Abstract: A method and apparatus are described for monitoring frame alignment status and correcting misalignment for film in a stereoscopic 3D projection system, in which gap indicia are provided onto the film in a gap between images, whether the images belong to the same frame or different adjacent frames. Thus, gap indicia are positioned in at least one of an interframe gap and an intraframe gap of the stereoscopic film. During projection of the film, presence or absence of the gap indicia is used to determine a frame alignment status. Frame misalignment is then easily diagnosed and corrected. Characteristics of the gap indicia are modified for improving frame alignment diagnosis.