pixel.into_color();
-                pixel.saturation = 0.0;
-                pixel.into_color()
-            }
-            Self::Hsv => {
-                let mut pixel: Hsv = pixel.into_color();
-                pixel.saturation = 0.0;
-                pixel.into_color()
-            }
-            Self::Cielab => {
-                let mut pixel: Lab = pixel.into_color();
-                pixel.a = 0.5;
-                pixel.b = 0.5;
-                pixel.into_color()
-            }
-            Self::Oklab => {
-                let mut pixel: Oklab = pixel.into_color();
-                pixel.a = 0.0;
-                pixel.b = 0.0;
-                pixel.into_color()
-            }
-        }
-    }
-}
-
-pub fn bw(image: &mut RgbaImage, method: BwMethod) {
-    for pixel in image.pixels_mut() {
-        let [r, g, b, _] = pixel.0;
-        let srgb = Srgb::new(r, g, b).into_format::<f32>();
-        let srgb = method.to_bw(srgb).into_format::<u8>();
-        pixel.0[0] = srgb.red;
-        pixel.0[1] = srgb.green;
-        pixel.0[2] = srgb.blue;
-    }
-}
+pub mod bw;
diff --git a/src/main.rs b/src/main.rs
index e2b1a4f..5af2199 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,6 +5,7 @@ use std::{
 
 use clap::Parser;
 use image::{ImageFormat, RgbaImage};
+use mark::bw;
 
 #[derive(Debug, Clone, Copy, clap::ValueEnum)]
 enum BwMethod {
@@ -16,15 +17,15 @@ enum BwMethod {
     Oklab,
 }
 
-impl From<BwMethod> for mark::BwMethod {
+impl From<BwMethod> for bw::Method {
     fn from(value: BwMethod) -> Self {
         match value {
-            BwMethod::SrgbAverage => mark::BwMethod::SrgbAverage,
-            BwMethod::LinSrgbAverage => mark::BwMethod::LinSrgbAverage,
-            BwMethod::Hsl => mark::BwMethod::Hsl,
-            BwMethod::Hsv => mark::BwMethod::Hsv,
-            BwMethod::Cielab => mark::BwMethod::Cielab,
-            BwMethod::Oklab => mark::BwMethod::Oklab,
+            BwMethod::SrgbAverage => bw::Method::SrgbAverage,
+            BwMethod::LinSrgbAverage => bw::Method::LinSrgbAverage,
+            BwMethod::Hsl => bw::Method::Hsl,
+            BwMethod::Hsv => bw::Method::Hsv,
+            BwMethod::Cielab => bw::Method::Cielab,
+            BwMethod::Oklab => bw::Method::Oklab,
         }
     }
 }
@@ -38,7 +39,7 @@ struct BwCmd {
 
 impl BwCmd {
     fn run(self, mut image: RgbaImage) -> RgbaImage {
-        mark::bw(&mut image, self.method.into());
+        bw::bw(&mut image, self.method.into());
         image
     }
 }