Autodesk 12812-051462-9011 User Guide 2 - Page 1153

■ Feature searching is performed on internally scaled-up versions of the target image feature and portions of the search window. These versions are scaled up in inverse proportion to the subpixel level using bicubic interpolation, which allows the feature tracker to search at this scaled pixel level. When manually positioning gizmos at keyframes, it's crucial that you zoom in enough so you can place the gizmo in the center of the feature to the degree of accuracy allowed by the selected subpixel level. If you don't do this, the tracker won't be able to track the correct center at the selected resolution. TIP Even though the feature is first tracked to the nearest pixel at whole pixel increments and then tracked at the subpixel level within that one pixel range, the tracking time can increase significantly in proportion to the inverse square of the subpixel level. For this reason it's suggested that you set the subpixel level to as low as possible. Usually 1/8 of a pixel is adequate for features well distributed around the scene. Use greater levels if the features are fairly close together, or move very slowly within the frame. You can also reduce feature tracking times by making the inner feature bounds box in the gizmo as tight as you can while still retaining a couple of pixels worth of surrounding contrast. Search times are proportional to the feature bounds box size. Track Range Use these when a feature moves off-screen or is not visible in the Movie window. Set Start Sets the start of the active frame range for a tracker. Set Stop Sets the end of the active frame range for a tracker. You use the Set Start and Set Stop buttons in the Setup box together with the Movie Stepper rollout to set the active range. Pressing the buttons will set the start or stop point at the frame currently shown in the Movie Stepper. Tracker Keyframes and Position Data Any time you manually position a tracker gizmo, such as at the first frame or at some tracking error, you create a special keyframe position. The tracker never attempts to reposition these keyframes, but uses them as start points for the frames that follow them up to the next keyframe. At keyframes, you can adjust the position and the feature or search bounds. 3834 | Chapter 15 Animation