Hello
I am struggling to understand what the “reprojection error curve” display button in the graph display shows and means please?
Thanks
I’m guessing that it will show curves of the reprojection error instead of showing the position of the trackers. And the reprojection error is the difference in position on the screen of the 3d reconstructed position of a tracker minus the actual position of the tracker. It would allow you to quickly see which trackers are giving a problem and when.
Thank you for the reply. To be honest I don’t reall understand wnaht you are saying. I am sure that there is a really good reason for this but it is virtually impossible to use as it does not tell you which of the trackers is not good. I notice that the K and camera settings influcence it but I don’t understand how to use it. Do you know where i could get more info as I am really struggling with a difficult track that is not working at the solve point.
Thanks
Whaat exactly is your problem? Do you not know how tracking works, or do you have a problem with a specific track.
Thank you for your interest. Below is the difficult track that I am trying to handle. I have got to an average that is as good as I can but there are part of the shot that even if they average well, are screwing the solve. I thought that the graphs could help me to understand why my solve is not working.
Also, the solve does not put the track points in a coherent order in the 3D space.
Lastly, when I re-solve the shot, I can get really incoherent averages. It can go from a 0.8 to a 125.5 just by adding a K to the solve. The blue lines go mad. As I don’t really understand what they show, I don’t get how they can help me optimise the track.
Thanks again
If your tracks look good, but your solve is bad, it could be that your shot just doesn’t have enough parallax. The camera should move in the shot. If it doesn’t move, but only pans, you should use a tripod solve.
There is a slight travelling motion, enough to get a parallax. The tripod mode gives me worse results.
Would you like to put you clip somewhere, so we can have a look at it?
That would be very kind.
You can download the elements at
140508_TRACK_TEST can be accessed at https://eshare.yr.com/publicController.do?folderName=20140508&fileName=iCuRn62Rjh
Thank you
From what I see on the file you have very few trackers. Granted tracking the grass is not easy as there are few distinctive things to anchor the trackers on. All of your trackers are on the left side of the frame. Try adding as many tracking points as you can, specially on the right side and as close as possible to the camera. The tree line will give you also a few solid tracking points to be able to estimate rotation. Track also the objects that appear and disappear behind the woman, even if they show for a few frames. Track the object for as long as you can until it disappears and track it again when it reappears and then use the “merge: join tracks” button. Feed the solver as much information as you can to lower the error rate.
Good luck
Thank you very much for your reply.
Admittedly I have kept only the 8 minimum tracks needed. They are also set only on one side. I had tried with many more tracks but my average was very poor. I also was struggling with either using lots of good short tracks or working longer tracks with offsets. The offsets are hard as you really need to find an object that is very close by to have any chance of getting a decent continuation. As for the short tracks, initially they were just screwing the average, With the weight option in 2.7, I have tried to use it intelligently but as I don’t really understand how it works, it becomes very frustrating.
All to say that the blue lines in the graph display, show how the track is but I have no idea how to interpret them to make the result better. I will keep going.
Thanks again
clowden007: Sorry for not expanding earlier on the graphs.
This is how I understand them. IF ANYONE HAS MORE INFO, PLEASE FEEL FREE TO CORRECT ME.
The red and green lines on the graph show you the speed for the trackers at a given frame. Green is vertical movement, Red is horizontal. Objects closer to camera will move faster than those far away. The curves are useful to see if the trackers are moving differently than the average. A line that spikes way out might mean a tracking error or that the object is moving differently than the rest. By selecting the curve you are selecting the corresponding tracker also. And then you can see if it is slilpping or what is the possible issue.
In this example most of the trackers follow the same pattern. The red line that strays from the norm is a tracker that is wandering off. By manually moving the trackers or retracking with different settings you can get the curve to match with the rest. The lone green line at the bottom is another example of tracking gone wrong
The bunch of blue lines appear once there is a camera solve and represent the reprojection error for each tracker.
“The reprojection error is a geometric error corresponding to the image distance between a projected point and a measured one” (wikipedia).
Blender wiki says:
“Reprojeciton error means the average distance between reconstructed 3D position of tracks projected back to footage and original position of tracks. Basically, reprojection error below 0.3 means accurate reprojection, 0.3-3.0 means quite nice solving which still can be used. Values above 3 means some tracks should be tracked more accurately, or that values for focal length or distortion coefficients were set incorrectly.”
The blue curves cannot be selected on the graph window but if you select a tracker on the tracking windwow the corresponding graphic will turn a darker blue.
Finding the trackers with the most reprojection errors is easier on the dopesheet for tracking data. It has a way to sort by Average error and you can see the numbers right next to the trackers name.
Finally, the lonely blue line that comes out when you click on the film strip is the average per frame error.
This curve is available once there is a camera solve. This is the one line that you want to be as flat as possible. The high points will show you where in your shot you might find problems.
Thank you so much for such a detailed explaination.