How SubPictures Work - Part 1

This is the first of a series of articles leading up to how menus work. But before we explain menus it is important to understand subpictures, as they are a major component of any menu.

Part I - Size Matters

Display modes

DVDs have 4 display modes, they are Normal, Widescreen, Auto-Letterboxed, and Auto-Pan & Scan (Auto-P/S). A Titleset (vts) can have an aspect ratio of either 4:3 or 16:9 (sometimes called anamorphic). 4:3 is the aspect ratio of standard television sets, and neither a standard or widescreen television needs any help displaying them. These movies are always displayed in Normal mode, which does not scale or crop the movie in any way.

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 subpicture layer.

Auto-Letterboxed reduces the height to 3/4, which, for NTSC, is 720x360 with 60 blank lines above and below. PAL is 720x432 with 72 blank lines above and below.

Auto-P/S displays a 540x480 (540x576) area of the movie stretched by 1/3 (or reclocked) to fill the display. The center of this area is encoded into the video stream. Since the area is displayed full height, the vertical offset should remain zero. The offset is given in 1/16 pixel increments, horizontal offset for 720 pixel wide frame may be from -1440 to +1440. (a centered frame has 90 pixels to either side not visible, 90 x 16 = 1440)

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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