The options on the Output Rect page of the Tracking property editor allow you to output a specific region of the image.
In the FxTree, double-click the Tracking operator to open its property editor and preview it in the Fx Viewer.
Click the Output Rect button to display the Output Rect page, select Apply to activate the output rectangle.
Set the Output Rect 
Left/Bottom/Top/Right values to define the boundaries of the output rectangle in pixels.
