diff --git a/Sources/geometrize/Bitmap.swift b/Sources/geometrize/Bitmap.swift
index eafefea..890fa7d 100644
--- a/Sources/geometrize/Bitmap.swift
+++ b/Sources/geometrize/Bitmap.swift
@@ -371,3 +371,30 @@ extension Bitmap {
     }
 
 }
+
+extension Bitmap {
+
+    /// Calculates the root-mean-square error between two bitmaps.
+    func differenceFull(with second: Bitmap) -> Double {
+        assert(width == second.width)
+        assert(height == second.height)
+
+        var total: Int64 = 0
+
+        for y in 0..<height {
+            for x in 0..<width {
+                let f: Rgba = self[x, y]
+                let s: Rgba = second[x, y]
+
+                let dr: Int32 = Int32(f.r) - Int32(s.r)
+                let dg: Int32 = Int32(f.g) - Int32(s.g)
+                let db: Int32 = Int32(f.b) - Int32(s.b)
+                let da: Int32 = Int32(f.a) - Int32(s.a)
+                total += Int64(dr * dr + dg * dg + db * db + da * da)
+            }
+        }
+
+        return sqrt(Double(total) / (Double(width) * Double(height) * 4.0)) / 255.0
+    }
+
+}
diff --git a/Sources/geometrize/Core.swift b/Sources/geometrize/Core.swift
index 8107dbf..7a4252e 100644
--- a/Sources/geometrize/Core.swift
+++ b/Sources/geometrize/Core.swift
@@ -112,35 +112,6 @@ func computeColor(
     return Rgba(r: r, g: g, b: b, a: alpha)
 }
 
-/// Calculates the root-mean-square error between two bitmaps.
-/// - Parameters:
-///   - first: The first bitmap.
-///   - second: The second bitmap.
-/// - Returns: The difference/error measure between the two bitmaps.
-func differenceFull(first: Bitmap, second: Bitmap) -> Double {
-    assert(first.width == second.width)
-    assert(first.height == second.height)
-
-    let width = first.width
-    let height = first.height
-    var total: Int64 = 0
-
-    for y in 0..<height {
-        for x in 0..<width {
-            let f: Rgba = first[x, y]
-            let s: Rgba = second[x, y]
-
-            let dr: Int32 = Int32(f.r) - Int32(s.r)
-            let dg: Int32 = Int32(f.g) - Int32(s.g)
-            let db: Int32 = Int32(f.b) - Int32(s.b)
-            let da: Int32 = Int32(f.a) - Int32(s.a)
-            total += Int64(dr * dr + dg * dg + db * db + da * da)
-        }
-    }
-
-    return sqrt(Double(total) / (Double(width) * Double(height) * 4.0)) / 255.0
-}
-
 /// Calculates the root-mean-square error between the parts of the two bitmaps within the scanline mask.
 /// This is for optimization purposes, it lets us calculate new error values only for parts of the image
 /// we know have changed.
diff --git a/Sources/geometrize/GeometrizeModelBase.swift b/Sources/geometrize/GeometrizeModelBase.swift
index f7d7dd2..ce957c5 100644
--- a/Sources/geometrize/GeometrizeModelBase.swift
+++ b/Sources/geometrize/GeometrizeModelBase.swift
@@ -8,7 +8,7 @@ class GeometrizeModelBase {
     init(targetBitmap: Bitmap) {
         self.targetBitmap = targetBitmap
         currentBitmap = Bitmap(width: targetBitmap.width, height: targetBitmap.height, color: targetBitmap.averageColor())
-        lastScore = differenceFull(first: targetBitmap, second: currentBitmap)
+        lastScore = targetBitmap.differenceFull(with: currentBitmap)
     }
 
