reducing number of SfM pairs if do not fit

src/main/java/com/elphel/imagej/tileprocessor/OpticalFlow.java

@@ -5041,6 +5041,13 @@ public class OpticalFlow {
 					*/
 //					int      num_avg_pairs = 16; // number of scene pairs to average
 					int      num_avg_pairs = clt_parameters.imp.sfm_num_pairs;  // number of scene pairs to average
+					if (num_avg_pairs > ((last_index - earliest_scene)/2)) {
+						num_avg_pairs = (last_index - earliest_scene)/2;
+						if (debugLevel > -3) {
+							System.out.println("**** The specified num_avg_pairs="+clt_parameters.imp.sfm_num_pairs+
+									" is more than half-range. Reducing to "+num_avg_pairs);
+						}
+					}
 					QuadCLT[][][] scenes_seq_pairs = new QuadCLT[3][num_avg_pairs][2];
 					for (int i = 0; i < num_avg_pairs; i++) {
 						scenes_seq_pairs[0][i][0] = quadCLTs[ref_index + num_avg_pairs - 1 - i];
@@ -5151,7 +5158,7 @@ public class OpticalFlow {
 						!batch_mode,    // boolean        test_motion_blur,
 						debugLevel) ;   // int            debugLevel)
 				// should update earliest_scene
-				if ((ref_index - earliest_scene + 1) < min_num_scenes) {
+				if ((last_index - earliest_scene + 1) < min_num_scenes) {
 					System.out.println("After reAdjustPairsLMAInterscene() total number of useful scenes = "+(ref_index - earliest_scene + 1)+
 							" < "+min_num_scenes+". Scrapping this series.");
 					if (start_ref_pointers != null) {