+ \item[Two pass tracking] Although this parameter can be enabled at any time, it is useful only while simultaneous tracking both translation and rotation, when
+ the rotation amount is large. The normal tracking algorithm firstly
+ examines translation, then rotation. However, comparison of a strongly
+ tilted picture with the reference one can lead to rather inaccurate
+ determination of the translation vector. The subsequent determination of
+ rotation will also inexact because of the preceding translation error. The
+ second tracking pass, if enabled, refines translation and rotation.
+
+ If motion and/or rotation noise level is set above 0\% and two pass tracking
+ is enabled, then both noise levels affect the first tracking pass only.
+ During the second tracking pass of two pass tracking both noise levels are
+ ignored.
+
+ It should be noted that two pass tracking is not twice slower than the usual
+ single pass tracking. Actually it is only about 40\% slower. As the second
+ pass serves only for motion refinement, it starts from the reduced search
+ range for block positions and rotation angles. And, as the first pass only
+ yields an approximate position, which will be later refined during the
+ second pass, it skips subpixel position search and examines rotation angles
+ with the accuracy reduced to 1 degree.
+ \item[Track single frame] When this option is used the motion between a single
+ starting frame and the frame currently under the insertion point is
+ calculated. The starting frame is specified in the Frame number box. The
+ motion calculated this way is taken as the absolute motion vector. The
+ absolute motion vector for each frame replaces the absolute motion vector
+ for the previous frame. Settling speed has no effect on it since it does
+ not contain any previous motion vectors. Playback can start anywhere on the
+ timeline since there is no dependence on previous results. We talk about
+ Keep shape and it is the most precise way to calculate the motion vector;
+ but it only works well when the object to be traced does not change along
+ the clip, remaining identical in shape, size and luminance.
+
+ Strong rotation of the picture can also have a bad impact on the tracking
+ accuracy. If it is the case, accuracy can be significantly improved by
+ enabling two pass tracking.
+
+ Frame number The number of the reference frame used for motion tracking in
+ the \textit{Track single frame mode} is specified in this input field. Frame
+ number is set as the 0-based absolute number starting from the very
+ beginning of the whole timeline. Instead of manual calculating, it is
+ possible to get frame number directly from the current cursor position by
+ pressing the \textit{Get current} button.
+
+ Get current Pressing this handy button provides a convenient way to set the
+ Frame number parameter to the value of the current cursor position.
+
+ Add (loaded) offset from tracked frame This toggle can help when the tracked
+ object changes shape, leaves the screen area or becomes obscured by other
+ objects somewhere in the middle of the motion effect. Sometimes it can help
+ also if a strong rotation causes the motion tracker to miss the right
+ tracked object and wildly spring away.
+
+ To overcome this problem, one has to define two keyframes: the first one at
+ the start of the motion effect, the second shortly before the problematic
+ location in the clip. At the first keyframe one defines the motion effect
+ parameters as usual, the \textit{Add offset} toggle being switched off. At the
+ second keyframe the match block can be redefined to cover another region of
+ the picture which remains visible during the following part of the effect.
+ The \textit{Frame number} parameter should be set several frames before position of this second keyframe, and the \textit{Add offset} toggle switched on.
+
+ After defining the two keyframes (even more can be added in the similar way)
+ one switches at all the defined keyframes Calculation mode to \textit{Save coords
+ to tracking file}, Action mode to \textit{Do Nothing}, rewinds the timeline cursor
+ to the beginning of the motion effect, and starts playback. The determined
+ motion vectors will be written to the tracking file, the \textit{Add offset}
+ parameter having no effect during the save stage.
+
+ After playback finishes and all the relevant motion vectors are saved in the
+ tracking file, one switches at all the defined keyframes Calculation mode to
+ \textit{Load coords from tracking file}, Action mode to whatever one needs, rewinds again to the beginning of the motion effect and starts playback or
+ rendering. After reaching the second keyframe, where \textit{Add offset} was
+ switched on, the program continues to follow the second object adding the
+ total motion and rotation, accumulated so far.
+
+ Of course, the \textit{Add offset} toggle can be used in the \textit{Track single fram}e mode only. In the other tracking modi such a technics is unnecessary.
+ \item[Track previous frame] Causes only the motion between the previous frame and
+ the current frame to be calculated (Follow shape). This is added to an
+ absolute motion vector to get the new motion from the start of the sequence
+ to the current position. After every frame processed this way, the block
+ position is shifted to always cover the same region of the image. Playback
+ must be started from the start of the motion effect in order to accumulate
+ all the necessary motion vectors. This method is less precise because you
+ have error propagation between frames. However, it is essential when the
+ object changes shape, size or luminance. Possible inaccuracies caused by
+ rotation of the picture can partly be reduced by enabling two pass tracking