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Interlaced and noninterlaced footage


    Another way to classify analog or digital footage is as interlaced or noninterlaced. Currently, most broadcast video is interlaced; each frame consists of two fields displayed in turn. The computer operating system and After Effects display noninterlaced video, also known as progressive scan, in which each frame is displayed completely from top to bottom. When you finish creating a movie for display on a television, After Effects can render it back to fields, maintaining high quality for broadcast display.

    Interlaced video consists of two fields, known either as Field 1 and Field 2, even and odd, or (as After Effects refers to them) upper and lower. When these fields are presented sequentially on an NTSC or PAL video monitor they create a smooth, unified picture. See Frames and fields.

    In interlaced video, a frame is divided into two fields. Each field contains every other horizontal line in the frame. A TV displays the first field of alternating lines over the entire screen, and then displays the second field to fill in the alternating gaps left by the first field. One NTSC video frame, displayed for approximately 1/30 of a second, contains two interlaced fields, displayed for approximately 1/60 of a second each.

    Interlaced video describes a frame with two passes of alternating scan lines.
    Interlaced video describes a frame with two passes of alternating scan lines.

    In noninterlaced video, scan lines are drawn in order from top to bottom, in one pass. Computer video and computer movie formats are generally noninterlaced. Motion-picture film is similar to noninterlaced video because it also displays an entire frame in one pass.

    Interlacing is a factor in image quality only for certain effects, such as rotating a frame or compositing video with digital effects. It is a characteristic of footage capture and display, not a structural component of file formats or media. You can easily play back a digitized NTSC or PAL movie (interlaced) on a computer display (noninterlaced), or display a 35mm photograph (noninterlaced) on an NTSC or PAL video monitor (interlaced).

    If you use interlaced video in a composition or want to use an After Effects movie in an interlaced-video medium such as NTSC, you must separate fields as you import and field-render the composition to a finished movie file. For more information, see Using interlaced video in After Effects and Testing the field-rendering order.