diff --git a/doc/index.md b/doc/index.md
new file mode 100644
index 0000000..9256125
--- /dev/null
+++ b/doc/index.md
@@ -0,0 +1,25 @@
+# Function Overview
+
+This document intended to assist developers who seek to extend the game by modifying source code in src/.
+
+## Classes
+
+### BackgroundImage
+
+Its constructor takes the following arguments:
+
+`int x, int y, BufferedImage backgroundImage, int width, int height, int parallaxRatio`
+
+Draws a background image that moves one pixel every `parallaxRatio` pixels when `BackgroundImage.draw()` is called.
+
+### Camera
+
+placeholder
+
+## Utility Functions
+
+These functions are located in the `public final` class UtilityFunction; please note that the UtilityFunction constructor is private to prevent initialization.
+
+### readFromFile
+
+placeholder
\ No newline at end of file
diff --git a/src/BackgroundImage.java b/src/BackgroundImage.java
index de43202..beb758a 100644
--- a/src/BackgroundImage.java
+++ b/src/BackgroundImage.java
@@ -4,22 +4,21 @@ import java.awt.image.BufferedImage;
 public class BackgroundImage {
     public int x, y;
     public int width, height;
+    public int parallaxRatio;
     public BufferedImage backgroundImage;
 
-    public BackgroundImage(int x, int y, BufferedImage backgroundImage, int width, int height) throws SpriteException {
-        int compressionRatio;
-
+    public BackgroundImage(int x, int y, BufferedImage backgroundImage, int width, int height, int parallaxRatio) {
         this.x = x;
         this.y = y;
         this.width = width;
         this.height = height;
 
         this.backgroundImage = backgroundImage;
-        // compressionRatio = Math.max(backgroundImage.getWidth() / width, backgroundImage.getHeight() / height);
-        // this.backgroundImage = backgroundImage.getSubimage((backgroundImage.getWidth() - width)/2,
-        //         (backgroundImage.getHeight() - height)/2, width, height);
+        this.parallaxRatio = parallaxRatio;
     }
     public void draw(Graphics g){
-        g.drawImage(backgroundImage, x, y, width, height, null);
+        g.drawImage(backgroundImage, x-GamePanel.camera.x/parallaxRatio % width, y, width, height, null);
+        // added to prevent the background image from disappearing
+        g.drawImage(backgroundImage, x-GamePanel.camera.x/parallaxRatio % width + width - 1, y, width, height, null);
     }
 }
diff --git a/src/GamePanel.java b/src/GamePanel.java
index 04f957e..8918f46 100644
--- a/src/GamePanel.java
+++ b/src/GamePanel.java
@@ -50,7 +50,7 @@ public class GamePanel extends JPanel implements Runnable, KeyListener{
 
   public GamePanel() throws IOException, SpriteException, UnsupportedAudioFileException, LineUnavailableException {
     camera = new Camera(0);
-    background = new BackgroundImage(0, 0, backgroundImage, GAME_WIDTH, GAME_HEIGHT);
+    background = new BackgroundImage(0, 0, backgroundImage, GAME_WIDTH, GAME_HEIGHT, 10);
     for (int i = 0; i < 11; i++) {
       try {
         BufferedImage sprite = getImage(String.format("img/walk/p1_walk%s.png", String.format("%1$2s", i+1).replace(' ', '0')));