Once you have finished tracking using a combination of automatic and manual tracking (see "Marker Tracking: Point Tracking" in the Table of Contents), points should be checked for high reprojection errors:

1) Select the Plot window (View -> Plot if not already open).

2) Select "Reprojection Error" from the Plot type drop-down menu, and select a point from the Marker drop-down menu:

3) Identify frames with unusually high reprojection errors in the plot.

Thresholds for acceptable reprojection errors will vary from point to point, but should generally be lower than 1 pixel. A better guideline is to look for areas in the reprojection error plot where the error jumps substantially compared to its baseline.

4) Set the error threshold in the plot window to a reasonable baseline. In the example plot, we can see that the reprojection error mostly stays below 0.6 pixels.

5) Starting from frame 1, use the 4 key to jump to the next frame that exceeds that threshold.

6) Use the arrow keys or click in the timeline to a frame with an acceptable error and open the Detailed View (View -> Detailed View), then advance to the high error frame.

In this example, the reprojection error of marker 1 increases starting around frame 259, when marker 1 passes very close to another marker in Camera 2.

7) Use the Detailed View to refine point placement; you can't refine points in the main view windows, although you can switch between points in that window. Use the epipolar line as a guide for the correct point placement, but put the locator in the center of the marker (rather than prioritizing placing it directly on the epipolar line).

8) Press the 2 key to track a frame forward and see if automatic tracking is working again. If not, continue correcting and refining for any frames with high reprojection errors until they are all reduced.

Keyboard Shortcuts

The following keyboard shortcuts are useful for automatic and manual point tracking:

• 1 and 2 track backward and forward one frame at a time
• 5 and 6 track backward and forward by several frames at a time (specify in XMALab -> Preferences -> General)
• Q and W or arrow keys advance a frame without tracking
• 3 moves to the last frame of the current tracked segment
• Shift+3 moves to the first frame of the current tracked segment
• Esc stops tracking
• Shift + left-click a point in the image to select it in the Points window
• Scroll to zoom in camera views

For a full list of available keyboard shortcuts in XMALab, go to View -> Help.

Quality Control

After manually correcting frames with high reprojection errors, performing a polynomial fit on each point should help to further reduce errors/noise within the tracked points. See "Quality Control: Refinement of Marker Position" in the Table of Contents.