diff --git a/vtm/src/org/oscim/renderer/GLViewport.java b/vtm/src/org/oscim/renderer/GLViewport.java
index 6094d46e..44f8bf62 100644
--- a/vtm/src/org/oscim/renderer/GLViewport.java
+++ b/vtm/src/org/oscim/renderer/GLViewport.java
@@ -68,7 +68,8 @@ public class GLViewport extends Viewport {
     }
 
     /**
-     * Set MVP offset in screen pixel coordinates
+     * Set MVP so that coordinates are in screen pixel coordinates with 0,0
+     * being center
      */
     public void useScreenCoordinates(int width, int height, Position position, float xOffset, float yOffset, float scale) {
         float invScale = 1f / scale;