In this very short video tutorial, we’ll take a look at one of the reasons why the error "At least 8 common tracks on both keyframes are needed for reconstruction" may appear when trying to reconstruct the motion of a real camera in Blender using Motion Tracking.
Video Transcript
Hello everyone! In this very short video tutorial, we’ll take a look at one of the reasons why the error "At least 8 common tracks on both keyframes are needed for reconstruction" may appear when trying to reconstruct the motion of a real camera in Blender using Motion Tracking.
The two keyframes Blender is referring to are the ones marked by two yellow lines in the progress bar at the bottom of the Movie Clip Editor. These are usually set automatically by Blender, but you can change them manually by adjusting the values of the Keyframe A and Keyframe B fields in the Solve tab of the editor. It’s generally recommended not to choose two keyframes that are too far apart in the Timeline, so Blender’s default settings work well in most cases.
Looking at the markers, automatically generated in this example using the Detect Features button in the Track tab, it seems there should be no problem tracking at least 8 between the two keyframes, as required by the software. And in fact, that’s true.
However, if we click Solve Camera Motion in the Solve tab, we get the infamous error message, with no further explanation.
In most cases, this error happens because we’ve skipped an intermediate step, which is somewhat hidden in the Track tab but clearly visible in the header of the Movie Clip Editor: it’s the Track Markers operation, found in the editor’s header or by expanding the Track section in the Track tab.
There are several tracking buttons that let you track the motion of selected markers. The Track Markers button allows you to track them throughout the entire duration of the clip. I’ll click on this button and perform the tracking over the whole video.
Most of the markers stop being tracked halfway through the clip. I won’t cover how to choose the markers or configure their parameters here, as those are topics for other tutorials. For now, we only care about having at least 8 well-tracked markers between Keyframes A and B, set here to Frame 1 and Frame 30 respectively, so that we can click Solve.
By manually scrolling through the Timeline, we can verify that there are well over 8 markers tracked between the two keyframes. If you don’t find 8 markers between the default Keyframes A and B, examine the video and adjust those two values to find a better range, then try calculating the tracking again, possibly using the buttons in the header to move forward or backward one frame at a time.
Now we can go to the Solve tab and confidently click on Solve Camera Motion. Since I haven’t cleaned up the trackers or configured their parameters properly, the Solve Error value will be unacceptably high, so the reconstruction won’t be accurate, but that, too, is a topic for another tutorial!
That’s it for the topic of this video. If you found it helpful, feel free to leave a Like and subscribe to the channel! Don’t forget to turn on notifications so you won’t miss future tutorials. See you next time!