Before you begin calibrating, make sure that your camera views are clear of any unwanted reflections, whether they originate from markers or reflective materials in the volume. In either case, try to physically remove or cover the source of any reflections that are shown in each camera view as they can be mistaken for markers by the cameras. You can check the source of these reflections by using Video Preview Mode (see Check volume coverage). If removing the source of the reflections is not possible, you must mask your cameras to remove the pixels caused by the reflections so that they don't influence the 3D data.
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