Autodesk 62205-051408-9001 User Guide - Page 719
Adjusting Tolerance, Using a Roaming Reference Box, Positioning the Reference and Tracker Boxes
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Advanced Tracking Techniques ❚❘❘ Tracker to lose the reference feature and set incorrect keyframes. Although you can manually correct these keyframes, it is easier to make the tracker box large enough to accommodate the motion of the reference feature and restart the analysis. Note: Processing time increases as the size of the tracker box increases. To adjust the size of the tracker box: 1. Pause the analysis. 2. Return to the first frame before the Tracker lost its reference feature. 3. Increase the size of the tracker box to prevent the Tracker from losing its reference feature. See Positioning the Reference and Tracker Boxes. 4. Restart the analysis. Adjusting Tolerance Reduce the tolerance value if the reference feature becomes hidden by another object for several frames and does not change direction during that time. This prevents the Tracker from setting incorrect keyframes while the reference feature is hidden. Adjusting the tolerance value can prevent or reduce the need to track manually. This strategy works best when the frame-to-frame motion of the reference feature (while it is hidden) has the same direction and speed as the frame-toframe motion it had just before it became hidden. The Tracker can extrapolate likely future locations of the reference feature while it is out of view. The position of the tracker box may then coincide with the position of the reference feature when it reappears. For example, say a truck enters the frame from the left in your clip. After several frames it enters a tunnel, stays in the tunnel for several frames, and then exits the tunnel on the right. You use the headlight on the truck as a reference feature. You pause the analysis as the headlight enters the tunnel, lower the tolerance until the Tracker cannot find a match, and then restart the analysis. When the headlight reappears, the tracker box is positioned over it, the Tracker finds a match, sets a keyframe, and continues tracking. Note: Tolerance is "animatable". That is, you can change the tolerance value at different points during the analysis. To adjust tolerance: 1. Analyze the clip with the default tolerance value. 2. Pause the analysis when the Tracker loses its reference feature. 3. In the Playback controls, click the Previous Keyframe button to return to the frame where the reference feature is first hidden by another object. 4. In the Tracker panel, reduce the tolerance value until the two diagonals in the tracker box disappear. The two diagonals indicate that a keyframe was set. Make sure you lower tolerance enough so that the Tracker does not make a bad match. 5. Restart the analysis. The analysis resumes. No keyframes are set while the reference feature is hidden. The Tracker extrapolates the estimated position of the reference feature until it reappears. Using a Roaming Reference Box Use a roaming reference box when the reference feature changes gradually from the first frame to the last frame in the clip, for example, if the reference feature is gradually rotating or changing size or shape. 703
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