Letterboxing and Aspect Ratios
Another very important consideration when trying to decide on the optimal format in which to encode your content is the aspect ratio of the original source content.
Aspect ratio simply refers to the ratio of the width of the image to the height of the image as shown on screen. There are three common aspect ratios in use for commercial video content today:
4:3 used for almost all standard-definition TV broadcast content. This is sometimes also referred to as 1.33:1:
Above is an example 4:3 video
16:9 used for almost all high-definition TV content (HDTV) and many theatrical DVD releases. This is sometimes also referred to as 1.78:1:
Above is an example 16:9 video
2.35:1 used for “Cinemascope” or “Panavision” movies on DVD:
Above is an example 2.35:1 video
For commercial DVDs, you will generally find the aspect ratio indicated on the back. For other types of video content, you can determine the aspect ratio yourself simply by dividing the width of a video by its height. For example a 640x480 video clip would have a 1.33:1 aspect ratio (640/480 = 1.33).
Since the aspect ratio of the content will not always match the aspect ratio of the actual output device, you will often get either “letterboxing” or cropping, depending on the device and its settings. Letterboxing is more common, which is the practice of adding black bars to the top and bottom of a widescreen video when playing it back on a standard TV.
A widescreen TV show, 16:9 aspect ratio, as shown on a 4:3 screen
A Cinemascope Movie, 2.35:1 aspect ratio, as shown on a 4:3 screen
Conversely, if you were to watch a standard 4:3 TV show on a widescreen TV, you will end up with something referred to as pillarboxing which places black bars at the sides of the image:
A standard 4:3 TV show as shown on a 16:9 widescreen TV
On the other hand, some TVs and other devices (such as an iPod) also provide the option for cropping content to fit the aspect ratio of the screen. As the name implies, cropping the content chops off the portion of the image that does not fit, expanding the image to the full size of the screen.
This will result in the loss of detail on the left and right sides when trying to display widescreen content on a 4:3 screen:
A widescreen TV show cropped for 4:3 display
A Cinemascope Movie, 2.35:1 aspect ratio, cropped for 4:3 display
Or, the top and bottom of the image in the case of trying to display 4:3 content to fill a 16:9 screen:
A standard 4:3 TV show cropped for 16:9 display
Anamorphic Encoding
Some newer video conversion tools now provide support for anamorphic encoding. This basically refers to encoding video content in one aspect ratio but setting it to display using another, through the use of non-square pixels.
A pixel is not necessarily always rendered as a perfectly square element of a picture, and many modern video playback applications and hardware support a pixel aspect ratio (PAR) setting. In fact, the aspect ratio of content as stored on a normal North American DVD is actually 1.5:1 (720 x 480), rather than either 16:9 or 4:3. A DVD player, however, produces an image in the appropriate aspect ratio by reading a flag within the content that tells the player what shape of pixels to use when playing back the video content. The result is that a 720x480 DVD image is actually rendered in 640 x 480 (4:3) or 854 x 480 (16:9).
Without getting into too much technical detail, the short explanation is that the use of anamorphic encoding can provide a proper widescreen presentation of a movie without having to force it to actually be encoded at the higher resolution. Since nothing is truly gained by increasing the encoded resolution of a video, it makes more sense to save the storage space and simply render the frame in its proper aspect ratio, in the same way that a DVD player does.
Hard Letterboxing
One other problem that you may encounter is that many DVDs, particularly older ones, were actually in what was known as a “hard letterbox” format, rather than a proper anamorphic widescreen format. In this case, even though the original video format may have been widescreen, the DVD video is not actually encoded in a widescreen aspect ratio. Rather, “letterboxed” movies were basically encoded onto the DVD in a 4:3 aspect ratio with the black bars at the top and bottom actually added to the movie. This would allow these movies to play in a widescreen format even on older DVD players that did not have a widescreen mode.
Unfortunately, however, when these DVDs are converted, most video converters will leave them in their original 4:3 aspect ratio, with the black bars as part of the frame. In this case, the resulting video will be treated as a 4:3 video, with the black bars continuously shown at the top and bottom of the frame. Cropping will not be available on the iPod, and the Apple TV and iPhone will show these videos by default in a black frame:
In this case, the black bars at the top and bottom are not being generated by the output device, but are in reality part of the video image.
When dealing with DVDs, the description on the packaging can often be helpful in determining what the source format is. Although the terminology differs among the different studios, many older DVDs that used the term “Letterboxed” referred to this type of encoding: a 4:3 image with the black bars added to the frame. On the other hand, the term “anamorphic widescreen” almost always refers to videos that are properly encoded in 1.85:1 or 2.35:1 aspect ratio. Note that this will be a much less common issue with DVDs that have been more recently published.
You can also check whether or not your video content is in a proper widescreen format by viewing it in a window on your computer through iTunes or QuickTime. When playing video in a window, QuickTime will not add any black bars, but will rather show the window itself in the proper aspect ratio.
|
