Anamorphic widescreen
Anamorphic widescreen (also called full-height anamorphic or FHA) is a process by which a comparatively wide widescreen image is horizontally compressed to fit into a storage medium (photographic film or MPEG-2 standard-definition frame, for example) with a narrower aspect ratio, reducing the horizontal resolution of the image while keeping its full original vertical resolution. Compatible play-back equipment (a projector with modified lens, or a digital video player or set-top box) can then expand the horizontal dimension to show the original widescreen image. This is typically used to allow one to store widescreen images on a medium that was originally intended for a narrower ratio, while using as much of the frame – and therefore recording as much detail – as possible.[1]
For the film format, see Anamorphic format.
The technique comes from cinema, when a film would be framed and recorded as widescreen but the picture would be "squashed together" using a special concave lens to fit into non-widescreen 1.37:1 aspect ratio film. This film can then be printed and manipulated like any other 1.37:1 film stock, although the images on it will appear to be squashed horizontally (or elongated vertically). An anamorphic lens on the projector in the cinema (a convex lens) corrects the picture by performing the opposite distortion, returning it to its original width and its widescreen aspect ratio.
The optical scaling of the lens to a film medium is considered more desirable than the digital counterpart, due to the amount of non-proportional pixel-decimated scaling that is applied to the width of an image to achieve (something of a misnomer) a so-called "rectangular" pixel widescreen image. The legacy ITU-R Rec. 601 4:3 image size is used for its compatibility with the original video bandwidth that was available for professional video devices that used fixed clock rates of a SMPTE 259M serial digital interface. One would produce a higher-quality upscaled 16:9 widescreen image by using either a 1:1 SD progressive frame size of 640×360 or for ITU-R Rec. 601 and SMPTE 259M compatibility a letterboxed frame size of 480i or 576i. Similar operations are performed electronically to allow widescreen material to be stored on formats or broadcast on systems that assume a non-widescreen aspect ratio, such as DVD or standard definition digital television broadcasting.
Laserdisc[edit]
While not anamorphic widescreen per se, many of the earliest Laserdisc offerings forwent the pan-and-scan cropping typical of home releases at the time, the mastering-technicians opting instead to simply squeeze the film's original aspect ratio down to 4:3. While this resulted in an image that was overly compressed on standard televisions, many later HDTVs can stretch out this picture, thus restoring the correct aspect ratio.
Later during the 1990s, a handful of Laserdiscs were released with proper anamorphic transfers.
Video was stretched vertically to fill the whole 4:3 picture of a Laserdisc (and add more information where black bars would be at the top and bottom) then either un-squeezed horizontally on a 16:9 TV set or using an anamorphic lens on a 4:3 video projector.
Blu-ray video[edit]
Unlike DVD, Blu-ray supports SMPTE HD resolutions of 720p and 1080i/p with a display aspect ratio of 16:9 and a pixel aspect ratio of 1:1, so widescreen video is scaled non-anamorphically (this is referred to as "square" pixels).
Blu-ray also supports anamorphic wide-screen, both at the DVD-Video/D-1 resolutions of 720×480 (NTSC) and 720×576 (PAL), and at the higher resolution of 1440×1080 (source aspect ratio of 4:3, hence a pixel aspect ratio of 4:3 = 16:9 / 4:3 when used as anamorphic 16:9). See Blu-ray Disc: Technical specifications for details.
Television[edit]
Major digital television channels in Europe (for example, the five major UK terrestrial TV channels of BBC One, BBC Two, ITV, Channel 4 and Channel 5), as well as Australia, carry anamorphic widescreen programming in standard definition. In almost all cases, 4:3 programming is also transmitted on the same channel. The SCART switching signal can be used by a set-top-box to signal the television which kind of programming (4:3 or anamorphic) is currently being received, so that the television can change modes appropriately. The user can often elect to display widescreen programming in a 4:3 letterbox format instead of pan and scan if they do not have a widescreen television.
TV stations and TV networks can also include Active Format Description (AFD) just as DVDs can. Many ATSC tuners (integrated or set-top box) can be set to respond to this, or to apply a user setting. This can sometimes be set on a per-channel basis, and often on a per-input basis, and usually easily with a button on the remote control. However, tuners often fail to allow this on SDTV (480i-mode) channels, so that viewers are forced to view a small picture instead of cropping the unnecessary sides (which are outside of the safe area), or zooming to eliminate the windowboxing that may be causing a small picture, or stretching/compressing to eliminate other format-conversion errors. The shrunken pictures are especially troublesome for smaller TV sets.
Many modern HDTV sets have the capability to detect black areas in any video signal, and to smoothly re-scale the picture independently in both directions (horizontal and vertical) so that it fills the screen. However, some sets are 16:10 (1.6:1) like some computer monitors, and will not crop the left and right edges of the picture, meaning that all programming looks slightly (though usually imperceptibly) tall and thin.
ATSC allows two anamorphic widescreen SDTV formats (interlaced and progressive scan) which are 704×480 (10% wider than 640×480); this is narrower than the 720×480 of DVD due to 16 pixels being consumed by overscan (nominal analogue blanking) – see overscan: analog to digital resolution issues. The format can also be used for fullscreen programming, and in this case it is anamorphic with pixels slightly taller (10:11, or 0.91:1) than their width.