added VARI

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@34153 15284696-431f-4ddb-bdfa-cd5b030d7da7

imagery/i.vi/i.vi.html

@@ -130,6 +130,15 @@ sr( redchan, nirchan )
 </pre>
 
 <pre>
+VARI: Visible Atmospherically Resistant Index
+VARI = (green - red ) / (green + red - blue)
+it was designed to introduce an atmospheric self-correction 
+Gitelson A.A., Kaufman Y.J., Stark R., Rundquist D., 2002.
+Novel algorithms for estimation of vegetation fraction 
+Remote Sensing of Environment (80), pp76-87. 
+</pre>
+
+<pre>
 WDVI: Weighted Difference Vegetation Index
 WDVI = nirchan - a * redchan
 if(soil_weight_line == None):

imagery/i.vi/vari.c

@@ -2,17 +2,19 @@
 #include<math.h>
 #include<stdlib.h>
 
-    /*VARI: ? */ 
-double va_ri(double redchan, double nirchan, double bluechan,
+/*VARI: Visible Atmospherically Resistant Index */ 
+double va_ri(double redchan, double bluechan,
 	      double greenchan) 
 {
+/* VARI is the Visible Atmospherically Resistant Index, it was 
+ * designed to introduce an atmospheric self-correction 
+ * Gitelson A.A., Kaufman Y.J., Stark R., Rundquist D., 2002.
+ * Novel algorithms for estimation of vegetation fraction 
+ * Remote Sensing of Environment (80), pp76-87.  */
     double result;
-
-     {
-
-    G_fatal_error("Not yet implemented");
-
-    } return result;
+    result = (greenchan - redchan ) / (greenchan + 
+		redchan - bluechan);
+    return result;
 }