Letterboxing is the practice of transferring film shot in a widescreen aspect ratio to standard-width video formats while preserving the film's original aspect ratio. The resulting videographic image has mattes (black bars) above and below it; these mattes are part of the image (i.e., of each frame of the video signal). LTBX is the identifying abbreviation for films and images so formatted. The term refers to the shape of a letter box, a slot in a wall or door through which mail is delivered, being rectangular and wider than it is high.
Letterboxing is used as an alternative to a full-screen, pan-and-scan transfer of a widescreen film image to videotape or videodisc. In pan-and-scan transfers, the original image is cropped to the narrower aspect ratio of the destination format, usually the 1.33:1 (4:3) ratio of the standard television screen, whereas letterboxing preserves the film's original image composition as seen in the cinema. Letterboxing was developed for use in 4:3 television displays before widescreen television screens were available, but it is also necessary to represent on a 16:9 widescreen display the unaltered original composition of a film with a wider aspect ratio, such as Panavision's 2.35:1 ratio.
Letterbox mattes are usually symmetrical (both the top and bottom mattes are roughly similar in size), but in some instances the picture can be elevated so the bottom matte is much larger, usually for the purpose of placing "hard" subtitles within the matte to avoid overlapping of the image. This was often done for letterbox widescreen anime on VHS, though the practice of "hiding" subtitles within the lower matte also is done with symmetrical mattes, albeit with less space available. The placing of "soft" subtitles within the picture or matte varies according to the DVD player being used, though it appears to be dependent on the movie for Blu-Ray disc.
An alternative to letterboxing is anamorphic widescreen presentation, which squeezes the picture horizontally to fit into a narrower aspect ratio. The player or receiver must correct this distortion by either stretching the image back to its original relative width, for display on widescreen televisions, or letterboxing it (during playback) for display on 4:3 video screens. This image transformation generally requires digital signal processing, so letterboxing was the only way in which films were presented in widescreen on home video prior to the DVD format (with a few exceptions outside the mass market, such as Squeeze LD). Anamorphic widescreen video recordings are sometimes called "anamorphically enhanced", in comparison to letterboxed versions. To represent a film wider than 16:9 (e.g., a 2.35:1 film) on a 16:9 display with no cropping, both anamorphic and letterbox techniques (or letterboxing alone) are required; using the anamorphic technique, the mattes will be smaller but still necessary.
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