Fixed NaN when rendering with zero Motion Blur

src/main/java/com/elphel/imagej/gpu/GpuQuad.java

@@ -3843,7 +3843,7 @@ public class GpuQuad{ // quad camera description
 			final int                 threadsMax)  // maximal number of threads to launch
 	{
 		final double min_sub = 1e-12;
-		final double min_len = 0.1; // pix
+		final double min_len = 0.001; // pix was 0.1
 		int num_pairs = Correlation2d.getNumPairs(num_cams);
 		//change to fixed 511?
 //		final int task_code = ((1 << num_pairs)-1) << GPUTileProcessor.TASK_CORR_BITS; //  correlation only
@@ -3895,18 +3895,17 @@ public class GpuQuad{ // quad camera description
 						// calculate offset for the secondary tile and weigh
 						double dx = mb_vectors[0][nTile];
 						double dy = mb_vectors[1][nTile];
-						double mb_len = Math.sqrt(dx*dx+dy*dy); // in pixels/s
-						dx /= mb_len; // unit vector
-						dy /= mb_len;
-						mb_len *= mb_tau; // now in pixels
-						/*
+						double mb_len = Math.sqrt(dx*dx+dy*dy); // in pixels/s -> 0
 						double [] centerXY_sub = centerXY;
 						if (mb_len < min_len) {
 							tp_task.setScale(1.0);
 							tp_task_sub.task = 0; // disable
-							tp_task.setScale(-min_sub);
+							tp_task_sub.setScale(-min_sub);
+							tp_task_sub.setCenterXY(centerXY);
 						} else {
-						*/
+							dx /= mb_len; // unit vector
+							dy /= mb_len;
+							mb_len *= mb_tau; // now in pixels
 							double mb_offs = 1.0; // try 1 pixel. Maybe adjust for non-ortho, e.g. sqrt(2) for diagonal?
 							double min_offs = mb_len_scale * mb_len;
 							if (mb_offs < min_offs) {
@@ -3914,7 +3913,7 @@ public class GpuQuad{ // quad camera description
 							}
 							dx *= mb_offs;
 							dy *= mb_offs;
-							double [] centerXY_sub = new double[] {centerXY[0]+dx,centerXY[1]+dy};
+							centerXY_sub = new double[] {centerXY[0]+dx,centerXY[1]+dy};
 							tp_task_sub.setCenterXY(centerXY_sub);
 							double exp_offs = Math.exp(-mb_offs/mb_len);
 							double gain =    1.0/(1.0 - exp_offs);
@@ -3924,7 +3923,7 @@ public class GpuQuad{ // quad camera description
 							}
 							tp_task.setScale(gain);
 							tp_task_sub.setScale(gain_sub);
-//						}
+						}
 						boolean bad_margins = false;
 						if (calcPortsCoordinatesAndDerivatives) { // for non-GPU?
 							double [][] disp_dist = new double[quad_main][]; // used to correct 3D correlations (not yet used here)