     /// Creates a model that will optimize for the given target bitmap, starting from the given initial bitmap.
@@ -19,7 +19,7 @@ class GeometrizeModelBase {
     init(target: Bitmap, initial: Bitmap) {
         targetBitmap = target
         currentBitmap = initial
-        lastScore = differenceFull(first: target, second: currentBitmap)
+        lastScore = target.differenceFull(with: currentBitmap)
         assert(target.width == currentBitmap.width)
         assert(target.height == currentBitmap.height)
     }
@@ -28,7 +28,7 @@ class GeometrizeModelBase {
     /// - Parameter backgroundColor: The starting background color to use.
     func reset(backgroundColor: Rgba) {
        currentBitmap.fill(color: backgroundColor)
-       lastScore = differenceFull(first: targetBitmap, second: currentBitmap)
+        lastScore = targetBitmap.differenceFull(with: currentBitmap)
    }
 
     var width: Int { targetBitmap.width }
diff --git a/Tests/geometrizeTests/CoreTests.swift b/Tests/geometrizeTests/CoreTests.swift
index e34f1fa..b87b57f 100644
--- a/Tests/geometrizeTests/CoreTests.swift
+++ b/Tests/geometrizeTests/CoreTests.swift
@@ -7,7 +7,7 @@ final class CoreTests: XCTestCase {
         let blackBitmap = Bitmap(width: 10, height: 10, color: .black)
 
         // Difference with itself is 0
-        XCTAssertEqual(differenceFull(first: blackBitmap, second: blackBitmap), 0)
+        XCTAssertEqual(blackBitmap.differenceFull(with: blackBitmap), 0)
 
         var blackBitmapOnePixelChanged = blackBitmap
         blackBitmapOnePixelChanged[0, 0] = .white
@@ -15,13 +15,13 @@ final class CoreTests: XCTestCase {
         blackBitmapTwoPixelsChanged[0, 1] = .white
 
         // Changing two pixels means there's more difference than changing one.
-        XCTAssertTrue(differenceFull(first: blackBitmap, second: blackBitmapTwoPixelsChanged) > differenceFull(first: blackBitmap, second: blackBitmapOnePixelChanged))
+        XCTAssertTrue(blackBitmap.differenceFull(with: blackBitmapTwoPixelsChanged) > blackBitmap.differenceFull(with: blackBitmapOnePixelChanged))
 
         // Now the same for white image
         let whiteBitmap = Bitmap(width: 10, height: 10, color: .white)
 
         // Difference with itself is 0
-        XCTAssertEqual(differenceFull(first: whiteBitmap, second: whiteBitmap), 0)
+        XCTAssertEqual(whiteBitmap.differenceFull(with: whiteBitmap), 0)
 
         var whiteBitmapOnePixelChanged = whiteBitmap
         whiteBitmapOnePixelChanged[0, 0] = .black
@@ -29,7 +29,7 @@ final class CoreTests: XCTestCase {
         whiteBitmapTwoPixelsChanged[0, 1] = .black
 
         // Changing two pixels means there's more difference than changing one.
-        XCTAssertTrue(differenceFull(first: whiteBitmap, second: whiteBitmapTwoPixelsChanged) > differenceFull(first: whiteBitmap, second: whiteBitmapOnePixelChanged))
+        XCTAssertTrue(whiteBitmap.differenceFull(with: whiteBitmapTwoPixelsChanged) > whiteBitmap.differenceFull(with: whiteBitmapOnePixelChanged))
     }
 
     func testDifferenceFullComparingResultWithCPlusPlus() throws {
@@ -37,7 +37,7 @@ final class CoreTests: XCTestCase {
         let bitmapFirst = Bitmap(stringLiteral: try String(contentsOf: firstUrl))
         let secondUrl = Bundle.module.url(forResource: "differenceFull bitmap second", withExtension: "txt")!
         let bitmapSecond = Bitmap(stringLiteral: try String(contentsOf: secondUrl))
-        XCTAssertEqual(differenceFull(first: bitmapFirst, second: bitmapSecond), 0.170819, accuracy: 0.000001)
+        XCTAssertEqual(bitmapFirst.differenceFull(with: bitmapSecond), 0.170819, accuracy: 0.000001)
     }
 
     func testDifferencePartialComparingResultWithCPlusPlus() throws {