CLAUDE: undistortPxPy(): invert correction direction (getRDistByR)

The RECTILINEAR nadir pass multiplied sampling coordinates by getRByRDist()
(distorted->ideal), but sampling positions must be moved to where the ideal
ray lands in the distorted image - the FORWARD distortion getRDistByR(),
same direction as GeometryCorrection.getImageCoordinates(). The old code
ADDED the common radial distortion to the output instead of removing it.

Evidence (graveyard ref 1763232420_274928, flat constant-disparity terrain
where translation flow must be uniform for a true pinhole image):
- plain NADIR-MERGED edge/center flow ratio 0.965 (expected 0.954 from the
  calibration polynomial: ~4.8 px barrel at the edge),
- old RECTILINEAR 0.952 - worse than uncorrected,
- COLMAP self-calibrated f=1003/k1=-0.385 on the old rectilinear stack.
Co-authored-by: Claude <claude@elphel.com>

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

@@ -11104,11 +11104,21 @@ java.lang.NullPointerException
     }
 
     /**
-     * Undistort pixel coordinates in a pXpYD tile array in-place.
-     * Converts distorted sensor coordinates to rectilinear (pinhole) coordinates
-     * using the scene's radial distortion calibration (getRByRDist).
-     * With Boson 640, the correction is <1 px but eliminates the systematic
-     * bowl effect in COLMAP reconstructions that use a PINHOLE camera model.
+     * Convert a pXpYD tile array (in-place) so that rendering it produces a
+     * RECTILINEAR (pinhole) output image.
+     *
+     * The tile coordinates are SAMPLING positions in the common (radially
+     * distorted) virtual-camera space.  For output pixel u (the tile grid) to
+     * show the content of the ideal pinhole ray through u, it must sample the
+     * distorted image where that ray actually lands: rD = R * (Rdist/R), i.e.
+     * apply the FORWARD distortion getRDistByR() - same direction as
+     * GeometryCorrection.getImageCoordinates() (world -> distorted pixel).
+     *
+     * (The previous version used getRByRDist() - the inverse - which ADDED the
+     * common radial distortion to the output instead of removing it: measured
+     * edge/center translation-flow ratio 0.952 vs 0.965 of the non-undistorted
+     * stack, and COLMAP self-calibrated k1=-0.38 on supposedly pinhole images.)
+     *
      * @param pXpYD  tile array: each non-null element is {pX, pY, disparity}
      * @param gc     GeometryCorrection providing distortion model and pixel geometry
      */
@@ -11122,11 +11132,11 @@ java.lang.NullPointerException
     		if (tile == null) continue;
     		double pXc = tile[0] - cx;
     		double pYc = tile[1] - cy;
-    		double rD_mm = Math.sqrt(pXc * pXc + pYc * pYc) * mmPerPix;
-    		if (rD_mm > 0) {
-    			double rND2R = gc.getRByRDist(rD_mm / distRad, false);
-    			tile[0] = pXc * rND2R + cx;
-    			tile[1] = pYc * rND2R + cy;
+    		double rND_mm = Math.sqrt(pXc * pXc + pYc * pYc) * mmPerPix;
+    		if (rND_mm > 0) {
+    			double rD2RND = gc.getRDistByR(rND_mm / distRad);
+    			tile[0] = pXc * rD2RND + cx;
+    			tile[1] = pYc * rD2RND + cy;
     		}
     	}
     }