Fixed FPN mitigation - using farthest scene if perfect is not available

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

@@ -1388,7 +1388,9 @@ public class Interscene {
 			for (;(fpn_index >= 0) && !need_fpn_mitigate[fpn_index]; fpn_index--);
 			// find closest but farther than fpn_mitigate_dist;
 			fpn_pairs[ipair][0] = fpn_index; 
-			fpn_pairs[ipair][1] = -1; 
+			fpn_pairs[ipair][1] = -1;
+			double max_dist = 0.0;
+			int best_cond_pair = -1;
 			double best_dist = Double.NaN;
 			for (int i = latest_scene; i >= earliest_scene; i--) {
 				double fpn_dist = quadCLTs[latest_scene].estimateAverageShift(
@@ -1397,6 +1399,7 @@ public class Interscene {
 						avg_z,                    // double average_z,
 						false,                    // boolean use_rot,
 						rectilinear);                   // boolean rectilinear)
+				
 				if (fpn_dist < fpn_mitigate_dist) {
 					fpn_dist = quadCLTs[i].estimateAverageShift(
 							scenes_xyzatr[i], // double [][] xyzatr0,
@@ -1411,8 +1414,17 @@ public class Interscene {
 						fpn_pairs[ipair][1] = i;
 						need_fpn_mitigate[fpn_pairs[ipair][0]] = false;
 					}
+				} else if (fpn_dist > max_dist) {
+					max_dist = fpn_dist;
+					best_cond_pair = i;
 				}
 			}
+			if (fpn_pairs[ipair][1] < 0) {
+				System.out.print("Failed to find a good pair for scene #"+fpn_pairs[ipair][0]+". ");
+				System.out.println("Using #"+best_cond_pair+" with distance = "+max_dist+" pix.");
+				fpn_pairs[ipair][1] = best_cond_pair;
+				need_fpn_mitigate[fpn_pairs[ipair][0]] = false;
+			}
 		}
 		return fpn_pairs;
 	}