button, or re-tracking follows the same erroneous path.No matter how sophisticated tracking becomes, some sequences are inevitably going to cause problems. There are a number of pre-tracking checks you can perform to assist Auto-Tracking:
• Play through the sequence before placing your tracking anchors
• Look for features that are consistent throughout the majority of the sequence
• Avoid occluded features where possible - see Dealing with Occlusions.
You can also adjust the way patterns are tracked and how often they are re-sampled, or grabbed, using the Settings tab and Auto-Tracking controls:
| 1. | Try adjusting the max iterations, epsilon, and max_error controls to improve the track accuracy: |
• max iterations - the maximum number of iterations before the tracking algorithm stops searching for features.
• epsilon/resolution - the error level at which Tracker is assumed to have found the feature - no further search for a better match is performed. Higher values may result in a faster but less accurate track.
• max_error - the error level at which Tracker stops searching for features.
| 2. | In the Auto-Tracking sub-menu, enable predict track to use the animation path to predict where the pattern may appear in the next frame. |
NOTE: If the track fails when prediction is enabled, click the Clear Forward button, or re-tracking follows the same erroneous path.
| 3. | Set the type of movement Tracker should expect in the pattern using the warp type dropdown: |
• Translate
• Translate/Rotate
• Translate/Scale
• Translate/Rotate/Scale
• Affine
Translation only is the easiest to calculate, but can lose the pattern if it rotates or scales. Affine can be used to attempt to preserve straight lines and relative distances, compensating for sheering.
| 4. | Try adjusting the pattern grabbing behavior, when or how often a new pattern should be grabbed from the sequence: |
• on first frame - the comparison pattern is grabbed on the first frame from with the pattern anchor. You might select this option if the feature translates but doesn’t rotate, scale, or sheer.
• every frame - the comparison pattern is grabbed at every frame in the sequence. This option takes longer to process, but can produce a smoother track.
• every n frames - allows you to set the frame interval between pattern grabs using the every n frames control.
• if error above - the comparison pattern is grabbed when the error value is greater than that specified by the when error > control. Setting this control to a low value grabs the pattern more often.
• if error below - the comparison pattern is grabbed when the error value is less than that specified by the when error < control.
• custom - this option enables all the pattern grab behavior controls, allowing you to fine-tune when the comparison pattern in grabbed through out the sequence.
| 5. | Enable when tracking is stopped to cause Tracker to re-grab the pattern at the current frame when tracking stops. |
| 6. | Enable when tracker is moved to cause Tracker to re-grab the pattern at the current frame when a tracking anchor is moved. |
Tracker’s offset capability allows you track an obscured feature using the relative position of another feature, providing that the distance between the two points remains constant.
| 1. | Track the pattern normally until the occlusion causes Tracker to fail. |
The zoom window helps to identify the failure point.
| 2. | Play though the sequence to identify a likely offset point - a pattern that remains equidistant from the original pattern grab. |
| 3. | Hold down Ctrl/Cmd and drag the tracking anchor to the offset position. |
The offset amount is recorded in the Tracks list and highlighted in yellow in the Viewer.
| 4. | Continue tracking as normal by clicking the track backward (Z) or forward (V) button.  |
Tracker combines the two tracks into a single continuous track.
| 5. | Use the clear backward and forward buttons to clear poor keyframes. Click clear all to remove all selected tracks and keyframes, excluding manually placed keyframes. |
NOTE: You can reset tracking anchor pattern and search areas by clicking 
.