A number of different screen formats are available for DVD-Video and special effects can be added. DVD-Video offers a range of screen formats plus the option of multiple camera angles and video stills.
Video Screen Formats
The aspect ratio of traditional TV pictures is 4:3 (horizontal:vertical) while movies are generally a wider aspect ratio, up to 2.35:1. A compromise format, 16:9, is now in use for widescreen TVs and, in Europe in particular, for an increasing number of television programs.
DVD-Video discs can contain video suitable for either 4:3 or widescreen TVs. A total of three screen formats are available.
- Widescreen, which is only suitable for a widescreen TV display, where a 16:9 image is displayed full screen.
- Letterbox, where the full widescreen image is displayed but leaving black bars at the top and bottom of the screen.
- Pan & Scan where a widescreen movie is displayed as a 4:3 image which is narrower than the original.
16:9 source video is anamorphically squeezed (because the image needs to conform to the 4:3 format on disc, there being the same number of pixels available per line for both 4:3 and 16:9 images) to 4:3 before MPEG-2 encoding, then displayed as letterbox or pan & scan on a normal TV or full size on a widescreen TV. A 16:9 image will comprise pixels that are not square, due to the anamorphic squeezing. Alternatively, a pan & scan version can be encoded on disc for display on normal TVs.
The range of on-disc and display formats are listed in the table below, which shows the options for displaying different source image formats on normal and widescreen displays.
|On-disc format||Resultant display|
|4:3 display||16:9 display|
|4:3 full frame||4:3 full screen||4:3|
|4:3 letterbox||4:3 letterbox||16:9 full screen|
|16:9 no pan & scan data||4:3 letterbox||16:9 full screen|
|16:9 with pan & scan data*||4:3 letterbox/pan & scan||16:9 full screen|
* This format requires data to define on a frame by frame basis which part of the image should be displayed. As yet there is probably no title available which uses this format. Instead, DVD-10 discs have been used to offer both pan & scan and widescreen versions on one disc.
Video Camera Angles
DVD-Video discs offer the possibility to include up to 9 different camera angles which can be seamlessly selected by the user. The different but parallel sequences (angles) are interleaved on the disc to allow rapid seeking between them. The result is that the overall data rate available is somewhat reduced to allow for the seek time needed to allow the laser to jump to the next part of each sequence or angle. The reduction in data rate depends on the number of angles included.
The above example shows how three angles are interleaved within the video stream (with the audio and any subpictures). Any particular angle (2 in this example) is played by jumping across the other two. The need to re-seek at intervals does reduce the overall bit rate.
Multiple camera angles can be seamless or non-seamless. Non-seamless transitions happen instantly but there is a break in playback. Seamless transitions are slower but there is no break in playback.
Also video sequences can be displayed as stills using the player's still-frame control. A video sequence can also be caused to freeze automatically by being programmed to do so. The quality of still frames depends on the video material. If a video sequence with movement is paused, any movement between the two fields will prevent the display of a perfectly still image. An example is a person walking or moving their arms. Some players allow a single field to be displayed (but repeated to give a complete frame) to avoid this problem, although the resolution is reduced.
Encoding problems can affect the display of stable still frames. For example incorrect field dominance can produce a still frame which comprises fields from two successive frames. The result will be particularly disturbing at a cut between two scenes. Video from film will not exhibit this problem as both fields were shot at the same time.
Player controls also allow the video to be played in slow motion (forward motion is relatively smooth as every frame is played but reverse motion plays only the I-frames) and fast forward or reverse (by displaying I-frames only).