16:9, on the other hand, can be displayed on a widescreen television properly, but needs some help on standard televisions. For widescreen televisions the Widescreen mode is used, which, like Normal mode, does not scale or crop the movie, allowing a widescreen television to show it properly.
Displaying 16:9 on a 4:3 display (television) requires either shrinking the picture vertically (Auto-Letterbox) or cropping and
stretching horizontally (Auto-P/S).
So, what's this got to do with the subpictures?
In order to allow use of the entire screen in Auto-Letterboxed, and full resolution in Auto-P/S, subpictures are NOT
resized and cropped along with the movie. Because of this, there may be as many as three subpictures streams for each selection (track).
One for Widescreen, another for Auto-Letterboxed, and a third for Auto-P/S. Each can be optimized for the display mode. For example,
Auto-Letterboxed subpictures can utilize the black areas above and below the movie area. Auto-P/S need not worry about being cropped off
on the sides as the action pans.
Are there any pitfalls?
Sure, especially when authoring a menu. Care must be taken to align the subpicture with the underlying picture, as there no longer is
a 1:1 relationship between them for Auto-Letterboxed and Auto-P/S. The movie frame has to be properly resized before adding the
What is the subpicture resolution?
The size of the subpicture area, which corresponds to the full display screen in any display mode, is 720x480 for NTSC, or 720x576 for
PAL. Subpictures (except menus) do not typically use the entire display area, and only that area which is used is encoded. Note,
though, that subpictures are limited to 478 lines in NTSC, and 573 lines in PAL. However, the subpicture window can be placed anywhere
on the 720x480 (720x576) screen.
What about half-resolution movies?
Movies which are 352x240 (NTSC) or 352x288 (PAL/SECAM) get resized before the subpicture is mixed into the video. The
subpicture area remains 720x480 (720x576).
Subpictures may be interlaced or half resolution
While the display area for a subpicture is always 720x480 or 720x576, there are two pointers in the subpicture
SET_DSPXA command. The first is for the top field of data, the second points to the bottom field.
For an interlaced subpicture these point to separate RLE compressed data. But if space is tight the pointers can point to the same data,
making for a half-resolution non-interlaced overlay.
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