Method to reference channels for Issue #619

java_package/brainflow/src/main/java/brainflow/DataFilter.java

@@ -1040,6 +1040,41 @@ public static double get_band_power (Pair<double[], double[]> psd, double freq_s
         return res[0];
     }
 
+    /**
+     * (re)reference channels
+     * 
+     * To eliminate ground-related noise, an online reference is chosen out of the recorded EEG channels. 
+     * This reference is used as “baseline” for all remaining EEG channels. 
+     * This means the signal at the other EEG channels is expressed as the difference from this reference.
+     * 
+     * Thie method averages data from reference_channels and then subtract the averaged data from channels_to_reference
+     * 
+     * @param data  data to process
+     * @param channels_to_reference channels to be referenced or expressed as difference from reference_channels averaged data
+     * @param reference_channels    channels to use as reference or baseline
+     * @return void
+     */
+    public static void reference (double[][] data, ArrayList<Integer> channels_to_reference, ArrayList<Integer> reference_channels)
+            throws BrainFlowError
+    {
+        // Calculate the average data points of reference_channels
+        double[] avg_reference = new double[data[0].length];
+        for (int channel : reference_channels) {
+            for (int i = 0; i < data[channel].length; i++) {
+                avg_reference[i] += data[channel][i];
+            }
+        }
+        for (int i = 0; i < avg_reference.length; i++) {
+            avg_reference[i] /= reference_channels.size();
+        }    
+        // Subtract the average of reference_channels from each channel in channels_to_reference
+        for (int channel : channels_to_reference) {
+            for (int i = 0; i < data[channel].length; i++) {
+                data[channel][i] -= avg_reference[i];
+            }
+        }
+    }
+
     /**
      * calculate nearest power of two
      */