diff --git a/source/net/yacy/peers/graphics/EncodedImage.java b/source/net/yacy/peers/graphics/EncodedImage.java
index 43f212396..ac4afbf82 100644
--- a/source/net/yacy/peers/graphics/EncodedImage.java
+++ b/source/net/yacy/peers/graphics/EncodedImage.java
@@ -35,7 +35,8 @@ public class EncodedImage {
     private boolean isStatic;
 
     /**
-     * set an encoded image; prefer this over methods with Image-source objects because png generation is faster when done from RasterPlotter sources
+     * set an encoded image; prefer this over methods with Image-source objects because png generation is faster when done from RasterPlotter sources. 
+     * Image ByteBuffer will be empty when encoding format is not supported.
      * @param sourceImage the image
      * @param targetExt the target extension of the image when converted into a file
      * @param isStatic shall be true if the image will never change, false if not
@@ -47,7 +48,7 @@ public class EncodedImage {
     }
     
     /**
-     * set an encoded image from a buffered image
+     * set an encoded image from a buffered image. Image ByteBuffer will be empty when encoding format is not supported.
      * @param sourceImage the image
      * @param targetExt the target extension of the image when converted into a file
      * @param isStatic shall be true if the image will never change, false if not
@@ -60,7 +61,7 @@ public class EncodedImage {
     }
     
     /**
-     * set an encoded image from a buffered image
+     * set an encoded image from a buffered image. Image ByteBuffer will be empty when encoding format is not supported.
      * @param sourceImage the image
      * @param targetExt the target extension of the image when converted into a file
      * @param isStatic shall be true if the image will never change, false if not
@@ -101,7 +102,7 @@ public class EncodedImage {
     }
     
     /**
-     * get the encoded image
+     * get the encoded image data (empty when encoding format is not supported)
      * @return the bytes of the image encoded into the target extension format
      */
     public ByteBuffer getImage() {
diff --git a/source/net/yacy/visualization/RasterPlotter.java b/source/net/yacy/visualization/RasterPlotter.java
index 9ccae13e8..2134188bc 100644
--- a/source/net/yacy/visualization/RasterPlotter.java
+++ b/source/net/yacy/visualization/RasterPlotter.java
@@ -906,11 +906,18 @@ public class RasterPlotter {
         return dest;
     }
 
+    /**
+     * Encode buffered image using specified format to a new ByteBuffer
+     * @param image image to encode
+     * @param targetExt format name. For example "png".
+     * @return a ByteBuffer instance containing encoded data, or empty if an error occured or target format is not supported.
+     */
     public static ByteBuffer exportImage(final BufferedImage image, final String targetExt) {
     	// generate an byte array from the given image
     	final ByteBuffer baos = new ByteBuffer();
     	ImageIO.setUseCache(false); // because we write into ram here
     	try {
+    		/* When no ImageIO writer is found image might no be written*/
     		ImageIO.write(image, targetExt, baos);
     		return baos;
     	} catch (final IOException e) {