diff --git a/com.minres.scviewer.database.ui.swt/src/com/minres/scviewer/database/swt/internal/WaveformCanvas.java b/com.minres.scviewer.database.ui.swt/src/com/minres/scviewer/database/swt/internal/WaveformCanvas.java
index 35ccff3..06687bc 100644
--- a/com.minres.scviewer.database.ui.swt/src/com/minres/scviewer/database/swt/internal/WaveformCanvas.java
+++ b/com.minres.scviewer.database.ui.swt/src/com/minres/scviewer/database/swt/internal/WaveformCanvas.java
@@ -235,6 +235,7 @@ public class WaveformCanvas extends Canvas{
 
     public void setZoomLevel(int level, long centerTime) {
     	long oldScaleFactor=scaleFactor;
+    	if(level<0) level = 0;
     	if(level<Constants.unitMultiplier.length*Constants.unitString.length){
     		this.level = level;
     		this.scaleFactor = (long) Math.pow(10, level/2);
diff --git a/com.minres.scviewer.database.ui.swt/src/com/minres/scviewer/database/swt/internal/WaveformViewer.java b/com.minres.scviewer.database.ui.swt/src/com/minres/scviewer/database/swt/internal/WaveformViewer.java
index cc557c7..7f0654f 100644
--- a/com.minres.scviewer.database.ui.swt/src/com/minres/scviewer/database/swt/internal/WaveformViewer.java
+++ b/com.minres.scviewer.database.ui.swt/src/com/minres/scviewer/database/swt/internal/WaveformViewer.java
@@ -180,9 +180,11 @@ public class WaveformViewer implements IWaveformViewer  {
 	            GC gc = e.gc;
 	            gc.setAlpha(128);
 	            int minX = Math.min(start.x, end.x);
-	            int maxX = Math.max(start.x, end.x);
-	            int width = maxX - minX;
-	            gc.fillRectangle(minX, 0, width, e.height);
+	            int width = Math.max(start.x, end.x) - minX;
+	            Point origin = waveformCanvas.getOrigin();
+	            int y_top = waveformCanvas.getRulerHeight()- origin.y;
+	            int y_bottom = waveformCanvas.getSize().y;
+	            gc.fillRectangle(minX, y_top, width,y_bottom);
 			}
 		}